A few days ago, I was passed out on my couch (sleep apnea) and for some reason I awoke and noticed that I had gotten a CSEM notification of a large earthquake offshore of Alaska. Well, after looking into that, I sent my boss, Rick, a text message: “8.2.”
https://earthquake.usgs.gov/earthquakes/eventpage/us6000f02w/executive
Rick Wilson runs the tsunami program at the California Geological Survey (CGS) and works with the California Governor’s Office of Emergency Services (Cal OES) to use official forecasts of tsunami size from the National Tsunami Warning Center (NTWC) to alert coastal emergency managers about the level of potential evacuation that they may want to act upon.
More about this process can be found here. Take a look at the CGS Special Report 236 to learn about the Tsunami Playbooks and the “FASTER” approach for tsunami evacuation guidance. Evacuation is something that is done at the local level, so CGS and Cal OES can only provide recommendations.
Needless to say, we were both at the ready to respond. Rick has hourly phone calls with the NTWC and follows up with phone calls and emails to specific interested parties (e.g. the emergency managers). We each went into tsunami response mode. I manage the Tsunami Event Response Team, which may be activated to collect observations of tsunami inundation or ocean currents.
I started looking at tide gage and DART Buoy data to see how large the tsunami was in the epicentral region. The M 8.2 was in the region of the 1938 M 8.2 earthquake which generated a transoceanic tsunami. I also looked into the literature about the 1938 tsunami, to see what size that tsunami was. The 1938 tsunami had a decimeter scale wave height (peak to trough) for gages in Alaska and in California (Johnson and Satake, 1994). Jeff Freymueller et al. (2021) had also recently worked on the 1938 earthquake source area and tsunami modeling as well.
The nearest tide gage for this 2021 event is at Sand Point, but the nearest gage in 1938 was in Unalaska. So, in order to get a modest comparison between 1938 and 2021, I felt a need to wait for the Unalaska data to trickle in. This may give us some idea whether the 1938 tsunami recorded in Crescent City and San Francisco might be a decent analogue. Of course, we need to get the official forecast from the NTWC prior to sending out any information. But, that process can take hours (over 3 hours in this case). So, we need to get our minds wrangled around the possibilities in the absence of more information.
Earthquake and Tectonic Background:
The plate boundary in the north Pacific is a convergent (pushing together) plate boundary where the Pacific plate on the south ‘subducts’ northwards beneath the North America plate on the north. The Alaska-Aleutian subduction zone forms a deep sea trench which can be seen in maps of the region. The subduction zone fault dips into the Earth, getting deeper to the north.
Between earthquakes (the interseismic period), the megathrust fault is seismogenically coupled (i.e. ‘locked’) just like velcro has the ability to hold together one’s wallet. The plates are always moving towards each other. Because the fault is locked, the crust surrounding the fault bends elastically to accommodate this convergent motion.
As the crust bends and flexes, it stores energy (i.e. tectonic strain). The part of the fault closest to the seafloor (the southernmost part of this subduction zone fault) gets pulled downwards, while the part of the crust further to the north flexes upwards.
The materials along the earthquake fault have properties that resist motion (like the velcro). But, as the plates converge and increase the amount of energy stored, the forces on the fault may exceed the strength of the fault. At this time, the fault slips, causing an earthquake.
The part of the fault that was being pulled downwards gets pushed upwards during the earthquake (the coseismic period), while the crust that was being flexed upwards between earthquakes thus subsides downwards during the earthquake.
The Alaska-Aleutian subduction zone has a history of subduction zone earthquakes and tsunami, plus there exists a prehistory of earthquakes and tsunami in some parts of this plate boundary. Geologists are often asked to determine the potential hazard of future earthquakes and tsunami and their answers are based on what we know from the past (using both historic and prehistoric data).
The 2021 M 8.2 earthquake happened in the same location as a 1938 M 8.2 earthquake, just to the east of a sequence of earthquakes from last year (22 July and 19 October 2020).
Tsunami:
When the earthquake fault slips, and the upper plate deforms, the vertical motion of the plate can elevate (or lower) the overlying ocean water. After the water changes position, it seeks to return to sea-level (an equipotential surface). If elevated, the water drops downwards and then oscillates up and down. This is the process that generates waves that radiate from the area with seafloor deformed by the earthquake.
-
Things that make a tsunami larger are [generally]:
- More vertical land motion (possibly from larger slip on the fault, e.g. from a larger magnitude earthquake)
- Deeper water (deeper water = more volume of water moving = more energy to create larger tsunami waves)
So let’s take a look at the things that may have affected the size of the tsunami from this 2021 M 8.2 earthquake.
First of all, based on the earthquake slip models (estimates of how the earthquake slipped, in meters, and how that slip varied along the fault) suggest that a majority of the largest slip happened beneath the continental shelf. The water depth on the shelf is similar to many shelfs worldwide, shallower than about 200 meters. How does this affect the size of the tsunami?
Well, I guess that is the main point, the ground deformation that generated the tsunami was beneath shallow water.
These slip models are based on a variety of data and most of the data are seismic data. Some tsunami are generated by slow slip (not generating seismic waves) on the shallow part of the fault. These are called tsunami earthquakes.
Because tsunami earthquakes may be generated by slip in this way, slip models using seismic data cannot resolve the location of the slip on the fault that created these tsunami. However, the tsunami from this 2021 M 8.2 earthquake were small. Therefore the updip part of the fault probably did not contribute significantly to the tsunamigenic ground deformation.
Below is my interpretive poster for this earthquake
- I plot the seismicity from the past 3 months, with diameter representing magnitude (see legend). I also include earthquake epicenters from 1921-2021 with magnitudes M ≥ 7.5.
- I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
- A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
- Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.
- I include outlines of the historic subduction zone earthquakes as prepared by Peter Haeussler from the USGS in Anchorage. He appears in the video about the 1964 earthquake below.
- Some of the tide gage and DART buoy locations are labeled.
- Note how there are still aftershocks from the 2018 M 7.9 earthquake sequence.
- This is the first poster I prepared.
- In the upper center is a low-angle oblique view of the plate boundary. Note the oceanic Pacific plate is subducting beneath the continental North America plate. As the plate goes down, the water embedded in the rocks and sediment are released into the overlying mantle wedge. This water causes the mantle to melt, which rises, erupts as lava and forms the volcanic chain we call the Aleutian Islands. I place a green star in the “epicentral” location of the 2021 M 8.2 earthquake.
- In the upper left corner is part of a figure from Witter et al. (2019) that shows sections of the megathrust fault relative to how much the fault is thought to be locked. This is called the coupling ratio. For a fault that is fully coupled (or locked), the ratio is 1.0. For a fault that is slipping about 50% and accumulating about 50% of the plate motion rate, the coupling ratio is 0.5. Many subduction zones have low coupling ratios of 0.2-0.6. The region of the fault west of the 1938 and 2021 M 8.2 earthquakes is called the Shumagin Gap, thought to be possibly aseismic (with a coupling ratio closer to 0). But the 2020 sequence of M 7.8 and 7.9 earthquakes filled much of this gap.
- In the upper right corner is a plot showing the earthquake shaking intensity using the Modified Mercalli Intensity Scale (MMI). This is a USGS model based on observations of intensity from thousands of earthquakes. Read more about MMI here.
- In the center right is a plot showing the aftershocks within a couple hours of the mainshock
- In the lower right corner is the initial record of the tsunami at the Sand Point tide gage (see map for gage location).
- I labeled the USGS slab 2.0 slab contours (Hayes et al., 2018). These depth contours represent the depth of the megathrust fault at these locations. The M 8.2 hypocentral depth is 32.2 km and the slab2 depth is about 35 km. Nice!
I include some inset figures. Some of the same figures are located in different places on the larger scale map below. I present 3 posters, each with slightly different information.
- Here is the map with 3 month’s seismicity plotted. There are 3 posters. The first one is something I put together around 2 hours after I awoke on the couch (abt 2am my time). I prepared the 2nd poster an hour later, which includes some information about tsunami prehistory. I prepared the 3rd poster late Sunday evening, about 3 days after the earthquake.
- This is the second map I prepared and some figures are the same as in the first poster.
- Below the low-angle oblique map is a slip model from the USGS. The color represents the amount of slip on the fault. Note that the maximum slip is close to the epicenter. This is not always the case, as for the 1938 event, it appears that the maximum slip was not where the mainshock epicenter was.
- In the upper left corner is a map from Nelson et al. (2015). Those authors studied the prehistoric tsunami records at Chrikof Island, an island about 200 km to the east of the 2021 M 8.2 epicenter. The lower map shows GPS derived plate motion rates.
- In the lower right corner is also from Nelson et al. (2015). On this plot, the vertical axis represents time with “today” at the top and over 5000 years ago at the bottom. The horizontal axis is space, west to east from left to right. Each colored symbol represents the time of a prehistoric tsunami. The vertical size of these symbols represents the uncertainty (or “error”) associated with those chronologic data. We can take the number of earthquakes or tsunami over a period of time to estimate how frequently those process happen over time.
- To the left is a more updated version of the Sand Point tide gage, showing a wave height (peak to trough) of about 45 cm. We cannot compare this to the 1938 tsunami as there was not a tide gage at Sand Point in 1938
- I prepared a 3rd poster, but updated it to this 4th poster.
- In the Intensity Data area, I added USGS “Did You Feel It?” data, which come from reports from real people. Learn more about dyfi here. The model data are the colored lines labeled in white and the dyfi data are colored polygons labeled in yellow.
- In the aftershocks plot, I added epicenters from the several days after the mainshock. I also added a transparent overlay of the USGS finite fault model (the slip model). Compare the overlap, or non-overlap, of the slip region and the aftershocks. Why do you think that they are not completely overlapping?
- In the lower right section are tide gage records from gages in the area included in the poster. I plot the tidal forecast (dark blue), the tide gage observed water surface elevation (medium blue), and the difference between these data (in light blue) which is a record of the tsunami (and other waves, like wind waves). I made a rough approximation estimate of the maximum wave height and labeled this in yellow. The San Point tide gage has a mx wave height of about 0.8 m!
- I also plot the data from the DART buoy 46403, which is the closest DART buoy to the mainshock epicenter. The DART buoy network is used to help calibrate tsunami forecast models during tsunami events. These are basically pressure transducers on the seafloor that measure changes in pressure caused by waves and atmospheric processes. The data plotted here are not tsunami data, but seismic wave data. One reason we know that this is not a tsunami is that the waveform initiated about 3 minutes after the earthquake. A tsunami would take longer to get to the buoy.
- In the upper left corner is a pair of maps that show USGS earthquake induced ground failure models. The map on the right shows what areas have likelihood of having landslides triggered by the 2021 M 8.2 earthquake. The panel on the right shows the possibility that areas might experience liquefaction induced by the earthquake.
- I added aftershocks associated with the 2020 M 7.8/7.5 sequence that filled the Shumagin Gap (green circles) and outlined the aftershock region for both 2020 and 2021 sequences. The 2021 sequence is not yet over. The largest aftershock so far has only been M 6.1. The 1938 M 8.2 event had a M~7 event 5 days after the mainshock. Stay tuned?
Tectonic Overview
Below is an educational video from the USGS that presents material about subduction zones and the 1964 earthquake and tsunami in particular.
Youtube Source IRIS
mp4 file for downloading.
-
Credits:
- Animation & graphics by Jenda Johnson, geologist
- Directed by Robert F. Butler, University of Portland
- U.S. Geological Survey consultants: Robert C. Witter, Alaska Science Center Peter J. Haeussler, Alaska Science Center
- Narrated by Roger Groom, Mount Tabor Middle School
This is a map from Haeussler et al. (2014). The region in red shows the area that subsided and the area in blue shows the region that uplifted during the earthquake. These regions were originally measured in the field by George Plafker and published in several documents, including this USGS Professional Paper (Plafker, 1969).
Here is a cross section showing the differences of vertical deformation between the coseismic (during the earthquake) and interseismic (between earthquakes).
This figure, from Atwater et al. (2005) shows the earthquake deformation cycle and includes the aspect that the uplift deformation of the seafloor can cause a tsunami.
Here is a figure recently published in the 5th International Conference of IGCP 588 by the Division of Geological and Geophysical Surveys, Dept. of Natural Resources, State of Alaska (State of Alaska, 2015). This is derived from a figure published originally by Plafker (1969). There is a cross section included that shows how the slip was distributed along upper plate faults (e.g. the Patton Bay and Middleton Island faults).
Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia, but the analogy works for Alaska also. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.
This is a photo that I took along the Seward HWY 1, that runs east of Anchorage along the Turnagain Arm. I attended the 2014 Seismological Society of America Meeting that was located in Anchorage to commemorate the anniversary of the Good Friday Earthquake. This is a ghost forest of trees that perished as a result of coseismic subsidence during the earthquake. Copyright Jason R. Patton (2014). This region subsided coseismically during the 1964 earthquake. Here are some photos from the paleoseismology field trip. (Please contact me for a higher resolution version of this image: quakejay at gmail.com)
This is another video about the 1964 Good Friday Earthquake and how we learned about what happened.
- Here is a map that shows historic earthquake slip regions as pink polygons (Peter Haeussler, USGS). Dr. Haeussler also plotted the magnetic anomalies (grey regions), the arc volcanoes (black diamonds), and the plate motion vectors (mm/yr, NAP vs PP).
- Here is the figure from Sykes et al. (1980) that shows the space time relations for historic earthquakes in relation to the map.
Above: Rupture zones of earthquakes of magnitude M > 7.4 from 1925-1971 as delineated by their aftershocks along plate boundary in Aleutians, southern Alaska and offshore British Columbia [after Sykes, 1971]. Contours in fathoms. Various symbols denote individual aftershock sequences as follows: crosses, 1949, 1957 and 1964; squares, 1938, 1958 and 1965; open triangles, 1946; solid triangles, 1948; solid circles, 1929, 1972. Larger symbols denote more precise locations. C = Chirikof Island. Below: Space-time diagram showing lengths of rupture zones, magnitudes [Richter, 1958; Kanamori, 1977 b; Kondorskay and Shebalin, 1977; Kanamori and Abe, 1979; Perez and Jacob, 1980] and locations of mainshocks for known events of M > 7.4 from 1784 to 1980. Dashes denote uncertainties in size of rupture zones. Magnitudes pertain to surface wave scale, M unless otherwise indicated. M is ultra-long period magnitude of Kanamori 1977 b; Mt is tsunami magnitude of Abe[ 1979]. Large shocks 1929 and 1965 that involve normal faulting in trench and were not located along plate interface are omitted. Absence of shocks before 1898 along several portions of plate boundary reflects lack of an historic record of earthquakes for those areas.
- Here is a great illustration that shows how forearc sliver faults form due to oblique convergence at a subduction zone (Lange et al., 2008). Strain is partitioned into fault normal faults (the subduction zone) and fault parallel faults (the forearc sliver faults, which are strike-slip). This figure is for southern Chile, but is applicable globally.
Proposed tectonic model for southern Chile. Partitioning of the oblique convergence vector between the Nazca plate and South American plate results in a dextral strike-slip fault zone in the magmatic arc and a northward moving forearc sliver. Modified after Lavenu and Cembrano (1999).
In 2016, there was an earthquake along the Alaska Peninsula, a M 7.1 on 2016.01.24. Here is my earthquake report for this earthquake. Here is a map for the earthquakes of magnitude greater than or equal to M 7.0 between 1900 and today. This is the USGS query that I used to make this map. One may locate the USGS web pages for all the earthquakes on this map by following that link.
Tsunami Data
I plot tide gage data for gages in the north and northeast Pacific Ocean. These data are from NOAA Tides and Currents, though are also available via the eu tide gage website here.
-
Each plot includes three datasets:
- The tidal forecasts are shown as a dark blue line.
- The actual observed water surface elevation is plotted in medium blue.
- By removing (subtracting) the tide forecast from the observed data, we get the signal from wind waves, tsunami, and atmospheric phenomena. This residual is plotted in light blue.
The scale for the tsunami wave height is on the right side of the chart.
Note the all tsunami wave height plots are the same vertical scale, except for Sand Point.
I measured the largest wave heights for each site, displayed in yellow.
Alaska
Here are the data from the DART buoy nearest the M 8.2. People often mistake these data for tsunami data, but this is generated by seismic waves.
One way to test one’s hypothesis about whether these buoy data are seismic waves or tsunami waves, one simply need to take a look at the time that the wave begins to be recorded by the DART buoy.
Seismic waves travel through water at about 1.5 kms per second. While tsunami wave velocity (based on the shallow water wave equation) for depths ranging from 200-4000 meters is between ~0.02 to 0.2 kms per second, much slower than seismic waves.
Surface Deformation
Below are surface deformation data generated by the USGS based on their finite fault model. The three panels show surface deformation in the north, east, and vertical directions.
North, East, and Up are positive (blue) while South, West, and Down are negative (red).
Note the upper panel and how the Pacific plate is moving to the north and the North America is moving south. Does this make sense?
The middle panel is interesting too, but skip to the lower panel, vertical. The accretionary prism (forming the continental slope), directly above the aftershocks and mainshock, rises up during the earthquake. The upper North America plate landward of the slip patch subsides. Does this make sense?
Earlier in this report we took a look at the geologic evidence for megathrust subduction zone earthquakes, evidence that records this “coseismic” subsidence.
Shaking Intensity and Potential for Ground Failure
- Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.
- Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides.
- I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.
There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:
FOS = Resisting Force / Driving Force
When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.
Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.
Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.
Some Relevant Discussion and Figures
- Johnson and Satake (1994) studied tsunami waveforms from the 10 November 1938 Alaska M 8.2 earthquake. Their analysis was designed to estimate the source for the tsunami. Below are some figures from their paper, with figure captions beneath each figure.
- This first plot shows the tsunami records from tide gages. This is the plot I used to consider the potential impact to the coast from the 2021 M 8.2 tsunami.
- Here is a map that shows the fault model that they used, as well as the amount of slip that they used for each fault element.
- This is a figure comparing their model results (synthetic = dashed) compared to the tide gage records (solid lines).
Digitized marigrams from 1938 Alaskan earthquake recorded in Crescent City, San Diego, and San Francisco. The tidal componenht asn ot beenr emoved.S tartt ime listedf or each record is the time in minutes from the origin time of the earthquaketo the startt ime of the digitizedr ecord.
Location of subfaults used in inversion of tsunami waveforms. Graph shows slip distribution in meters.
Observed and synthetic waveforms from inversion for four subfaults. Start time of each record is different. The arrows indicate the parts of the waveforms used for the inversion.
- Freymueller et al. (2021) also studied the 1938 M 8.2 event, seeking to resolve the slip on the fault using tsunami modeling.
- Below are figures with their captions in blockquote.
- Here are some maps showing 2 of the slip distrubutions that they used for their modeling.
- Here are some maps showing vertical seafloor displacements for some of their tsunami scenarios.
- Here are plots that show some results of their modeling. The tide gage data are plotted in black and their simulated waves are plotted with red and blue lines.
Example slip distributions for two of the slip models, shallow eastern and shallow far eastern. For each model the slip is the product of a function f(x) representing the along-strike variation and g(y) representing the downdip variation, and then scaled to a constant magnitude MW 8.25. The functions f(x) and g(y) are based on relations in Freund and Barnett [1976]. For the central and western models, the rupture area is the same as for the eastern model, but the area of higher slip is shifted to the west. For the mid-depth and deep models, the main area of high slip is shifted downdip.
Vertical seafloor displacements caused by representative slip scenarios. On the left side, the slip is concentrated in the east and the deep, mid-depth and shallow slip distribution scenarios are shown. On the right, the Western, Central and Far Eastern slip distribution scenarios are shown assuming the shallow rupture. Displacements are in meters. Red contours show depth to the plate interface from 0 to 80 km with a 10 km increment.
Tide gauge data and model predictions for the eastern and far eastern source models.
Here is an animation from one of the Ferymueller et al. (2021) models for the 1938 M 8.2 tsunami.
- Nelson et al. (2015) presented their evidence for prehistoric tsunami on Chirikof Island, an island in the forearc in the eastern part of the 1938 earthquake slip patch.
- They found evidence for many tsunami over a timespan from before 5000 years ago.
- Below are some figures from their paper, with figure captions in blockquote.
- This figure shows the tectonic setting and the area of their field study.
- Here is a plot that shows the timing for the prehistoric tsunami inferred by these authors. The vertical axis is the time scale, with “today” at the top. Each colored pattern represents the age range for a tsunami deposit.
- These data are plotted left to right, west to east, so we can compare tsunami records at different locations along the margin. These comparisons are important so that we can test different hypotheses about how subduciton faults may slip over time. In the 2021 case, the slip area was close to the 1938 earthquake. But, did has this always occured here?
A) Location of Chirikof Island within the plate tectonic setting of the Alaska-Aleutian subduction zone. Rupture areas for great twentieth century earthquakes on the megathrust are in pink. (B) Velocity field of the Alaska Peninsula and the eastern Aleutian Islands observed by global positioning system (GPS) (Fournier and Freymueller, 2007). Colors show inferred rupture areas for earthquakes in 1788 (green) and 1938 (orange). Both A and B are modified from Witter et al. (2014). The section of the megathrust between Kodiak Island and the Shumagin Islands has been referred to as the Semidi segment (e.g., Shennan et al., 2014b). (C) Physiography of Chirikof Island (Google Earth image, 2012) showing the location of our study area at Southwest Anchorage, a prominent moraine, a fault scarp (facing southeast) that probably records the 1880 earthquake, the New Ranch valley reconnaissance core site, and UNAVCO GPS station AC13 (http:// pbo .unavco .org /station /overview /AC13). In the eighteenth and nineteenth centuries, Chirikof Island was known to native Alutiiq and Russians as Ukamuk Island.
Age probability distributions for probable (red) and possible (orange) tsunami deposits at Southwest Anchorage (labels as in Fig. 11) compared with age distributions for possible tsunami deposits at Sitkinak Island (Briggs et al., 2014a) and with age estimates for great earthquakes and tsunamis on Kodiak Island (from studies referenced on this figure;
Fig. 1). Dotted horizontal lines show our correlation of evidence for some younger earthquakes and tsunamis. Times of great earthquakes inferred from episodes of village abandonment determined from archaeological stratigraphy in the eastern Alaska-Aleutian megathrust region are also shown (Hutchinson and Crowell, 2007).
- Summary of the 1964 Earthquake
- 2021.07.29 M 8.2 Alaska
- 2020.10.19 M 7.5 Alaska POSTER
- 2020.07.22 M 7.8 Alaska POSTER
- 2019.04.02 M 6.5 Aleutians
- 2019.03.26 M 5.2 Alaska
- 2018.12.20 M 7.4 Bering Kresla
- 2018.12.20 M 7.3 Bering Kresla UPDATE #1
- 2018.11.30 M 7.0 Alaska
- 2018.08.15 M 6.6 Aleutians
- 2018.08.12 M 6.4 North Alaska
- 2018.08.12 M 6.4 North Alaska UPDATE #1
- 2018.01.23 M 7.9 Gulf of Alaska
- 2018.01.23 M 7.9 Gulf of Alaska UPDATE #1
- 2018.01.23 M 7.9 Gulf of Alaska UPDATE #2
- 2017.07.17 M 7.7 Aleutians
- 2017.07.17 M 7.7 Aleutians UPDATE #1
- 2017.06.02 M 6.8 Aleutians
- 2017.05.08 M 6.2 Aleutians
- 2017.05.01 M 6.3 British Columbia
- 2017.03.29 M 6.6 Kamchatka
- 2017.03.02 M 5.5 Alaska
- 2016.09.05 M 6.3 Bering Kresla (west of Aleutians)
- 2016.04.13 M 5.7 & 6.4 Kamchatka
- 2016.04.02 M 6.2 Alaska Peninsula
- 2016.03.27 M 5.7 Aleutians
- 2016.03.12 M 6.3 Aleutians
- 2016.01.29 M 7.2 Kamchatka
- 2016.01.24 M 7.1 Alaska
- 2015.11.09 M 6.2 Aleutians
- 2015.11.02 M 5.9 Aleutians
- 2015.11.02 M 5.9 Aleutians (update)
- 2015.07.27 M 6.9 Aleutians
- 2015.05.29 M 6.7 Alaska Peninsula
- 2015.05.29 M 6.7 Alaska Peninsula (animations)
- 1964.03.27 M 9.2 Good Friday
Alaska | Kamchatka | Kurile
General Overview
Earthquake Reports
Social Media
#EarthquakeReport for M8.2 #Earthquake and probable #Tsunami offshore of #Alaskahttps://t.co/mFtEoigFQB
read more about the tectonics herehttps://t.co/L4RHgNdex7 pic.twitter.com/Kgp6HxzSQ6
— Jason "Jay" R. Patton (@patton_cascadia) July 29, 2021
From @BNONews
BREAKING: Tsunami sirens sound in Kodiak, Alaska after a major magnitude 8.2 earthquake struck off the coast; risk being evaluated for the Pacific pic.twitter.com/amxpLGX70s— Desianto F. Wibisono (@TDesiantoFW) July 29, 2021
#EarthquakeReport preliminary interpretive poster for M 8.2 #Earthquake #tsunami offshore of #Alaska
in region 1938 M 8.2 generated #Tsunami with wave hts 5-10cm in #California (Johnson and Satake, '96)https://t.co/mFtEoigFQB
tectonic background:https://t.co/L4RHgNdex7 pic.twitter.com/uQ2ur85EaC— Jason "Jay" R. Patton (@patton_cascadia) July 29, 2021
Watch the waves from the M8.2 earthquake just offshore Alaska roll across the seismic stations in North America.
(Credit @IRIS_EPO) pic.twitter.com/8qQeV4qBZY
— Dr. Kasey Aderhold (@KaseyAderhold) July 29, 2021
Here's the #USGS MT for the recent M 8.2 on Fig. 1 of Freymueller et al. 2021 (https://t.co/FN8owbDqEY). Orange outline is aftershocks of the 1938 M 8.2. Red lines are 1 m contours of 1938 slip models. Grey is slip deficit inferred from geodesy. Obvious similarities 1938 -> 2021! pic.twitter.com/DIUh4YVhXc
— Rich Briggs (@rangefront) July 29, 2021
UPDATE: The timing and form of this signal looks like it is the DART response to the seismic waves directly from the earthquake, NOT to a tsunami wave. pic.twitter.com/bxeF5TPqjv
— Anthony Lomax 😷🇪🇺🌍 (@ALomaxNet) July 29, 2021
Small tsunami waves continue arriving at Sand Point & other coastal areas of Alaska. Tomorrow these waves will create swirly currents in boat harbors up & down the west coast, so tie up your boats real good. pic.twitter.com/nofvKqJoU5
— Brian Olson (@mrbrianolson) July 29, 2021
And since I have a drone workshop to attend tomorrow, I will bow out now and get some sleep.
My initial guess that today's event may have been similar to the 1938 M8.2 earthquake still looks like it has some merit.
Follow https://t.co/A1MNRg1WKF for updates on tsunami warnings. pic.twitter.com/g4qME2w0SI— Jascha Polet (@CPPGeophysics) July 29, 2021
@NOAA Tsunami Warning System has issued a tsunami watch for the West Coast. The warning for Hawaii has been cancelled, because the waves are focused east of Hawaii and the event isn't that large. @NWS_NTWC pic.twitter.com/h2KRBmOKNL
— Dr. Lucy Jones (@DrLucyJones) July 29, 2021
A Tsunami Warning remains in effect. A Tsunami Advisory also remains in effect. pic.twitter.com/QLTiROkiri
— NWS Anchorage (@NWSAnchorage) July 29, 2021
What a tsunami warning sounds like… they tested this earlier today too but this time is for real! M8.2, looks like on the subduction zone interface.
(and look at those pretty peonies! 🌸) pic.twitter.com/1HPy8tBUC2
— Dr. Kasey Aderhold (@KaseyAderhold) July 29, 2021
Records of tsunami deposits show significant tsunamis in 1788, 1880 and 1938 (https://t.co/NsFfTuqigs), indicating recurrence intervals of large earthquakes in the Semidi segment every 58-92 years. We are now 83 years since 1938, so that seems roughly consistent. pic.twitter.com/CGIM40Fv0g
— Dr Stephen Hicks 🇪🇺 (@seismo_steve) July 29, 2021
#EarthquakeReport for M 8.2 #Earthquake and #Tsunami offshore of #Alaska
updated poster with Sand Point tide gage data@USGS_Quakes slip model
tsunami prehistory and history for region doi:10.1130/GES01108.1https://t.co/mFtEoigFQB
more background here https://t.co/L4RHgNdex7 pic.twitter.com/HXpQUVSWFE— Jason "Jay" R. Patton (@patton_cascadia) July 29, 2021
Preliminary finite fault for this morning's M8.2 earthquake is available. Rupture primarily to the NE of the hypocenter, away from the Shumagin Gap.https://t.co/dVkYuR2kPC pic.twitter.com/idGBqxRhbX
— Dr. Dara Goldberg (@dara_berg_) July 29, 2021
All #Tsunami alerts for the #Alaska coastline have been cancelled.
Remember, strong and unusual currents may continue for several hours. If you have damage, please report it to your local officials.
Stay safe, get some rest, and we'll keep the watch for you. Good night. https://t.co/wzUBu4ysK3
— NWS Tsunami Alerts (@NWS_NTWC) July 29, 2021
Tonight's M8.2 event occurred close to the rupture area of the 2020 M7.8 earthquake and was the largest U.S. earthquake in 50 years. We'll continue to update as this sequence unfolds, but here is a short piece on our website with what we know so far. https://t.co/PzHaaQ8Zbl pic.twitter.com/vcM8fq9IV7
— Alaska Earthquake Center (@AKearthquake) July 29, 2021
Some Perryville M 8.2 thoughts: One of the arresting things about Chirikof coastal geology is that the island is clearly sinking like a stone today, evident in geodesy and coastal geology. Figure from Nelson et al. 2015 https://t.co/vGKDp0WYuN
*BUT* that isn't the entire story pic.twitter.com/LAWLqE1Su3
— Rich Briggs (@rangefront) July 29, 2021
The two closest sites to the M8.2 Alaska earthquake today show some decent surface wave signals. There are several other closer sites that should give us better insight. @UNAVCO pic.twitter.com/lN22i7arEP
— Brendan Crowell (@bwcphd) July 29, 2021
8.2 Earthquake is the largest in Alaska since 1965. I was sitting in the upper wheelhouse of my 125' steel schooner ALEUTIAN EXPRESS at Chignik Harbor and the whole boat bounced and vibrated for about a minute. 14' range of gradual Tsunami one foot every 4 minutes both directions pic.twitter.com/IlYox48ejg
— John Clutter (@AleutianExpress) July 29, 2021
Interesting look at the tide gauge in Eureka this morning.
That perturbation over the last couple of hours is likely associated with the small tsunami waves from Alaska.
This is a great reminder that tsunami danger can last well after the specific 'arrival time' #cawx pic.twitter.com/JloflW8aa5
— NWS Eureka (@NWSEureka) July 29, 2021
Additional Information about the M 8.2 earthquake that occurred 50 miles south of the Alaska Peninsula last night. https://t.co/2Jn2DLAV8M #Earthquake #Alaska pic.twitter.com/s1DDPmmaXG
— USGS (@USGS) July 29, 2021
Alaska has a M7 earthquake every 2 years on average. So why the big deal about this M8.2? There is a BIG DIFFERENCE between a M7 and a M8.
Use this “spaghetti magnitude” scale to visualize the difference. #AlaskaQuake pic.twitter.com/DT3tBzRkxs
— Dr. Wendy Bohon (@DrWendyRocks) July 29, 2021
Preliminary Finite Fault Model of the Mw 8.2 Alaska event. @dara_berg_ @geosmx pic.twitter.com/WZNgmu9HWo
— Sebastian Riquelme (@accelerogram) July 29, 2021
Clear NE propagation from the M8.2 in Alaska, but look at 102 sec- action to E way updip by the trench. Early aftershock or where rupture finally expired? It's small amplitude, but coherent and seen by 4 very different arrays. I await better analyses.https://t.co/6hFfZ64Elw pic.twitter.com/hacucSnOil
— Alex Hutko (@alexanderhutko) July 29, 2021
Good morning all! The tsunami waves are still bouncing around the Aleutian Islands in Alaska (max height measured was ~2 feet). The tsunami turned out not to be very big & all @NWS_NTWC alerts for the US west coast are CANCELLED. 🚨NO alerts for CA, OR, WA. #earthquake pic.twitter.com/uEHSdzzvv9
— Brian Olson (@mrbrianolson) July 29, 2021
Waves from the recent M8.2 #Alaska #earthquake rolling through North America. Different colors correspond to different types of seismic waves. @IRIS_EPO pic.twitter.com/RJBlGh7zFg
— UMN Seismology (@UMNseismology) July 29, 2021
Good morning PNW- ICYMI, last night there was a M8.2 earthquake off the Alaska Peninsula. Here, you can see waves from it (bottom) compared to a nearby Alaskan M6.8 (top, similar to our 2001 Nisqually M6.8) at station LEBA near the SW Washington coast. pic.twitter.com/GCCAjbYpII
— PNSN (@PNSN1) July 29, 2021
Here I show the cross-section through the Alaska seismicity with projected mechanisms. The largest two events are yesterday's M8.2 quake and last year's M7.8, both subduction interface events. For reference, a cartoon of the shallow subduction zone from https://t.co/Gces1m71C8 pic.twitter.com/bgchTpPT5n
— Jascha Polet (@CPPGeophysics) July 29, 2021
You may not have felt it, but a groundwater well in Washington County, Maryland did! An 8.2 magnitude earthquake rocked southern Alaska overnight and the water level in our well sloshed almost a foot. https://t.co/kafsMsaaph. For more real-time well data https://t.co/w56ACDNk4h pic.twitter.com/87fVSz0MLz
— @USGS_MD_DE_DC (@USGS_MD_DE_DC) July 29, 2021
15 second sample rate data for AB13 is now available for the M8.2 Alaska earthquake, we see a pretty appreciable SE offset with 10 cm of subsidence. The event started NE of AC12 and ruptured to the NE, so this site is in the middle of it all. @UNAVCO pic.twitter.com/8nTTZstIDX
— Brendan Crowell (@bwcphd) July 30, 2021
The largest earthquake to hit the U.S. in the last few decades took place in Alaska yesterday. The Mw 8.2 quake broke the Aleutian megathrust in the Shumagin seismic gap. The rupture did not propagate to the trench, causing only a minor tsunami. Figure by @QQtecGeodesy pic.twitter.com/02ylFet8P6
— Sylvain Barbot (@quakephysics) July 30, 2021
Recent Earthquake Teachable Moment for the M8.2 #AlaskaEarthquakehttps://t.co/2sFE9QDrNb pic.twitter.com/aLtYLIm61i
— IRIS Earthquake Sci (@IRIS_EPO) July 30, 2021
14+ hours after the #alaska earthquake and there is still a tsunami bouncing around at the closest tide gauge (small tsunami) pic.twitter.com/3xtjS4hhge
— Bill Barnhart (@SeismoSARus) July 29, 2021
There was a bit of confusion and misinformation with the Alaska earthquake last night, so how about us geoscientists put together a thread of seismologists/tsunami experts to follow. I'll start: @CPPGeophysics @SeismoSue @seismo_steve #Earthquake #alaskaearthquake pic.twitter.com/nfbQvfbnQy
— Dr Janine Krippner (@janinekrippner) July 29, 2021
As of 12 hours following the M8.2 we've located ~140 aftershocks. The locations and magnitudes are subject to change upon further review, but look to be occurring to the east of 2020 sequence. The map here shows 2020 in gray and the recent aftershocks in red. pic.twitter.com/hQ93k7HVUZ
— Alaska Earthquake Center (@AKearthquake) July 29, 2021
Last night's magnitude 8.2 earthquake serves as a powerful reminder of the restlessness of our planet's surface—and it presents an exciting opportunity to peer deeper at our planet’s inner workings.
Learn more about Alaska's shakes in my latest @NatGeo https://t.co/gnPANhqWW1
— Dr. Maya Wei-Haas (@WeiPoints) July 29, 2021
Our event page for last night's M8.2 earthquake in Alaska is posted and will be updated as data are made available: https://t.co/XSq1nVyBuU pic.twitter.com/NcUqOKBdGT
— UNAVCO (@UNAVCO) July 29, 2021
Slip contours for the July 2020 and 2021 megathrust #earthquakes One begins where the other ends. @bwcphd @dara_berg_ pic.twitter.com/URdqVX2r2R
— Sean (@tsuphd) July 29, 2021
(1/3) The "Lame Monster": Today's largest US earthquake in >50 years did not make a large tsunami. Why?
B/c most of elastic energy was released deeper in the Earth.These are computer models of the tsunami from the M8.3 earthquake in Alaska#AlaskaQuake #alaskatsunami pic.twitter.com/QlJSWJMaqG
— Amir Salaree (@amirsalaree) July 29, 2021
🌊The entire #California coast is a #tsunami hazard area.
🌊The July 27 M8.2 earthquake in #Alaska generated minor tsunami waves that are still being recorded on tide gages here.
🌊Head to ➡️https://t.co/UUkQsqYcAk to learn more about your tsunami risk. pic.twitter.com/pHjrTeOv5o
— California Department of Conservation (@CalConservation) July 30, 2021
GPS receivers can be used as seismometers. In blue are the 5 Hz velocities recovered on Kodiak Island with the variometric approach for the M8.2 earthquake yesterday. In red, the collocated accelerometer, S19K, downsampled to 5 Hz. pic.twitter.com/jxJR1v7Fj1
— Brendan Crowell (@bwcphd) July 30, 2021
A notable characteristic of the M8.2 Alaska earthquake is that it was relatively deep and doesn’t appear to have ruptured the shallow plate boundary. Could overpressured sediments on the shallow plate boundary inhibited shallow slip? Check out this seismic image updip of event. pic.twitter.com/HRQEPxrAZk
— Donna Shillington (@djshillington) July 30, 2021
We finally have some preliminary coseismic offsets for the M8.2 Alaska earthquake. AB13 has a 43 cm offset to the SE. pic.twitter.com/RSttwW4nBl
— Brendan Crowell (@bwcphd) July 30, 2021
While the M8.2 was the largest earthquake in the U.S. in 50 years, Alaska has experienced some significantly sized events during that time. The plot here shows the largest Alaska earthquake magnitude each year since 1964. Since 2000, we're experienced at least a M6.4 annually. pic.twitter.com/Iq9pFanPqi
— Alaska Earthquake Center (@AKearthquake) July 30, 2021
Whopper M8.2 earthquake in Alaska moved GPS stations, revealing the broad pattern and extent of deformation.
Stations near Denali NP ~900 km moved a few mm… See https://t.co/4zpOW4m1pJ for more info and data. pic.twitter.com/j1GSIazVfJ— Bill Hammond (@BillCHammond) July 30, 2021
(1/6) DART Seismology:
How the tsunami sensors near Alaska picked up the seismic surface waves from the M8.3 Alaska earthquake!
The tails in the records are mixes of surface waves and the tsunami.#alaskaearthquake #alaska_tsunami @NOAAResearch @NWS_PTWC @IRIS_EPO pic.twitter.com/OWI7e47dNq
— Amir Salaree (@amirsalaree) July 31, 2021
#EarthquakeReport and #TsunamiReport for M8.2 #Earthquake offshore of #Alaska
updated interpretive poster
'21 sequence matches '38 sequence for both ~slip patch and ~tsunami size https://t.co/pE3zA9HHFShttps://t.co/mFtEoigFQB
tectonic background here:https://t.co/L4RHgNdex7 pic.twitter.com/iTMmm5u2LQ— Jason "Jay" R. Patton (@patton_cascadia) August 2, 2021
#Sentinel1 co-seismic interferograms (ascending track) over western Alaska, show ground deformation towards the southern coast, above the main M8.2 #earthquake fault rupture. Aftershock epicenters (yellow) from USGS. pic.twitter.com/RtavuJZGSZ
— Sotiris Valkaniotis (@SotisValkan) August 2, 2021
The M 8.2 Chignik earthquake that occurred off the Alaskan Peninsula on July 28 was the largest US earthquake in 50 years. This 2013 simulation from the same region shows how a hypothetical M 9.1 (almost 30x stronger!) earthquake can create a far-reaching tsunami. @USGS_Quakes pic.twitter.com/tLtxWxoal7
— USGS Coastal Change (@USGSCoastChange) July 30, 2021
Updated finite fault model (joint inversion of regional and teleseismic data) is now available: https://t.co/K0kXumE6Pv pic.twitter.com/y7Z9vIF6Lu
— Dr. Dara Goldberg (@dara_berg_) August 3, 2021
#EarthquakeReport & #TsunamiReport for M8.2 Perrysville #Earthquake and transpacific #Tsunami
updated poster including @USGS_Quakes @dara_berg_ updated slip model
also, surface deformation data
I prepared a report and will update morehttps://t.co/y1RwyZjKOA pic.twitter.com/lj8qIk7vQl
— Jason "Jay" R. Patton (@patton_cascadia) August 4, 2021
- Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
- Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
- Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
- Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
- Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
- Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
- Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
- Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
- Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
- Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
- Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
- Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198
- Abe, K., 1972. Lithospheric Normal Faulting Beneath the Aleutian Trench in Phys. Earth Planet. Interiors, v. 5, p. 1990-198.
- Atwater, B.F., Yamaguchi, D.K., Bondevik, S., Barnhardt, W.A., Amidon, L.J., Benson, B.E., Skjerdal, G., Shulene, J.A., and Nanalyama ,F., 2001. Rapid resetting of an estuarine recorder of the 1964 Alaska earthquake in Geology, v. 113, no. 9, p. 1193-1204.
- Bassett and Watts, 2015 A. Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief in Geochemistry, Geophysics, Geosystems, v. 16, doi:10.1002/2014GC005684.
- Bassett, D. and Watts, A.B., 2015 B. Gravity anomalies, crustal structure, and seismicity at subduction zones: 2. Interrelationships between fore-arc structure and seismogenic behavior in Geochemistry, Geophysics, Geosystems, v. 16, doi:10.1002/2014GC005685.
- Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Hayes, G.P., Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 Aleutian arc and vicinity: U.S. Geological Survey Open-File Report 2010–1083-B, scale 1:5,000,000.
- Brown, J.R., Prejan, S.G., Beroza, G.C., Gomberg, J.S., and Hauessler,m P.J., 2013. Deep low-frequency earthquakes in tectonic tremor along the Alaska-Aleutian subduction zone in JGR Solid Earth, v. 118, p. 1079-1090, doi:10.1029/2012JB009459
- Freymueller, J.T., Suleimani, E.N., and Nocolski, D.J., 2021. Constraints on the Slip Distribution of the 1938 MW 8.3 Alaska Peninsula Earthquake From Tsunami Modeling in GRL, v. 48, no. 9, https://doi.org/10.1029/2021GL092812
- Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
- Gaedicke, C., Baranov, B., Seliverstov, N., Alexeiev, D., Tsukanov, N., Freitag, R., 2000. Structure of an active arc-continent collision area: the Aleutian-Kamchatka junction. Tectonophysics 325, 63–85
- Haeussler, P., Leith, W., Wald, D., Filson, J., Wolfe, C., and Applegate, D., 2014. Geophysical Advances Triggered by the 1964 Great Alaska Earthquake in EOS, Transactions, American Geophysical Union, v. 95, no. 17, p. 141-142.
- Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
- Ikuta, R., Mitsui, Y., Kurokawa, Y., and Ando, M., 2015. Evaluation of strain accumulation in global subduction zones from seismicity data in Earth, Planets and Space, v. 67, DOI 10.1186/s40623-015-0361-5
- Johnson, J.M. and Satake, K., 1994. Rupture extent of the 1938 Alaskan earthquake as inferred from tsunami waveforms in GRL, v. 21, no. 8, p. 733-736
- Koulakov, I.Y., Dobretsov, N.L., Bushenkova, N.A., and Yakovlev, A.V., 2011. Slab shape in subduction zones beneath the Kurile–Kamchatka and Aleutian arcs based on regional tomography results in Russian Geology and Geophysics, v. 52, p. 650-667.
- Konstantnovskaia, 2001. Arc-continent collision and subduction reversal in the Cenozoic evolution of the Northwest Pacific: an example from Kamchatka (NE Russia) in Tectonophysics, v. 333, p. 75-94.
- Krutikov, L., Stone, D.B., and Minyuk, P., 2008. New Paleomagnetic Data From the Central Aleutian Arc: Evidence and Implications for Block Rotations in Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179 American Geophysical Union. 10.1029/179GM07
- Lange, D., Cembrano, J., Rietbrock, A., Haberland, C., Dahm, T., and Bataille, K., 2008. First seismic record for intra-arc strike-slip tectonics along the Liquiñe-Ofqui fault zone at the obliquely convergent plate margin of the southern Andes in Tectonophysics, v. 455, p. 14-24
- Lay, T., H. Kanamori, C. J. Ammon, A. R. Hutko, K. Furlong, and L. Rivera, 2009. The 2006 – 2007 Kuril Islands great earthquake sequence in J. Geophys. Res., 114, B11308, doi:10.1029/2008JB006280.
- Lay, T., Ye, L., Bai, Y., Cheung, K. F., Kanamori, H., Freymueller, J., … Kogan, M. G. (2017). Rupture along 400 km of the Bering fracture zone in the Komandorsky Islands earthquake (MW 7.8) of 17 July 2017. Geophysical Research Letters, 44, 12,161–12,169. https://doi.org/10.1002/2017GL076148
- Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
- Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
- Nelson, A.R., Briggs, R.W., Dura, T., Engelhart, S.E., Gelfenbaum, G., Bradley, L.-A., Forman, S.L., Vane, C.H., and Kelley, K.A., 2015, Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska: Geosphere, v. 11, no. 4, p. 1172–1203, doi:10.1130/GES01108.1.
- Plafker, G., 1969. Tectonics of the March 27, 1964 Alaska earthquake: U.S. Geological Survey Professional Paper 543–I, 74 p., 2 sheets, scales 1:2,000,000 and 1:500,000, http://pubs.usgs.gov/pp/0543i/.
- Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
- Portnyagin, M. and Manea, V.C., 2008. Mantle temperature control on composition of arc magmas along the Central Kamchatka Depression in Geology, v. 36, no. 7, p. 519-522.
- Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., Furlong, K.P., and Benz, H.M., 2010. Seismicity of the Earth 1900-2007, Kuril-Kamchatka arc and vicinity: U.S. Geological Survey Open-File Report 2010-1083-C, 1 map sheet, scale 1:5,000,000.
- Saltus, R.W., and Barnett, A., 2000. Eastern Aleutian Volcanic Arc Digital Model – Version 1.0: U.S. Geological Survey Open-File Report 00
- Shevchenko, V.I., Lukk, A.A., and Prilepin, M.T., 2006. The Sumatra Earthquake of December 26, 2004, as an Event Unrelated to the Plate-Tectonic Process in the Lithosphere in Physics of the Solid Earth, v. 42, no. 12, p. 1018–1037.
- Stauder, W., 1968. Mechanism of the Rat Island Earthquake Sequence of February 4, 1965, with Relation to Island Arcs and Sea-Floor Spreading in JGR, v. 73, no. 12, p. 3847-3858
- Sykes, L.R., Kissinger, J.B>, House, L., Davies, J.N>, and Jacob, K.H., 1980. Rupture Zones and Repeat Times of Great Earthquakes Along the Alaska-Aleutian Arc, 1784-1980, in Maurice Ewing Series, Earthquake Prediction, An International Review, AGU
- Torsvik, T. H. et al., 2017. Pacific plate motion change caused the Hawaiian-Emperor Bend in Nat. Commun., v. 8, doi: 10.1038/ncomms15660
- Wilson, J. Tuzo, 1963. “A possible origin of the Hawaiian Islands” in Canadian Journal of Physics. v. 41, p. 863–870 doi:10.1139/p63-094.
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
- Sorted by Magnitude
- Sorted by Year
- Sorted by Day of the Year
- Sorted By Region
A couple days ago, in my inbox, there was an email from the Pacific Tsunami Warning Center about an earthquake along the Aleutian Islands, near Rat Island, Alaska. However, this earthquake was not along the megathrust subduction zone fault there and it was rather deep (~19 km). Also, this earthquake was strike-slip (not thrust or reverse), so probably did not produce much vertical ground motion. These two factors combined (deep and strike-slip) suggest to me that there would not be a tsunami generated from this earthquake. BUT we learn new things every month. I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.
Above: Rupture zones of earthquakes of magnitude M > 7.4 from 1925-1971 as delineated by their aftershocks along plate boundary in Aleutians, southern Alaska and offshore British Columbia [after Sykes, 1971]. Contours in fathoms. Various symbols denote individual aftershock sequences as follows: crosses, 1949, 1957 and 1964; squares, 1938, 1958 and 1965; open triangles, 1946; solid triangles, 1948; solid circles, 1929, 1972. Larger symbols denote more precise locations. C = Chirikof Island. Below: Space-time diagram showing lengths of rupture zones, magnitudes [Richter, 1958; Kanamori, 1977 b; Kondorskay and Shebalin, 1977; Kanamori and Abe, 1979; Perez and Jacob, 1980] and locations of mainshocks for known events of M > 7.4 from 1784 to 1980. Dashes denote uncertainties in size of rupture zones. Magnitudes pertain to surface wave scale, M unless otherwise indicated. M is ultra-long period magnitude of Kanamori 1977 b; Mt is tsunami magnitude of Abe[ 1979]. Large shocks 1929 and 1965 that involve normal faulting in trench and were not located along plate interface are omitted. Absence of shocks before 1898 along several portions of plate boundary reflects lack of an historic record of earthquakes for those areas.
Proposed tectonic model for southern Chile. Partitioning of the oblique convergence vector between the Nazca plate and South American plate results in a dextral strike-slip fault zone in the magmatic arc and a northward moving forearc sliver. Modified after Lavenu and Cembrano (1999).
Location map for the Aleutian Islands. The outline blocks and shaded summit basins are from Geist et al. [1988], showing a possible rotation mechanism. The heavy arrows show the mean rotations with respect to North America indicated by paleomagnetic data, the lighter arrows the motion of the Pacific plate with respect to North America. (inset) General location map modified from Chapman and Solomon [1976], Mackey et al. [1997], and Pedoja et al. [2006]. Solid lines show boundaries of plates and blocks: NA, North American Plate; B, Bering Block; PA, Pacific Plate; OKH, Okhotsk Plate; KI, Komandorsky Island Block; EUA, Eurasian Plate
Map showing the boundaries of clockwise-rotating and westward translating blocks that comprise the Aleutian Ridge [from Geist et al. 1988]. Summit basins and transverse Pacific slope canyons are extensional structures that formed in the wake of these rotating and translating blocks. Arrows show relative plate motion between the Pacific and North American plates; convergence is increasingly oblique to the west. The central Aleutian sector lies within the Andreanof block located between Adak Canyon and Amukta Basin. A prominent summit basin has formed in the eastern part of the block (the composite Amlia and Amukta Basins). However, a summit basin is not present in the western part of the Andreanof block between Adak and Atka Islands. Asterisks show the location of active and dormant volcanoes; the star denotes the approximate location of the 1986 Andreanof earthquake.
(a) Bilateral slip model for the 2017 earthquake and USGS/NEIC catalog seismicity from 1900 to 16 July 2017 (blue circles, scaled proportional to magnitude, with events larger than M ~ 7 being labeled), along with all moment tensor solutions from the GCMT catalog from 1976 to 16 July 2017 (red-filled compressional quadrant focal mechanisms). (b) Foreshock seismicity on 17 July 2017 (blue circles) and aftershock seismicity in the first 2 weeks (magenta circles) along with the MW 6.3 foreshock GCMT focal mechanism (cyan focal mechanism). The large focal mechanism is the W-phase moment tensor from this study. The boxes indicate short-period radiators from the Eurasia-Greenland back projection, and stars indicate radiators from the North American back projection (Figure 3). The slip distribution is shown in detail in Figure S12. White vectors indicate the relative motion of the Pacific Plate to North America (almost identical to that relative to the Bering Plate). The large red star indicates the main shock epicenter.
Predicted tsunami from the bilateral faulting model. (a) Final seafloor deformation with the red star indicating the epicenter and the dashed line delineating projection of the faulting model on the seafloor. (b) Predicted tsunami amplitude and DART stations (circles) considered in this study. (c) Comparison of filtered sea surface recordings (black) at DART stations with predictions (red) along with corresponding amplitude spectra (right). The recorded and predicted time series were filtered to remove signals shorter than 5 min period and the full 5 h time series were used in the computation of the amplitude spectra. The strike-slip faulting and position of the stations result in weak tsunami waves, but the timing and height of long-period arrivals provide bounds on the source.
Aleutian subduction zone. Symbols as in Figure 3. (a) Residual free-air gravity anomaly and seismicity. The outer-arc high, trench-parallel fore-arc ridge and block-bounding faults are dashed in blue, black, and red, respectively. Annotations are AP = Amchitka Pass; BHR = Black-Hills Ridge; SS = Sunday Sumit Basin; PD = Pratt Depression. (b) Published asperities and slip-distributions/aftershock areas for large magnitude earthquakes. (c) Cross sections showing residual bathymetry (green), residual free-air gravity anomaly (black), and the geometry of the seismogenic zone [Hayes et al., 2012].
Schematic diagram summarizing the key spatial associations interpreted between the morphology of the fore-arc and variations in the seismogenic behavior of subduction megathrusts.
Geodynamic setting of Kamchatka in framework of the Northwest Pacific. Modified after Nokleberg et al. (1994) and Kharakhinov (1996)). Simplified cross-section line I-I’ is shown in Fig. 2. The inset shows location of Sredinny and Eastern Ranges. [More figure caption text in the publication].
The Cenozoic evolution in the Northwest Pacific. Plate kinematics is shown in hotspot reference frame after (Engebretson et al., 1985). Keys distinguish zones of active volcanism (thick black lines), inactive volcanic belts (thick gray lines), deformed arc terranes (hatched pattern), subduction zones: active (black triangles), inactive *(empty triangles). In letters: sa = Sikhote-aline, bs = Bering shelf belts; SH = Shirshov Ridge; V = Vitus arch; KA = Kuril; RA = Ryukyu’ LA = Luzon; IBMA = Izu-Bonin-Mariana arcs; WPB = Western Philippine, BB = Bowers basins.
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)
Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy — Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
At shortly before 13:30 today in northern Alaska there was a large earthquake, with a magnitude of M=5.1. I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version.
Map of a portion of the field epicenter. Alaska earthquake of 7 April 1958. (Compiled from vertical air photos and USGS Alaska Topographic Series 1:63,360, Melozitna and Kateel River Quadrangles, 1954.
Isoseismal map of the intensities of the April 7, 1958 earthquake, (Modified Mercalli scale).
Surface of one of the major sand flows covering an area greater than 1 square mile. The silty sand has a relatively uniform thickness of approximately 2½ feet.
A conical collapse nearly 20 feet deep. It occurred approximately 200 yards from the nearest sand flow.
Cross-section A-A’ showing the arrested sand dune deposits resting on the alluvium below. Location of the cross-section is shown on the map (figure 5). [Figure 5 is the map and legend.]
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)
Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy — Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
Large earthquake strikes near Kobuk National Preserve in remote part of Alaska https://t.co/fHqOrc1nt5 — Jason "Jay" R. Patton (@patton_cascadia) March 27, 2019
Here I summarize Earth’s significant seismicity for 2018. I limit this summary to earthquakes with magnitude greater than or equal to M 6.5. I am sure that there is a possibility that your favorite earthquake is not included in this review. Happy New Year. One year of #earthquakes recorded by @INGVterremoti in Italy. About 2500 events with magnitude equal or larger than M2, about seven per day. Data source https://t.co/g1RvR2A989) #Italia #terremoto #Italy #earthquake pic.twitter.com/ft8GAsFjKA — iunio iervolino (@iuniervo) December 31, 2018 Earthquakes of 2018: a quick post summarising global seismic activity last year (i.e., the figures I showed you yesterday). https://t.co/ahdwpf1OFv pic.twitter.com/S438okD8QQ — Chris Rowan (@Allochthonous) January 1, 2019 Global #earthquakes by Magnitude (M5+) by year (2000-18), showing remarkable consistency from geologic forcing. Whereas patterns are understood, they do not permit short-term, local predictions; instead, be informed and be prepared. #geohazards @IRIS_EPO @USGS pic.twitter.com/BmtXhhUvWF — Ben van der Pluijm 🌎 (@vdpluijm) January 2, 2019 The pattern of shallow earthquakes (depth < 33 km) is typical, with much of the country susceptible to regular shallow seismicity, with lower rates in Northland/Auckland and southeast Otago. pic.twitter.com/3jip8Lyje9 — John Ristau 🇨🇦 🇳🇿 (@SinistralSeismo) January 3, 2019
Just a couple hours ago there was an earthquake along the Swan fault, which is the transform plate boundary between the North America and Caribbean plates. The Cayman trough (CT) is a region of oceanic crust, formed at the Mid-Cayman Rise (MCR) oceanic spreading center. To the west of the MCR the CT is bound by the left-lateral strike-slip Swan fault. To the east of the MCR, the CT is bound on the north by the Oriente fault. We had a damaging and (sadly) deadly earthquake in southern Peru in the last 24 hours. This is an earthquake, with magnitude M 7.1, that is associated with the subduction zone forming the Peru-Chile trench (PCT). The Nazca plate (NP) is subducting beneath the South America plate (SAP). There are lots of geologic structures on the Nazca plate that tend to affect how the subduction zone responds during earthquakes (e.g. segmentation). This earthquake appears to be located along a reactivated fracture zone in the GA. There have only been a couple earthquakes in this region in the past century, one an M 6.0 to the east (though this M 6.0 was a thrust earthquake). The Gulf of Alaska shear zone is even further to the east and has a more active historic fault history (a pair of earthquakes in 1987-1988). The magnetic anomalies (formed when the Earth’s magnetic polarity flips) reflect a ~north-south oriented spreading ridge (the anomalies are oriented north-south in the region of today’s earthquake). There is a right-lateral offset of these magnetic anomalies located near the M 7.9 epicenter. Interesting that this right-lateral strike-slip fault (?) is also located at the intersection of the Gulf of Alaska shear zone and the 1988 M 7.8 earthquake (probably just a coincidence?). However, the 1988 M 7.8 earthquake fault plane solution can be interpreted for both fault planes (it is probably on the GA shear zone, but I don’t think that we can really tell). As a reminder, if the M 7.9 earthquake fault is E-W oriented, it would be left-lateral. The offset magnetic anomalies show right-lateral offset across these fracture zones. This was perhaps the main reason why I thought that the main fault was not E-W, but N-S. After a day’s worth of aftershocks, the seismicity may reveal some north-south trends. But, as a drama student in 7th grade (1977), my drama teacher (Ms. Naichbor, rest in peace) asked our class to go stand up on stage. We all stood in a line and she mentioned that this is social behavior, that people tend to stand in lines (and to avoid doing this while on stage). Later, when in college, professors often commented about how people tend to seek linear trends in data (lines). I actually see 3-4 N-S trends and ~2 E-W trends in the seismicity data. There was just now an earthquake in Oaxaca, Mexico between the other large earthquakes from last 2017.09.08 (M 8.1) and 2017.09.08 (M 7.1). There has already been a M 5.8 aftershock.Here is the USGS website for today’s M 7.2 earthquake. This morning (local time in California) there was an earthquake in Papua New Guinea with, unfortunately, a high likelihood of having a good number of casualties. I was working on a project, so could not immediately begin work on this report. We had an M 6.8 earthquake near a transform micro-plate boundary fault system north of New Ireland, Papua New Guinea today. Here is the USGS website for this earthquake. The New Britain region is one of the more active regions in the world. See a list of earthquake reports for this region at the bottom of this page, above the reference list. Well, those earthquakes from earlier, one a foreshock to a later one, were foreshocks to an earthquake today! Here is my report from a couple days ago. The M 6.6 and M 6.3 straddle today’s earthquake and all have similar hypocentral depths. A couple days ago there was a deep focus earthquake in the downgoing Nazca plate deep beneath Bolivia. This earthquake has an hypocentral depth of 562 km (~350 miles). There has been a swarm of earthquakes on the southeastern part of the big island, with USGS volcanologists hypothesizing about magma movement and suggesting that an eruption may be imminent. Here is a great place to find official USGS updates on the volcanism in Hawaii (including maps). This version includes earthquakes M ≥ 3.5 (note the seismicity offshore to the south, this is where the youngest Hawaii volcano is). Below are a series of plots from tide gages installed at several sites in the Hawaii Island Chain. These data are all posted online here and here. Yesterday morning, as I was recovering from working on stage crew for the 34th Reggae on the River (fundraiser for the non profit, the Mateel Community Center), I noticed on social media that there was an M 6.9 earthquake in Lombok, Indonesia. This is sad because of the likelihood for casualties and economic damage in this region. Well, yesterday while I was installing the final window in a reconstruction project, there was an earthquake along the Aleutian Island Arc (a subduction zone) in the region of the Andreanof Islands. Here is the USGS website for the M 6.6 earthquake. This earthquake is close to the depth of the megathrust fault, but maybe not close enough. So, this may be on the subduction zone, but may also be on an upper plate fault (I interpret this due to the compressive earthquake fault mechanism). The earthquake has a hypocentral depth of 20 km and the slab model (see Hayes et al., 2013 below and in the poster) is at 40 km at this location. There is uncertainty in both the slab model and the hypocentral depth. We just had a Great Earthquake in the region of the Fiji Islands, in the central-western Pacific. Great Earthquakes are earthquakes with magnitudes M ≥ 8.0. This ongoing sequence began in late July with a Mw 6.4 earthquake. Followed less than 2 weeks later with a Mw 6.9 earthquake. We just had a M 7.3 earthquake in northern Venezuela. Sadly, this large earthquake has the potential to be quite damaging to people and their belongings (buildings, infrastructure). Well, this earthquake, while having a large magnitude, was quite deep. Because earthquake intensity decreases with distance from the earthquake source, the shaking intensity from this earthquake was so low that nobody submitted a single report to the USGS “Did You Feel It?” website for this earthquake. Following the largest typhoon to strike Japan in a very long time, there was an earthquake on the island of Hokkaido, Japan today. There is lots on social media, including some spectacular views of disastrous and deadly landslides triggered by this earthquake (earthquakes are the number 1 source for triggering of landslides). These landslides may have been precipitated (sorry for the pun) by the saturation of hillslopes from the typhoon. Based upon the USGS PAGER estimate, this earthquake has the potential to cause significant economic damages, but hopefully a small number of casualties. As far as I know, this does not incorporate potential losses from earthquake triggered landslides [yet]. Today, there was a large earthquake associated with the subduction zone that forms the Kermadec Trench. Well, around 3 AM my time (northeastern Pacific, northern CA) there was a sequence of earthquakes including a mainshock with a magnitude M = 7.5. This earthquake happened in a highly populated region of Indonesia. Here is a map that shows the updated USGS model of ground shaking. The USGS prepared an updated earthquake fault slip model that was additionally informed by post-earthquake analysis of ground deformation. The original fault model extended from north of the epicenter to the northernmost extent of Palu City. Soon after the earthquake, Dr. Sotiris Valkaniotis prepared a map that showed large horizontal offsets across the ruptured fault along the entire length of the western margin on Palu Valley. This horizontal offset had an estimated ~8 meters of relative displacement. InSAR analyses confirmed that the coseismic ground deformation extended through Palu Valley and into the mountains to the south of the valley. Synthetic Aperture Radar (SAR) is a remote sensing method that uses Radar to make observations of Earth. These observations include the position of the ground surface, along with other information about the material properties of the Earth’s surface. Landslides during and following the M=7.5 earthquake in central Sulawesi, Indonesia possibly caused the majority of casualties from this catastrophic natural disaster. Volunteers (citizen scientists) have used satellite aerial imagery collected after the earthquake to document the spatial extent and magnitude of damage caused by the earthquake, landslides, and tsunami.
Nowicki Jessee and others (2018) is the preferred model for earthquake-triggered landslide hazard. Our primary landslide model is the empirical model of Nowicki Jessee and others (2018). The model was developed by relating 23 inventories of landslides triggered by past earthquakes with different combinations of predictor variables using logistic regression. The output resolution is ~250 m. The model inputs are described below. More details about the model can be found in the original publication. We modify the published model by excluding areas with slopes <5° and changing the coefficient for the lithology layer "unconsolidated sediments" from -3.22 to -1.36, the coefficient for "mixed sedimentary rocks" to better reflect that this unit is expected to be weak (more negative coefficient indicates stronger rock).To exclude areas of insignificantly small probabilities in the computation of aggregate statistics for this model, we use a probability threshold of 0.002.
Zhu and others (2017) is the preferred model for liquefaction hazard. The model was developed by relating 27 inventories of liquefaction triggered by past earthquakes to globally-available geospatial proxies (summarized below) using logistic regression. We have implemented the global version of the model and have added additional modifications proposed by Baise and Rashidian (2017), including a peak ground acceleration (PGA) threshold of 0.1 g and linear interpolation of the input layers. We also exclude areas with slopes >5°. We linearly interpolate the original input layers of ~1 km resolution to 500 m resolution. The model inputs are described below. More details about the model can be found in the original publication.
In this region of the world, the Solomon Sea plate and the South Bismarck plate converge to form a subduction zone, where the Solomon Sea plate is the oceanic crust diving beneath the S.Bismarck plate. This region of the Pacific-North America plate boundary is at the northern end of the Cascadia subduction zone (CSZ). To the east, the Explorer and Juan de Fuca plates subduct beneath the North America plate to form the megathrust subduction zone fault capable of producing earthquakes in the magnitude M = 9 range. The last CSZ earthquake was in January of 1700, just almost 319 years ago. Before I looked more closely, I thought this sequence might be related to the Kefallonia fault. I prepared some earthquake reports for earthquakes here in the past, in 2015 and in 2016. There was a M = 6.8 earthquake along a transform fault connecting segments of the Mid Atlantic Ridge recently. Today’s earthquake occurred along the convergent plate boundary in southern Alaska. This subduction zone fault is famous for the 1964 March 27 M = 9.2 megathrust earthquake. I describe this earthquake in more detail here. There was a sequence of earthquakes along the subduction zone near New Caledonia and the Loyalty Islands. A large earthquake in the region of the Bering Kresla fracture zone, a strike-slip fault system that coincides with the westernmost portion of the Aleutian trench (which is a subduction zone further to the east). This magnitude M = 7.0 earthquake is related to the subduction zone that forms the Philippine trench (where the Philippine Sea plate subducts beneath the Sunda plate). Here is the USGS website for this earthquake.
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)
Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.
We just had a large earthquake in the region of the Bering Kresla fracture zone, a strike-slip fault system that coincides with the westernmost portion of the Aleutian trench (which is a subduction zone further to the east). I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0 in one version.
Tectonic setting of the Sredinny and Ganal Massifs in Kamchatka. Kamchatka/Aleutian junction is modified after Gaedicke et al. (2000). Onland geology is after Bogdanov and Khain (2000). 1, Active volcanoes (a) and Holocene monogenic vents (b). 2, Trench (a) and pull-apart basin in the Aleutian transform zone (b). 3, Thrust (a) and normal (b) faults. 4, Strike-slip faults. 5–6, Sredinny Massif. 5, Amphibolite-grade felsic paragneisses of the Kolpakovskaya series. 6, Allochthonous metasedimentary and metavolcanic rocks of the Malkinskaya series. 7, The Kvakhona arc. 8, Amphibolites and gabbro (solid circle) of the Ganal Massif. Lower inset shows the global position of Kamchatka. Upper inset shows main Cretaceous-Eocene tectonic units (Bogdanov and Khain 2000): Western Kamchatka (WK) composite unit including the Sredinny Massif, the Kvakhona arc, and the thick pile of Upper Cretaceous marine clastic rocks; Eastern Kamchatka (EK) arc, and Eastern Peninsulas terranes (EPT). Eastern Kamchatka is also known as the Olyutorka-Kamchatka arc (Nokleberg et al. 1998) or the Achaivayam-Valaginskaya arc (Konstantinovskaya 2000), while Eastern Peninsulas terranes are also called Kronotskaya arc (Levashova et al. 2000).
Kamchatka subduction zone. A: Major geologic structures at the Kamchatka–Aleutian Arc junction. Thin dashed lines show isodepths to subducting Pacific plate (Gorbatov et al., 1997). Inset illustrates major volcanic zones in Kamchatka: EVB—Eastern Volcanic Belt; CKD—Central
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)
Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.
Mw=7.3, KOMANDORSKIYE OSTROVA REGION (Depth: 18 km), 2018/12/20 17:01:54 UTC – Full details here: https://t.co/pUYUdEnFtb pic.twitter.com/u9Uv1X4v4u — Earthquakes (@geoscope_ipgp) December 20, 2018 very strong #earthquake offshore #Kamchatka, #Russia, minor, regional #tsunami expected. Fortunately, region not well inhabitat @Quake_Tracker @LastQuake @JuskisErdbeben @UKEQ_Bulletin pic.twitter.com/DwCE4NuAOd — CATnews (@CATnewsDE) December 20, 2018 Seismic waves from the M7.4 Russia earthquake have rolled across Canada during the past hour (not felt here). The fastest travelling waves took about 7 minutes to travel from Kamchatka to Dawson, Yukon. — John Cassidy (@earthquakeguy) December 20, 2018
What a day. I started by waking up about 5:43 AM (about, heheh), which was 17 minutes before my alarm was set. I had a job interview at 8:30. Further down on this page, I include additional materials that were developed in the past year. I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version. Below is an educational video from the USGS that presents material about subduction zones and the 1964 earthquake and tsunami in particular. FOS = Resisting Force / Driving Force Well, I now have the job that I was being interviewed for one year ago today.
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)
Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.
A 7.0 magnitude earthquake hit Anchorage, Alaska. Marty Raney of @HomesteadRescue captured some of the damage nearby. #akearthquake pic.twitter.com/WyQ8qV1VWr — Discovery (@Discovery) December 1, 2018 Alaska Daily News reporting major ground failure on local roads after M7.0 in Anchorage this morninghttps://t.co/TToLTjplQc — Rob Witter (@WitterBanter) November 30, 2018 Not well constrained First-motion mechanism: Mwp6.9 #earthquake Southern Alaska https://t.co/kCIw9Vypa6 pic.twitter.com/OLaQslaRJy — Anthony Lomax 🌍🇪🇺 (@ALomaxNet) November 30, 2018 Okay, now I understand why that section of the road failed. Liquefaction my friends, liquefaction. Photo courtesy Caryn Orvis. pic.twitter.com/vo3rk2Mfrx — Jackie Caplan-Auerbach (@geophysichick) December 1, 2018 Intermediate depth M7.0 earthquake near Anchorage, Alaska has been followed by numerous aftershocks (in purple), most at similar depths and a few greater than M5 pic.twitter.com/LCFeEkqF5x — Jascha Polet (@CPPGeophysics) November 30, 2018 "Exotic" M=7.0 earthquake strikes beneath Anchorage, Alaska https://t.co/gNHojFtgzR — Alison Bird (@alisonlbird) November 30, 2018 For his always excellent Landslide Blog, @davepetley has a post about slope failures from the earthquake, including a roundup of the best road damage photos. https://t.co/WiKP2mcRQI — AK Earthquake Center (@AKearthquake) December 1, 2018 Great situational awareness for AK slab event from USGS Ground Failure product, now a card on the event pageshttps://t.co/9RsarVGluP See potential liquefaction map in particular for this one Great job @KateAllstadt and Jonathan Godt and team! pic.twitter.com/7eFwLYZtMo — Rich Briggs (@rangefront) November 30, 2018 This is 30 seconds of east-west earthquake shaking across Anchorage, from our strong motion network. The severity of shaking varied based on the location, and some areas experienced shaking exceeding 20%g. pic.twitter.com/CmXKrM8sGh — AK Earthquake Center (@AKearthquake) December 1, 2018 What did I do with my Friday night? I stayed up to gather the most up-to-date information about the 7.0M quake that just struck Anchorage for @ForbesScience. This explains: -What caused it — Dr Robin George Andrews (@SquigglyVolcano) December 1, 2018 This is another ground motion visualization showing the motion of the ground recorded by the USArray during the Anchorage earthquake (https://t.co/RIcNz4bgWq). #AnchorageEarthquake #earthquake pic.twitter.com/5ZbzvXOj5l — IRIS Earthquake Sci (@IRIS_EPO) December 1, 2018 Here's a map showing the magnitude 7.0 along with all of the aftershocks we reviewed in the 7 hours after the quake. Most aftershocks have not yet been reviewed and are not on this map, but these precise locations give a good overview of the sequence so far. pic.twitter.com/FM7oZL7t4n — AK Earthquake Center (@AKearthquake) December 1, 2018 Aerial view of #AlaskaEarthquake damage on the Glenn Highway. pic.twitter.com/UlkMkMLky5 — Governor Bill Walker (@AkGovBillWalker) November 30, 2018 Here’s the islanded car at the wrecked anchorage off ramp. pic.twitter.com/626As53hzF — Nat Herz (@Nat_Herz) November 30, 2018 Earthquake just happened right now i ’m actually shaking pic.twitter.com/PoZGOlJGWS — Alyson Petrie (@AlysonPetrie7) November 30, 2018 Click on this thread to see additional tweets about the 1 year reflection of this earthquake sequence. #OTD #earthquakeOTD last year there was an #earthquake within the subducting Pacific plate beneath #Anchorage #Alaska i was also being interviewed for my job i have now more in report here https://t.co/L4RHgNdex7 pic.twitter.com/TUXTyTQ1Z4 — Jason "Jay" R. Patton (@patton_cascadia) November 30, 2019
Return to the Earthquake Reports page. Thanks to Jamie Gurney, I took a looksie at the Kaktovik earthquake sequence again. He had interpreted this sequence to possibly represent an extensional step over in a right-lateral (dextral) strike-slip tectonic fault system. An analysis of the Kaktovik eqs 9 days on: the reviewed eqs show a WNW-ESE dextral strike-slip system, not 2 orthogonal faults as suggested. A possible extensional stepover nr the 2nd M6 may have arrested the rupture of the 1st quake @AKearthquake @kaseyaderhold @patton_cascadia pic.twitter.com/uYfqwstzkM — Jamie Gurney (@UKEQ_Bulletin) August 21, 2018 However, methinks that there is also evidence for a series of ~north-south oriented faults. This is based largely on a series of small earthquakes that appear to be oriented along north-south trends. There are a great number of analogies for this, most remarkably the 2012 Sumatra Outer Rise sequence. I discuss these Sumatra earthquakes more on this page and present some of those figures below. I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 7/14-8/21 with magnitudes M ≥ 1.0. Here are some figures that present the material about the 2012 Outer Rise sequence offshore of Sumatra.
Spatiotemporal distribution of HF radiation imaged by the (left) European and (right) Japanese networks. Colored circles and squares indicate the positions of primary and secondary peak HF radiation (from movies S1 and S2, respectively). Their size is scaled by beamforming amplitude, and their color indicates timing relative to hypocentral time (color scale in center). The secondary peaks of the MUSIC pseudo-spectrum are those at least 50% as large as the main peak in the same frame. The brown shaded circles in the right figure are the HF radiation peaks from the Mw 8.2 aftershock observed from Japan. The colored contours in the Sumatra subduction zone (left) represent the slip model of the 2004 Mw 9.1 Sumatra earthquake (28). The figure background is colored by the satellite gravity anomaly (left) inmilligalileos (mgals) (color scale on bottom left) and the magnetic anomaly (right) in nanoteslas (color scale on bottom right). Black dots are the epicenters of the first day of aftershocks from the U.S. National Earthquake Information Center catalog. The big and small white stars indicate the hypocenter of the mainshock and Mw 8.2 aftershock. The moment tensors of the Mw 8.6 mainshock, Mw 8.2 aftershock, and double CMT solutions of the mainshock are shown as colored pink, yellow, red, and blue beach balls. The red line in the top left inset shows the boundary between the India (IN) and Sundaland (SU) plates (29). The patterned pink area is the diffuse deformation zone between the India and Australia plate. The red rectangular zone indicates the study area. The top right inset shows the interpreted fault planes (gray dashed lines) and rupture directions (colored arrows).
Recent stress changes in the Indian Ocean. (a) Total stresses induced by the 2004 [Chlieh et al., 2007], 2005 [Konca et al., 2007], and January M7.2 (http://earthquake.usgs.gov/earthquakes/eqinthenews/2012/usc0007ir5/finite_fault. php) earthquakes, resolved at the 20 km hypocentral depth of the mainshock on the orientation of the initial WNW-ESE (red) fault plane [Meng et al., 2012]. Gray circles mark the first 12 days of the aftershock sequence (NEIC catalog). (b) Coseismic stresses induced by the 2004 and 2005 earthquakes. The yellow focal mechanisms highlight the strike-slip earthquakes during the first year following the 2004 earthquake and the blue focal mechanisms depict the remaining strike-slip events before the 2012 mainshock (Global CMT catalog). (c) Cumulative postseismic stresses induced by the 2004 and 2005 earthquakes at the time of the 2012 earthquake.
Well, yesterday while I was installing the final window in a reconstruction project, there was an earthquake along the Aleutian Island Arc (a subduction zone) in the region of the Andreanof Islands. Here is the USGS website for the M 6.6 earthquake. This earthquake is close to the depth of the megathrust fault, but maybe not close enough. So, this may be on the subduction zone, but may also be on an upper plate fault (I interpret this due to the compressive earthquake fault mechanism). The earthquake has a hypocentral depth of 20 km and the slab model (see Hayes et al., 2013 below and in the poster) is at 40 km at this location. There is uncertainty in both the slab model and the hypocentral depth. I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version.
Profile and section of coseismic deformation associated with the 1964 Alaska earthquake across the Aleutian arc (oriented NW-SE through Middleton and Montague Islands). Profile of horizontal and vertical components of coseismic slip (above) and inferred slip partitioning between the megathrust and intraplate faults (below). From Plafker (1965, 1967; 1972)
WMV file for downloading.
Map of the Aleutian–Bering region and location of the study area (rectangle). Lines with barbs indicate subduction zones: (1) Kamchatka Trench and (2) Aleutian Trench; lines with sense of displacement mark fracture zones (FZs): (3) Steller, (4) Pikezh and (5) Bering FZs. Single arrows show relative direction of convergence of the Pacific (P) and North American (NA) plates. Bathymetric contours are in meters.
The main tectonic features of the Kamchatka–Aleutian junction area modified from Seliverstov (1983), Seliverstov et al. (1988) and Baranov et al. (1991). The eastern side of the Central Kamchatka depression is bounded by normal faults. Contour interval is 1000 m. Lines A and B indicate the locations of profiles shown in Fig. 3; the rectangle marks the location of the area shown in Fig. 4.
Rupture zones of the major earthquakes in the Kamchatka–Aleutian junction area [according to Vikulin (1997)]. Earthquakes with a magnitude of Mw>7 are shown.
Proposed tectonic model for southern Chile. Partitioning of the oblique convergence vector between the Nazca plate and South American plate results in a dextral strike-slip fault zone in the magmatic arc and a northward moving forearc sliver. Modified after Lavenu and Cembrano (1999).
Geodynamic setting of Kamchatka in framework of the Northwest Pacific. Modified after Nokleberg et al. (1994) and Kharakhinov (1996)). Simplified cross-section line I-I’ is shown in Fig. 2. The inset shows location of Sredinny and Eastern Ranges. [More figure caption text in the publication].
The Cenozoic evolution in the Northwest Pacific. Plate kinematics is shown in hotspot reference frame after (Engebretson et al., 1985). Keys distinguish zones of active volcanism (thick black lines), inactive volcanic belts (thick gray lines), deformed arc terranes (hatched pattern), subduction zones: active (black triangles), inactive *(empty triangles). In letters: sa = Sikhote-aline, bs = Bering shelf belts; SH = Shirshov Ridge; V = Vitus arch; KA = Kuril; RA = Ryukyu’ LA = Luzon; IBMA = Izu-Bonin-Mariana arcs; WPB = Western Philippine, BB = Bowers basins.
Aleutian subduction zone. Symbols as in Figure 3. (a) Residual free-air gravity anomaly and seismicity. The outer-arc high, trench-parallel fore-arc ridge and block-bounding faults are dashed in blue, black, and red, respectively. Annotations are AP = Amchitka Pass; BHR = Black-Hills Ridge; SS = Sunday Sumit Basin; PD = Pratt Depression. (b) Published asperities and slip-distributions/aftershock areas for large magnitude earthquakes. (c) Cross sections showing residual bathymetry (green), residual free-air gravity anomaly (black), and the geometry of the seismogenic zone [Hayes et al., 2012].
Schematic diagram summarizing the key spatial associations interpreted between the morphology of the fore-arc and variations in the seismogenic behavior of subduction megathrusts.
Above: Rupture zones of earthquakes of magnitude M > 7.4 from 1925-1971 as delineated by their aftershocks along plate boundary in Aleutians, southern Alaska and offshore British Columbia [after Sykes, 1971]. Contours in fathoms. Various symbols denote individual aftershock sequences as follows: crosses, 1949, 1957 and 1964; squares, 1938, 1958 and 1965; open triangles, 1946; solid triangles, 1948; solid circles, 1929, 1972. Larger symbols denote more precise locations. C = Chirikof Island. Below: Space-time diagram showing lengths of rupture zones, magnitudes [Richter, 1958; Kanamori, 1977 b; Kondorskay and Shebalin, 1977; Kanamori and Abe, 1979; Perez and Jacob, 1980] and locations of mainshocks for known events of M > 7.4 from 1784 to 1980. Dashes denote uncertainties in size of rupture zones. Magnitudes pertain to surface wave scale, M unless otherwise indicated. M is ultra-long period magnitude of Kanamori 1977 b; Mt is tsunami magnitude of Abe[ 1979]. Large shocks 1929 and 1965 that involve normal faulting in trench and were not located along plate interface are omitted. Absence of shocks before 1898 along several portions of plate boundary reflects lack of an historic record of earthquakes for those areas.
Aftershock areas of earthquakes of magnitude M > 7.4 in the Aleutians, southern Alaska and offshore British Columbia from 1938 to 1979, after Sykess [1971] and McCann et al. [1979]. Heavy arrows denote motion of Pacific plate with respect to North American plate as calculated by Chase [1978]. Two thousand fathom contour is shown for Aleutian trench. Ms and Mw denote magnitude scales described by Kanamori [1977b].
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)
Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.
Today in "Alaska is really big": Sunday's magnitude 6.4 earthquake in the northern Brooks Range and today's magnitude 6.6 earthquake in the Andreanof Islands occurred over 1,600 miles apart. pic.twitter.com/mfyfxdJfpr — AK Earthquake Center (@AKearthquake) August 15, 2018 These seismic waves (generated by a M6.6 earthquake near the Aleutian Islands) rolled through #Victoria a few minutes ago… — John Cassidy (@earthquakeguy) August 15, 2018
Well, I awakened shortly after this M 6.4 earthquake hit the northern part of Alaska, along the north Slope, north of the Brooks Range. I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version.
Generalized geologic map of northeastern Alaska, showing the location of the Arctic National Wildlife Refuge (ANWR), the northeastern Brooks Range, and other features specifically mentioned in the text.
Tectonic map of the northeastern Brooks Range, showing the location of the Sadlerochit and Shublik mountain ranges, Ignek Valley, the Beli Unit #1 well, seismic line 84-6, and other features mentioned in the text. Map modified from Wallace and Hanks (1990).
Presentation of known structures recognized within the Sadlerochit Mountains region. (A) Balanced cross section through the northern part of northeastern Brooks Range (modified from Wallace, 1993). Each basement-cored anticlinorium is interpreted to mark a horse in a duplex formed above a detachment at depth in basement (dark shading). The roof thrust in Kayak Shale terminates to north in the Sadlerochit Mountains owing to depositional discontinuity. All structures shown are interpreted to be Cenozoic in age. (B) Reproduced interpretation of seismic line 84-6 by Potter et al. (1999, plate BD2), indicating that basement rocks were involved in deformation beneath the coastal plain to the north of the Sadlerochit Mountains (at same scale as A).
Simplified geologic map of the Shublik and Sadlerochit Mountains, northeastern Brooks Range, Alaska. The Kikiktat volcanics are shown in green and outcrop in the hanging wall of large N-directed Cretaceous–Tertiary Brookian thrust sheets. Geologic is mapping by Strauss and Macdonald, with modifications from Robinson et al. (1989) and Bader and Bird (1986).
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)
Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.
Mw=6.1, NORTHERN ALASKA (Depth: 9 km), 2018/08/12 21:15:00 UTC – Full details here: https://t.co/5MGjW6asRG pic.twitter.com/2o7sjaY4rz — Earthquakes (@geoscope_ipgp) August 12, 2018 Thirty minutes of ground motion data from seismic stations across Alaska. The red star is the epicenter, and the dots are stations. The animation includes two aftershocks, a M4.9 at 7:14 am and a M4.7 at 07:18. pic.twitter.com/Q7Wy0ikGRj — AK Earthquake Center (@AKearthquake) August 12, 2018 M6.4 #earthquake in Alaska's Arctic National Wildlife Refuge (ANWR) this morning. A subset of Transportable Array stations show the data coming in. Close stations like C26K and C27K (also in ANWR) have a shorter signal because there was less time for different waves to separate. pic.twitter.com/HAzo8csMCw — Dr. Kasey Aderhold (@kaseyaderhold) August 12, 2018 Ground shaking from the M6.1 #Alaska earthquake was easily recorded on NRCan seismic stations form coast to coast to coast (from Inuvik to Victoria to Halifax). The seismic surface waves only took ~20 minutes to travel to Nova Scotia. — John Cassidy (@earthquakeguy) August 12, 2018 Auto solution FMNEAR (Géoazur) with regional records for M 6.4 – 84km SW of Kaktovik, Alaska, 69.624°N 145.247°W (PDE USGS used to trigger inversion). pic.twitter.com/nLi29wjA70 — Bertrand Delouis (@BertrandDelouis) August 12, 2018 we also start testing automated slipmaps at Géoazur. Here the result for with regional records for M 6.4 – 84km SW of Kaktovik, Alaska, 69.624°N 145.247°W (PDE USGS used to trigger inversion), using the FM solution from FMNEAR previously twitted. EW plane selected as rupt plane. pic.twitter.com/Wvhh45rz1J — Bertrand Delouis (@BertrandDelouis) August 12, 2018 M6.4 earthquake in northern #Alaska – details from the USGS:https://t.co/PYiuZCEdkB — John Cassidy (@earthquakeguy) August 12, 2018 Shallow, crustal earthquakes are extremely common throughout Alaska and do not indicate induced seismicity. The crustal, strike-slip mechanism for this quake aligns with previous earthquakes in the area and is completely consistent with a natural source. — AK Earthquake Center (@AKearthquake) August 12, 2018 Another M6.4 earthquake in Alaska, just to the NNE of the one earlier this morning. These recordings of the shaking at Inuvik and #Victoria a few minutes ago. — John Cassidy (@earthquakeguy) August 12, 2018 My @raspishake had a very clean catch of the M6.1 Kaktovik, Alaska EQ at 14:58:54 (7:48am PDT). The first waves (P-waves) from this strike-slip quake arrived at my station at 15:05:50. That ~7min travel time places the epicenter ~2450mi away, which Google Earth Confirms. pic.twitter.com/4xB9gajZkD — Ryan Hollister (@phaneritic) August 12, 2018 M6.4 #Alaska #earthquake and aftershocks from USGS & EMSC with fault layer from Koehler etal. 2013 & Plafker 1994. Main epicenter w/some additional geological figures. pic.twitter.com/UViycnTrLt — Sotiris Valkaniotis (@SotisValkan) August 12, 2018 Through our partnership with Alyeska, we have instruments at pump stations along the Trans Alaska Pipeline. Via Lea Gardine, this figure shows ground motion from the M6.4 quake as it traveled from the Beaufort Sea south to the Gulf of Alaska. pic.twitter.com/aAobIKt0q7 — AK Earthquake Center (@AKearthquake) August 12, 2018 A 6.4 magnitude earthquake has hit a remote part of Alaska. There have been no reports of injuries or damage. https://t.co/FmqKRefRjd pic.twitter.com/vgllmzXV8w — CNN (@CNN) August 12, 2018 Strong 6.5 #earthquake #Terremoto #Temblor near Kaktovic #Alaska right now 🚨 pic.twitter.com/niaFDTcSja — Teacher From PR 🌞🌊 (@MaestroDEPR) August 12, 2018 Here's a map from Helena's piece showing earthquakes in that region recorded since 1970. Altogether we've recorded around 4,500 earthquakes up there. We've picked up 16 aftershocks from the M6.4 so far today, but manual scans and other analysis will find more. pic.twitter.com/Xin0OB8k0V — AK Earthquake Center (@AKearthquake) August 12, 2018 Seismo Blog: When on Sunday morning shortly before 7 am local time, the citizens of Fairbanks in Alaska awoke to noticeable ground shaking, many of them thought these seismic waves were coming from the south… https://t.co/Aijoz6JYDh pic.twitter.com/EkS0WWo7oe — Berkeley Seismo Lab (@BerkeleySeismo) August 13, 2018
UPDATES Below is a list of all the reports associated with this earthquake sequence. I thought it would be interesting to see the seismicity with time. Perhaps this could help us learn about the fault sources associated with this earthquake sequence. Magnitude 7.9 earthquake: Midnight shaking and confusion in an Alaskan Cabin | https://t.co/1twVj9WIVY https://t.co/6EwODnWQ0y via @temblor — temblor (@temblor) January 24, 2018 the Alaska earthquake generated a small tsunami detected by @Ocean_Networks sensors off Canada's westcoast #tsunami pic.twitter.com/MK0K0SLOi8 — Kate Moran (@katemoran) January 23, 2018 Here's a time-magnitude plot of the sequence so far. 17 aftershocks with magnitudes in the 4-5 range. Also a map, but most of these have not been reviewed yet, so locations may be off. Of course there are many smaller aftershocks that we're not getting automatic locations for. pic.twitter.com/jnA9aDSZiQ — AK Earthquake Center (@AKearthquake) January 23, 2018 Why Did Alaska’s Big Quake Lead to a Tiny Tsunami? #AlaskaEarthquakehttps://t.co/wdA0YoWOhZ — IRIS Earthquake Sci (@IRIS_EPO) January 25, 2018 Here's an article about the rogue buoy data point: https://t.co/0PZTSVJFoD — Brian Brettschneider (@Climatologist49) January 25, 2018 Is this a foreshock to the Mw 7.9 Gulf of Alaska quake? 30 October 2017, ~5km south on the apparent (& hypothesized by me) N-S striking sinistral strike-slip fault on which the quake initiated: https://t.co/YdTO2zBaBL Compare seismograms of that quake (left) w/ 24 Jan M4.1 eq pic.twitter.com/gHdfE5NkSc — J H Gurney (@UKEQ_Bulletin) January 25, 2018 Did Tuesday’s M=7.9 Kodiak #earthquake nudge the Alaskan Megathrust closer to failure? https://t.co/nQqBkjI1Hv | @temblor | #alaskaearthquake pic.twitter.com/CpVAu4RjRT — IRIS Earthquake Sci (@IRIS_EPO) January 27, 2018
Earthquake Report: central Aleutians
https://earthquake.usgs.gov/earthquakes/eventpage/us2000k9d7/executive
There was a subduction zone earthquake nearby on 15 August 2018. Learn more about the subduction zone in my earthquake report for this M 6.6 earthquake here.
There was a similar earthquake in 2017 further to the west, which was also a strike-slip earthquake and it produced a small sized tsunami (Lay et al., 2017). However, the 17 July 2017 magnitude M 7.9 earthquake was much larger in magnitude. Here is my earthquake report and update for this 2017 earthquake. These reports include information about the intersection of the Aleutian and Kuril plate boundaries.
The majority of the Aleutian Islands are volcanic arc islands formed as a result of the subduction of the Pacific plate beneath the North America plate. To the west, there is another subduction zone along the Kuril and Kamchatka volcanic arcs. These subduction zones form deep sea trenches (the deepest parts of the ocean are in subduction zone trenches).
In the eastern part of the Aleutian/Alaska subduction zone (e.g. Alaska Peninsula or Prince William Sound), the plates converge in the direction of subduction (perpendicular to the fault orientation or “strike”). Further to the west, the plates converge obliquely compared to the fault orientation.
This oblique convergence results in the development of additional special faults that accommodate the plate convergence not perpendicular to the faults. These are typically strike-slip faults parallel to the subduction zone (they accommodate the proportion of relative motion parallel to the fault), called forearc sliver faults.
Along the central and western Aleutian plate boundary, this strike-slip relative motion also creates blocks in the upper North America plate that rotate relative to the forearc sliver fault. Imagine how ball bearings rotate when the two planes that they are contained within move relative to each other.Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
Magnetic Anomalies
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
Geologic Fundamentals
Compressional:
Extensional:
Alaska | Kamchatka | Kurile
General Overview
Earthquake Reports
Social Media
References:
Return to the Earthquake Reports page.
Earthquake Report: Northern Alaska
https://earthquake.usgs.gov/earthquakes/eventpage/ak0193wxcfea/executive
Many of us are familiar with the Good Friday earthquake, a megathrust subduction zone earthquake. This earthquake has a birthday tomorrow, from 27 March, 1964 (55 years ago).
The M=9.2 1964 temblor created a tsunami that traveled across the Pacific Ocean. More about the Good Friday earthquake and tsunami can be found here.
Alaska has a variety of major fault systems in addition to the subduction zone. There are also large strike-slip faults (move side by side) such as the Denali fault and the Kaltag fault. There are even more strike slip systems too, like the Queen Charlotte / Fairweather fault in southeastern Alaska and the Bering-Kresla shear zone in the extreme western part of the Aleutian Islands. Alaska is so cool, they even have extensional (normal) earthquakes, such as on 1 December 2018.
Recently, there was a series of strike-slip earthquakes in the Gulf of Alaska probably related to reactivation of pre-existing structures in the Pacific plate. We continue to have aftershocks in this area.
Also, there is an ongoing sequence of earthquakes (now, maybe it is a swarm?) in northeastern Alaska. The largest quake was in August last year (2018), with a magnitude of M=6.3.
Today’s earthquake happened away from one of the mapped faults in the USGS Quaternary Active Fault and Fold Database (the Kaltag fault). The earthquake mechanism shows this earthquake may have been a slightly oblique normal type of an earthquake. I placed strike-slip arrows on the 2 possible nodal planes.but this is mainly a normal earthquake.
There was also a normal earthquake in 1958, when a M=7.1 quake struck about 50 km (35 miles) to the southeast of today’s quake. However, the 1958 event was oriented perpendicular to today’s quake. Below are some observations made following the 1958 earthquake. There was evidence of liquefaction, with sand volcanoes about a meter thick extending for hundreds of meters laterally.
I need to get to bed, but will try to write more tomorrow.Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
Magnetic Anomalies
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
Geologic Fundamentals
Compressional:
Extensional:
Alaska | Kamchatka | Kurile
General Overview
Earthquake Reports
Social Media
References:
Return to the Earthquake Reports page.
Earthquake Report: 2018 Summary
However, our historic record is very short, so any thoughts about whether this year (or last, or next) has smaller (or larger) magnitude earthquakes than “normal” are limited by this small data set.
Here is a table of the earthquakes M ≥ 6.5.
Here is a plot showing the cumulative release of seismic energy. This summary is imperfect in several ways, but shows how only the largest earthquakes have a significant impact on the tally of energy release from earthquakes. I only include earthquakes M ≥ 6.5. Note how the M 7.5 Sulawesi earthquake and how little energy was released relative to the two M = 7.9 earthquakes.
Below is my summary poster for this earthquake year
This is a video that shuffles through the earthquake report posters of the year
2018 Earthquake Report Pages
Other Annual Summaries
2018 Earthquake Reports
General Overview of how to interact with these summaries
Background on the Earthquake Report posters
Magnetic Anomalies
2018.01.10 M 7.6 Cayman Trough
Based upon our knowledge of the plate tectonics of this region, I can interpret the fault plane solution for this earthquake. The M 7.6 earthquake was most likely a left-lateral strike-slip earthquake associated with the Swan fault.
2018.01.14 M 7.1 Peru
In the region of this M 7.1 earthquake, two large structures in the NP are the Nazca Ridge and the Nazca fracture zone. The Nazca fracture zone is a (probably inactive) strike-slip fault system. The Nazca Ridge is an over-thickened region of the NP, thickened as the NP moved over a hotspot located near Salas y Gomez in the Pacific Ocean east of Easter Island (Ray et al., 2012).
There are many papers that discuss how the ridge affects the shape of the megathrust fault here. The main take-away is that the NR is bull dozing into South America and the dip of the subduction zone is flat here. There is a figure below that shows the deviation of the subducting slab contours at the NR.
Well, I missed looking further into a key update paper and used figures from an older paper on my interpretive poster yesterday. Thanks to Stéphane Baize for pointing this out! Turns out, after their new analyses, the M 7.1 earthquake was in a region of higher seismogenic coupling, rather than low coupling (as was presented in my first poster).
Also, Dr. Robin Lacassin noticed (as did I) the paucity of aftershocks from yesterday’s M 7.1. This was also the case for the carbon copy 2013 M 7.1 earthquake (there was 1 M 4.6 aftershock in the weeks following the M 7.1 earthquake on 2013.09.25; there were a dozen M 1-2 earthquakes in Nov. and Dec. of 2013, but I am not sure how related they are to the M 7.1 then). I present a poster below with this in mind. I also include below a comparison of the MMI modeled estimates. The 2013 seems to have possibly generated more widespread intensities, even though that was a deeper earthquake.
2018.01.23 M 7.9 Gulf of Alaska
This is strange because the USGS fault plane is oriented east-west, leading us to interpret the fault plane solution (moment tensor or focal mechanism) as a left-lateral strike-slip earthquake. So, maybe this earthquake is a little more complicated than first presumed. The USGS fault model is constrained by seismic waves, so this is probably the correct fault (east-west).
I prepared an Earthquake Report for the 1964 Good Friday Earthquake here.
So, that being said, here is the animation I put together. I used the USGS query tool to get earthquakes from 1/22 until now, M ≥ 1.5. I include a couple inset maps presented in my interpretive posters. The music is copyright free. The animations run through twice.
Here is a screenshot of the 14 MB video embedded below. I encourage you to view it in full screen mode (or download it).
2018.02.16 M 7.2 Oaxaca, Mexico
The SSN has a reported depth of 12 km, further supporting evidence that this earthquake was in the North America plate.
This region of the subduction zone dips at a very shallow angle (flat and almost horizontal).
There was also a sequence of earthquakes offshore of Guatemala in June, which could possibly be related to the M 8.1 earthquake. Here is my earthquake report for the Guatemala earthquake.
The poster also shows the seismicity associated with the M 7.6 earthquake along the Swan fault (southern boundary of the Cayman trough). Here is my earthquake report for the Guatemala earthquake.2018.02.25 M 7.5 Papua New Guinea
This M 7.5 earthquake (USGS website) occurred along the Papua Fold and Thrust Belt (PFTB), a (mostly) south vergent sequence of imbricate thrust faults and associated fold (anticlines). The history of this PFTB appears to be related to the collision of the Australia plate with the Caroline and Pacific plates, the delamination of the downgoing oceanic crust, and then associated magmatic effects (from decompression melting where the overriding slab (crust) was exposed to the mantle following the delamination). More about this can be found in Cloos et al. (2005).
The aftershocks are still coming in! We can use these aftershocks to define where the fault may have slipped during this M 7.5 earthquake. As I mentioned yesterday in the original report, it turns out the fault dimension matches pretty well with empirical relations between fault length and magnitude from Wells and Coppersmith (1994).
The mapped faults in the region, as well as interpreted seismic lines, show an imbricate fold and thrust belt that dominates the geomorphology here (as well as some volcanoes, which are probably related to the slab gap produced by crust delamination; see Cloos et al., 2005 for more on this). I found a fault data set and include this in the aftershock update interpretive poster (from the Coordinating Committee for Geoscience Programmes in East and Southeast Asia, CCOP).
I initially thought that this M 7.5 earthquake was on a fault in the Papuan Fold and Thrust Belt (PFTB). Mark Allen pointed out on twitter that the ~35km hypocentral depth is probably too deep to be on one of these “thin skinned” faults (see Social Media below). Abers and McCaffrey (1988) used focal mechanism data to hypothesize that there are deeper crustal faults that are also capable of generating the earthquakes in this region. So, I now align myself with this hypothesis (that the M 7.5 slipped on a crustal fault, beneath the thin skin deformation associated with the PFTB. (thanks Mark! I had downloaded the Abers paper but had not digested it fully.2018.03.08 M 6.8 New Ireland
The main transform fault (Weitin fault) is ~40 km to the west of the USGS epicenter. There was a very similar earthquake on 1982.08.12 (USGS website).
This earthquake is unrelated to the sequence occurring on the island of New Guinea.
Something that I rediscovered is that there were two M 8 earthquakes in 1971 in this region. This testifies that it is possible to have a Great earthquake (M ≥ 8) close in space and time relative to another Great earthquake. These earthquakes do not have USGS fault plane solutions, but I suspect that these are subduction zone earthquakes (based upon their depth).
This transform system is capable of producing Great earthquakes too, as evidenced by the 2000.11.16 M 8.0 earthquake (USGS website). This is another example of two Great earthquakes (or almost 2 Great earthquakes, as the M 7.8 is not quite a Great earthquake) are related. It appears that the M 8.0 earthquake may have triggered teh M 7.8 earthquake about 3 months later (however at first glance, it seemed to me like the strike-slip earthquake might not increase the static coulomb stress on the subduction zone, but I have not spent more than half a minute thinking about this).Main Interpretive Poster with emag2
Earthquakes M≥ 6.5 with emag2
2018.03.26 M 6.6 New Britain
Today’s M 6.6 earthquake happened close in proximity to a M 6.3 from 2 days ago and a M 5.6 from a couple weeks ago. The M 5.6 may be related (may have triggered these other earthquakes), but this region is so active, it might be difficult to distinguish the effects from different earthquakes. The M 5.6 is much deeper and looks like it was in the downgoing Solomon Sea plate. It is much more likely that the M 6.3 and M 6.6 are related (I interpret that the M 6.3 probably triggered the M 6.6, or that M 6.3 was a foreshock to the M 6.6, given they are close in depth). Both M 6.3 and M 6.6 are at depths close to the depth of the subducting slab (the megathrust fault depth) at this location. So, I interpret these to be subduction zone earthquakes.
2018.03.26 M 6.9 New Britain
2018.04.02 M 6.8 Bolivia
We are still unsure what causes an earthquake at such great a depth. The majority of earthquakes happen at shallower depths, caused largely by the frictional between differently moving plates or crustal blocks (where earth materials like the crust behave with brittle behavior and not elastic behavior). Some of these shallow earthquakes are also due to internal deformation within plates or crustal blocks.
As plates dive into the Earth at subduction zones, they undergo a variety of changes (temperature, pressure, stress). However, because people cannot directly observe what is happening at these depths, we must rely on inferences, laboratory analogs, and other indirect methods to estimate what is going on.
So, we don’t really know what causes earthquakes at the depth of this Bolivia M 6.8 earthquake. Below is a review of possible explanations as provided by Thorne Lay (UC Santa Cruz) in an interview in response to the 2013 M 8.3 Okhotsk Earthquake.
2018.05.04 M 6.9 Hawai’i
Hawaii is an active volcanic island formed by hotspot volcanism. The Hawaii-Emperor Seamount Chain is a series of active and inactive volcanoes formed by this process and are in a line because the Pacific plate has been moving over the hotspot for many millions of years.
Southeast of the main Kilauea vent, the Pu‘u ‘Ö‘ö crater saw an elevation of lava into the crater, leading to overtopping of the crater (on 4/30/2018). Seismicity migrated eastward along the ERZ. This morning, there was a M 5.0 earthquake in the region of the Hilina fault zone (HFZ). I was getting ready to write something up, but I had other work that I needed to complete. Then, this evening, there was a M 6.9 earthquake between the ERZ and the HFZ.
There have been earthquakes this large in this region in the past (e.g. the 1975.1.29 M 7.1 earthquake along the HFZ). This earthquake was also most likely related to magma injection (Ando, 1979). The 1975 M 7.1 earthquake generated a small tsunami (Ando, 1979). These earthquakes are generally compressional in nature (including the earthquakes from today).
Today’s earthquake also generated a tsunami as recorded on tide gages throughout Hawaii. There is probably no chance that a tsunami will travel across the Pacific to have a significant impact elsewhere.Temblor Reports:
2018.05.05 Pele, the Hawai’i Goddess of Fire, Lightning, Wind, and Volcanoes
2018.05.06 Pele, la Diosa Hawaiana del Fuego, los Relámpagos, el Viento y los Volcanes de Hawái
2018.08.05 M 6.9 Lombok, Indonesia
However, it is interesting because the earthquake sequence from last week (with a largest earthquake with a magnitude of M 6.4) were all foreshocks to this M 6.9. Now, technically, these were not really foreshocks. The M 6.4 has an hypocentral (3-D location) depth of ~6 km and the M 6.9 has an hypocentral depth of ~31 km. These earthquakes are not on the same fault, so I would interpret that the M 6.9 was triggered by the sequence from last week due to static coulomb changes in stress on the fault that ruptured. Given the large difference in depths, the uncertainty for these depths is probably not sufficient to state that they may be on the same fault (i.e. these depths are sufficiently different that this difference is larger than the uncertainty of their locations).
I present a more comprehensive analysis of the tectonics of this region in my earthquake report for the M 6.4 earthquake here. I especially address the historic seismicity of the region there. This M 6.9 may have been on the Flores thrust system, while the earthquakes from last week were on the imbricate thrust faults overlying the Flores Thrust. See the map from Silver et al. (1986) below. I include the same maps as in my original report, but after those, I include the figures from Koulani et al. (2016) (the paper is available on researchgate).2018.08.15 M 6.6 Aleutians
The Andreanof Islands is one of the most active parts of the Aleutian Arc. There have been many historic earthquakes here, some of which have been tsunamigenic (in fact, the email that notified me of this earthquake was from the ITIC Tsunami Bulletin Board).
Possibly the most significant earthquake was the 1957 Andreanof Islands M 8.6 Great (M ≥ 8.0) earthquake, though the 1986 M 8.0 Great earthquake is also quite significant. As was the 1996 M 7.9 and 2003 M 7.8 earthquakes. Lest we forget smaller earthquakes, like the 2007 M 7.2. So many earthquakes, so little time.2018.08.18 M 8.2 Fiji
This earthquake is one of the largest earthquakes recorded historically in this region. I include the other Large and Great Earthquakes in the posters below for some comparisons.
Today’s earthquake has a Moment Magnitude of M = 8.2. The depth is over 550 km, so is very very deep. This region has an historic record of having deep earthquakes here. Here is the USGS website for this M 8.2 earthquake. While I was writing this, there was an M 6.8 deep earthquake to the northeast of the M 8.2. The M 6.8 is much shallower (about 420 km deep) and also a compressional earthquake, in contrast to the extensional M 8.2.
This M 8.2 earthquake occurred along the Tonga subduction zone, which is a convergent plate boundary where the Pacific plate on the east subducts to the west, beneath the Australia plate. This subduction zone forms the Tonga trench.2018.08.19 M 6.9 Lombok, Indonesia
Today there was an M 6.3 soon followed by an M 6.9 earthquake (and a couple M 5.X quakes).
These earthquakes have been occurring along a thrust fault system along the northern portion of Lombok, Indonesia, an island in the magamatic arc related to the Sunda subduction zone. The Flores thrust fault is a backthrust to the subduction zone. The tectonics are complicated in this region of the world and there are lots of varying views on the tectonic history. However, there has been several decades of work on the Flores thrust (e.g. Silver et al., 1986). The Flores thrust is an east-west striking (oriented) north vergent (dipping to the south) thrust fault that extends from eastern Java towards the Islands of Flores and Timor. Above the main thrust fault are a series of imbricate (overlapping) thrust faults. These imbricate thrust faults are shallower in depth than the main Flores thrust.
The earthquakes that have been happening appear to be on these shallower thrust faults, but there is a possibility that they are activating the Flores thrust itself. Perhaps further research will illuminate the relations between these shallower faults and the main player, the Flores thrust.
2018.08.21 M 7.3 Venezuela
The northeastern part of Venezuela lies a large strike-slip plate boundary fault, the El Pilar fault. This fault is rather complicated as it strikes through the region. There are thrust faults and normal faults forming ocean basins and mountains along strike.
Many of the earthquakes along this fault system are strike-slip earthquakes (e.g. the 1997.07.09 M 7.0 earthquake which is just to the southwest of today’s temblor. However, today’s earthquake broke my immediate expectations for strike-slip tectonics. There is a south vergent (dipping to the north) thrust fault system that strikes (is oriented) east-west along the Península de Paria, just north of highway 9, east of Carupano, Venezuela. Audenard et al. (2000, 2006) compiled a Quaternary Fault database for Venezuela, which helps us interpret today’s earthquake. I suspect that this earthquake occurred on this thrust fault system. I bet those that work in this area even know the name of this fault. However, looking at the epicenter and the location of the thrust fault, this is probably not on this thrust fault. When I initially wrote this report, the depth was much shallower. Currently, the hypocentral (3-D location) depth is 123 km, so cannot be on that thrust fault.
The best alternative might be the subduction zone associated with the Lesser Antilles.2018.08.24 M 7.1 Peru
While doing my lit review, I found the Okal and Bina (1994) paper where they use various methods to determine focal mechanisms for the some deep earthquakes in northern Peru. More about focal mechanisms below. These authors created focal mechanisms for the 1921 and 1922 deep earthquakes so they could lean more about the 1970 deep earthquake. Their seminal work here forms an important record of deep earthquakes globally. These three earthquakes are all extensional earthquakes, similar to the other deep earthquakes in this region. I label the 1921 and 1922 earthquakes a couplet on the poster.
There was also a pair of earthquakes that happened in November, 2015. These two earthquakes happened about 5 minutes apart. They have many similar characteristics, suggest that they slipped similar faults, if not the same fault. I label these as doublets also.
So, there may be a doublet companion to today’s M 7.1 earthquake. However, there may be not. There are examples of both (single and doublet) and it might not really matter for 99.99% of the people on Earth since the seismic hazard from these deep earthquakes is very low.
Other examples of doublets include the 2006 | 2007 Kuril Doublets (Ammon et al., 2008) and the 2011 Kermadec Doublets (Todd and Lay, 2013).2018.09.05 M 6.6 Hokkaido, Japan
This earthquake is in an interesting location. to the east of Hokkaido, there is a subduction zone trench formed by the subduction of the Pacific plate beneath the Okhotsk plate (on the north) and the Eurasia plate (to the south). This trench is called the Kuril Trench offshore and north of Hokkaido and the Japan Trench offshore of Honshu.
One of the interesting things about this region is that there is a collision zone (a convergent plate boundary where two continental plates are colliding) that exists along the southern part of the island of Hokkaido. The Hidaka collision zone is oriented (strikes) in a northwest orientation as a result of northeast-southwest compression. Some suggest that this collision zone is no longer very active, however, there are an abundance of active crustal faults that are spatially coincident with the collision zone.
Today’s M 6.6 earthquake is a thrust or reverse earthquake that responded to northeast-southwest compression, just like the Hidaka collision zone. However, the hypocentral (3-D) depth was about 33 km. This would place this earthquake deeper than what most of the active crustal faults might reach. The depth is also much shallower than where we think that the subduction zone megathrust fault is located at this location (the fault formed between the Pacific and the Okhotsk or Eurasia plates). Based upon the USGS Slab 1.0 model (Hayes et al., 2012), the slab (roughly the top of the Pacific plate) is between 80 and 100 km. So, the depth is too shallow for this hypothesis (Kuril Trench earthquake) and the orientation seems incorrect. Subduction zone earthquakes along the trench are oriented from northwest-southweast compression, a different orientation than today’s M 6.6.
So today’s M 6.6 earthquake appears to have been on a fault deeper than the crustal faults, possibly along a deep fault associated with the collision zone. Though I am not really certain. This region is complicated (e.g. Kita et al., 2010), but there are some interpretations of the crust at this depth range (Iwasaki et al., 2004) shown in an interpreted cross section below.Temblor Reports:
2018.09.06 Violent shaking triggers massive landslides in Sapporo Japan earthquake
2018.09.09 M 6.9 Kermadec
This earthquake was quite deep, so was not expected to generate a significant tsunami (if one at all).
There are several analogies to today’s earthquake. There was a M 7.4 earthquake in a similar location, but much deeper. These are an interesting comparison because the M 7.4 was compressional and the M 6.9 was extensional. There is some debate about what causes ultra deep earthquakes. The earthquakes that are deeper than about 40-50 km are not along subduction zone faults, but within the downgoing plate. This M 6.9 appears to be in a part of the plate that is bending (based on the Benz et al., 2011 cross section). As plates bend downwards, the upper part of the plate gets extended and the lower part of the plate experiences compression.2018.09.28 M 7.5 Sulawesi
This area of Indonesia is dominated by a left-lateral (sinistral) strike-slip plate boundary fault system. Sulawesi is bisected by the Palu-Kola / Matano fault system. These faults appear to be an extension of the Sorong fault, the sinistral strike-slip fault that cuts across the northern part of New Guinea.
There have been a few earthquakes along the Palu-Kola fault system that help inform us about the sense of motion across this fault, but most have maximum magnitudes mid M 6.
GPS and block modeling data suggest that the fault in this area has a slip rate of about 40 mm/yr (Socquet et al., 2006). However, analysis of offset stream channels provides evidence of a lower slip rate for the Holocene (last 12,000 years), a rate of about 35 mm/yr (Bellier et al., 2001). Given the short time period for GPS observations, the GPS rate may include postseismic motion earlier earthquakes, though these numbers are very close.
Using empirical relations for historic earthquakes compiled by Wells and Coppersmith (1994), Socquet et al. (2016) suggest that the Palu-Koro fault system could produce a magnitude M 7 earthquake once per century. However, studies of prehistoric earthquakes along this fault system suggest that, over the past 2000 years, this fault produces a magnitude M 7-8 earthquake every 700 years (Bellier et al., 2006). So, it appears that this is the characteristic earthquake we might expect along this fault.
Most commonly, we associate tsunamigenic earthquakes with subduction zones and thrust faults because these are the types of earthquakes most likely to deform the seafloor, causing the entire water column to be lifted up. Strike-slip earthquakes can generate tsunami if there is sufficient submarine topography that gets offset during the earthquake. Also, if a strike-slip earthquake triggers a landslide, this could cause a tsunami. We will need to wait until people take a deeper look into this before we can make any conclusions about the tsunami and what may have caused it.
My 2018.10.01 BC Newshour Interview
InSAR Analysis
Interferometric SAR (InSAR) utilizes two separate SAR data sets to determine if the ground surface has changed over time, the time between when these 2 data sets were collected. More about InSAR can be found here and here. Explaining the details about how these data are analyzed is beyond the scope of this report. I rely heavily on the expertise of those who do this type of analysis, for example Dr. Eric Fielding.
M 7.5 Landslide Model vs. Observation Comparison
Until these landslides are analyzed and compared with regions that did not fail in slope failure, we will not be able to reconstruct what happened… why some areas failed and some did not.
There are landslide slope stability and liquefaction susceptibility models based on empirical data from past earthquakes. The USGS has recently incorporated these types of analyses into their earthquake event pages. More about these USGS models can be found on this page.
I prepared some maps that compare the USGS landslide and liquefaction probability maps. Below I present these results along with the MMI contours. I also include the faults mapped by Wilkinson and Hall (2017). Shown are the cities of Donggala and Palu. Also shown are the 2 tide gage locations (Pantoloan Port – PP and Mumuju – M). I also used post-earthquake satellite imagery to outline the largest landslides in Palu Valley, ones that appear to be lateral spreads.
Temblor Reports:
2018.09.28 The Palu-Koro fault ruptures in a M=7.5 quake in Sulawesi, Indonesia, triggering a tsunami and likely more shocks
2018.10.03 Tsunami in Sulawesi, Indonesia, triggered by earthquake, landslide, or both
2018.10.16 Coseismic Landslides in Sulawesi, Indonesia
2018.10.10 M 7.0 New Britain, PNG
The subduction zone forms the New Britain Trench with an axis that trends east-northeast. To the east of New Britain, the subduction zone bends to the southeast to form the San Cristobal and South Solomon trenches. Between these two subduction zones is a series of oceanic spreading ridges sequentially offset by transform (strike slip) faults.
Earthquakes along the megathrust at the New Britain trench are oriented with the maximum compressive stress oriented north-northwest (perpendicular to the trench). Likewise, the subduction zone megathrust earthquakes along the S. Solomon trench compress in a northeasterly direction (perpendicular to that trench).
There is also a great strike slip earthquake that shows that the transform faults are active.
This earthquake was too small and too deep to generate a tsunami.Temblor Reports:
2018.10.10 M 7.5 Earthquake in New Britain, Papua New Guinea
2018.10.22 M 6.8 Explorer plate
The Juan de Fuca plate is created at an oceanic spreading center called the Juan de Fuca Ridge. This spreading ridge is offset by several transform (strike-slip) faults. At the southern terminus of the JDF Ridge is the Blanco fault, a transtensional transform fault connecting the JDF and Gorda ridges.
At the northern terminus of the JDF Ridge is the Sovanco transform fault that strikes to the northwest of the JDF Ridge. There are additional fracture zones parallel and south of the Sovanco fault, called the Heck, Heckle, and Springfield fracture zones.
The first earthquake (M = 6.6) appears to have slipped along the Sovanco fault as a right-lateral strike-slip earthquake. Then the M 6.8 earthquake happened and, given the uncertainty of the location for this event, occurred on a fault sub-parallel to the Sovanco fault. Then the M 6.5 earthquake hit, back on the Sovanco fault.2018.10.25 M 6.8 Greece
Both of those earthquakes were right-lateral strike-slip earthquakes associated with the Kefallonia fault.
However, today’s earthquake sequence was further to the south and east of the strike-slip fault, in a region experiencing compression from the Ionian Trench subduction zone. But there is some overlap of these different plate boundaries, so the M 6.8 mainshock is an oblique earthquake (compressional and strike-slip). Based upon the sequence, I interpret this earthquake to be right-lateral oblique. I could be wrong.
Temblor Reports:
2018.10.26 Greek earthquake in a region of high seismic hazard
2018.11.08 M 6.8 Mid Atlantic Ridge (Jan Mayen fracture zone)
North of Iceland, the MAR is offset by many small and several large transform faults. The largest transform fault north of Iceland is called the Jan Mayen fracture zone, which is the location for the 2018.11.08 M = 6.8 earthquake.
2018.11.30 M 7.0 Alaska
During the 1964 earthquake, the downgoing Pacific plate slipped past the North America plate, including slip on “splay faults” (like the Patton fault, no relation, heheh). There was deformation along the seafloor that caused a transoceanic tsunami.
The Pacific plate has pre-existing zones of weakness related to fracture zones and spreading ridges where the plate formed and are offset. There was an earthquake in January 2016 that may have reactivated one of these fracture zones. This earthquake (M = 7.1) was very deep (~130 km), but still caused widespread damage.
The earthquake appears to have a depth of ~40 km and the USGS model for the megathrust fault (slab 2.0) shows the megathrust to be shallower than this earthquake. There are generally 2 ways that may explain the extensional earthquake: slab tension (the downgoing plate is pulling down on the slab, causing extension) or “bending moment” extension (as the plate bends downward, the top of the plate stretches out.Temblor Reports:
2018.11.30 Exotic M=7.0 earthquake strikes beneath Anchorage, Alaska
2018.12.11 What the Anchorage earthquake means for the Bay Area, Southern California, Seattle, and Salt Lake City
2018.12.05 M 7.5 New Caledonia
This part of the plate boundary is quite active and I have a number of earthquake reports from the past few years (see below, a list of earthquake reports for this region).
But the cool thing from a plate tectonics perspective is that there was a series of different types of earthquakes. At first view, it appears that there was a mainshock with a magnitude of M = 7.5. There was a preceding M 6.0 earthquake which may have been a foreshock.
The M 7.5 earthquake was an extensional earthquake. This may be due to either extension from slab pull or due to extension from bending of the plate. More on this later.
Following the M 7.5, there was an M 6.6 earthquake, however, this was a thrust or reverse (compressional) earthquake. The M 6.6 may have been in the upper plate or along the subduction zone megathrust fault, but we won’t know until the earthquake locations are better determined.
A similar sequence happened in October/November 2017. I prepared two reports for this sequence here and here. Albeit, in 2017, the thrust earthquake was first (2017.10.31 vs. 2017.11.19).
There have been some observations of tsunami. Below is from the Pacific Tsunami Warning Center.
2018.12.20 M 7.4 Bering Kresla
This earthquake happened in an interesting region of the world where there is a junction between two plate boundaries, the Kamchatka subduction zone with the Aleutian subduction zone / Bering-Kresla Shear Zone. The Kamchatka Trench (KT) is formed by the subduction (a convergent plate boundary) beneath the Okhotsk plate (part of North America). The Aleutian Trench (AT) and Bering-Kresla Shear Zone (BKSZ) are formed by the oblique subduction of the Pacific plate beneath the Pacific plate. There is a deflection in the Kamchatka subduction zone north of the BKSZ, where the subduction trench is offset to the west. Some papers suggest the subduction zone to the north is a fossil (inactive) plate boundary fault system. There are also several strike-slip faults subparallel to the BKSZ to the north of the BKSZ.
UPDATE #1
2018.12.29 M 7.0 Philippines
The earthquake was quite deep, which makes it less likely to cause damage to people and their belongings (e.g. houses and roads) and also less likely that the earthquake will trigger a trans-oceanic tsunami.
Here are the tidal data:
Geologic Fundamentals
Compressional:
Extensional:
Return to the Earthquake Reports page.
Earthquake Report: Bering Kresla / Pacific plate
At first, when I noticed the location, I hypothesized that this may be a strike-slip earthquake. womp womp. The earthquake mechanism from the USGS shows that this M = 7.4 earthquake was a normal fault earthquake (extension).
This earthquake happened in an interesting region of the world where there is a junction between two plate boundaries, the Kamchatka subduction zone with the Aleutian subduction zone / Bering-Kresla Shear Zone. The Kamchatka Trench (KT) is formed by the subduction (a convergent plate boundary) beneath the Okhotsk plate (part of North America). The Aleutian Trench (AT) and Bering-Kresla Shear Zone (BKSZ) are formed by the oblique subduction of the Pacific plate beneath the Pacific plate. There is a deflection in the Kamchatka subduction zone north of the BKSZ, where the subduction trench is offset to the west. Some papers suggest the subduction zone to the north is a fossil (inactive) plate boundary fault system. There are also several strike-slip faults subparallel to the BKSZ to the north of the BKSZ.
Today’s M = 7.4 earthquake shows northwest-southeast directed extension. This is consistent with slab tension in the direction of the Kurile subduction zone. It may also represent extension due to bending in the Pacific plate, but this seems less likely to me. Basically, the Pacific plate, as it subducts beneath the Okhotsk plate, the downgoing slab (the plate) exerts forces on the rest of the plate that pulls it down, into the subduction zone.
A second cool thing about this earthquake is that this may be evidence that the Kuril subduction zone extends north of the intersection of the BKSZ with Kamchatka. I discussed this in my earthquake report from 2017 here.
There are a couple analogy earthquakes, but one is the best. There were several strike-slip earthquakes nearby in 1982, 1987, and 1999. However, there was a M = 6.2 earthquake in almost the same location as the M = 7.4 from today. This M = 6.2 earthquake was slightly deeper (33 km) relative to the M = 7.4 (9.6 km).Check out my update here
Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
Magnetic Anomalies
Age of Oceanic Lithosphere
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
Kamchatka Depression (rift-like tectonic structure, which accommodates the northern end of EVB); SR—Sredinny Range. Distribution of Quaternary volcanic rocks in EVB and SR is shown in orange and green, respectively. Small dots are active vol canoes. Large circles denote CKD volcanoes: T—Tolbachik; K l — K l y u c h e v s k o y ; Z—Zarechny; Kh—Kharchinsky; Sh—Shiveluch; Shs—Shisheisky Complex; N—Nachikinsky. Location of profiles shown in Figures 2 and 3 is indicated. B: Three dimensional visualization of the Kamchatka subduction zone from the north. Surface relief is shown as semi-transparent layer. Labeled dashed lines and color (blue to red) gradation of subducting plate denote depths to the plate from the earth surface (in km). Bold arrow shows direction of Pacific Plate movement.
Geologic Fundamentals
Compressional:
Extensional:
Alaska | Kamchatka | Kurile
General Overview
Earthquake Reports
Social Media
Quake details: https://t.co/sCHEMhsY7g
Tsunami info: https://t.co/kIFgUWkdzj pic.twitter.com/15ixlcPRld
References:
Return to the Earthquake Reports page.
Earthquake Report: Alaska
I went to the interview for a position working on tsunami geology. During the interview, everyone started getting phone calls and emails, there was an earthquake in Alaska. The main interviewer had to leave the interview to take a few calls. Pretty funny, before they left, they asked me what would I do. Perfect timing.
We all broke out our phones and started reviewing the early reports and hypothesizing. I thought this may be related to the earthquake in 2016, though that was much deeper.
Much has been written about this earthquake and I include tweets to summaries below in the social media section.
Today’s earthquake occurred along the convergent plate boundary in southern Alaska. This subduction zone fault is famous for the 1964 March 27 M = 9.2 megathrust earthquake. I describe this earthquake in more detail here.
During the 1964 earthquake, the downgoing Pacific plate slipped past the North America plate, including slip on “splay faults” (like the Patton fault, no relation, heheh). There was deformation along the seafloor that caused a transoceanic tsunami.
The Pacific plate has pre-existing zones of weakness related to fracture zones and spreading ridges where the plate formed and are offset. There was an earthquake in January 2016 that may have reactivated one of these fracture zones. This earthquake (M = 7.1) was very deep (~130 km), but still caused widespread damage.
There was also an earthquake associated with the faults in the Pacific plate, which is still having asftershocks, earlier this year. Here is my earthquake report for the 2018.01.24 M 7.9 earthquake. I prepared two update reports here and here.
Today’s earthquake was not on the megathrust fault interface and is extensional. I always have fun chatting with people new to subduction zones when we get to see an extensional earthquake at a convergent plate boundary. Because the earthquake was a normal earthquake (extensional) and it was rather deep, the possibility of a tsunami was quite small. However, there was a possibility that landslides could have triggered tsunami. However, these would be localized near the epicentral region.
The earthquake appears to have a depth of ~40 km and the USGS model for the megathrust fault (slab 2.0) shows the megathrust to be shallower than this earthquake. There are generally 2 ways that may explain the extensional earthquake: slab tension (the downgoing plate is pulling down on the slab, causing extension) or “bending moment” extension (as the plate bends downward, the top of the plate stretches out.UPDATE – 1 year later
Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
Magnetic Anomalies
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
Youtube Source IRIS
mp4 file for downloading.
Credits:
Earthquake Triggered Landslides
UPDATE – 1 year later 2019.11.30
Head over to the University of Alaska Fairbanks, Alaska Earthquake Center, to see a one year review of this earthquake sequence (which is still having aftershocks).
The USGS Alaska Science Center also has an excellent review of this earthquake sequence here.
Some of the material in this update came from the days immediately following the earthquake, but did not get into the Earthquake Report.
There was an Earthquake Symposium about this earthquake sequence earlier this year. Head over there to see the presentations from that symposium.
Geologic Fundamentals
Compressional:
Extensional:
Alaska | Kamchatka | Kurile
General Overview
Earthquake Reports
Social Media
-What the near future might hold
-What you shouldn't believehttps://t.co/6059QWfzfF
UPDATE 2019.11.30
lots of science from @AKearthquake @USGSBigQuakes @uafairbanks
References:
Earthquake Report: Alaska Update
I do not include much background material on the tectonics of this region in this report. However, there is substantial material in my original earthquake report here. I will include some of the material in the report today, but head on over to that original report for more information.
Gurney hypothesized that the sequence was a step over and did not have evidence for conjugate faults. I partially agree with this hypothesis. Their tweet is here:
I agree that there is evidence for a step over. Some of this evidence is laid out here:
I prepared an animation that shows (1) the earthquakes through time and (2) an interpretation of these earthquakes. I present this interpretive poster below.
Here is the animation (download it here a 10 MB mp4 file).
Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange).
I placed white dashed lines where there are linear trends in seismicity, or where larger earthquakes appear to be aligned (and supported by the fault mechanisms). Below I include the same poster without these hypothesized fault lines.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Here are some of the interpretive posters from my earlier report here.
Sumatra Analogue
Alaska | Kamchatka | Kurile Earthquake Reports
General Overview
Earthquake Reports
Arctic
General Overview
Earthquake Reports
Social Media
References:
Earthquake Report: Andreanof Islands, Aleutians
The Andreanof Islands is one of the most active parts of the Aleutian Arc. There have been many historic earthquakes here, some of which have been tsunamigenic (in fact, the email that notified me of this earthquake was from the ITIC Tsunami Bulletin Board).
Possibly the most significant earthquake was the 1957 Andreanof Islands M 8.6 Great (M ≥ 8.0) earthquake, though the 1986 M 8.0 Great earthquake is also quite significant. As was the 1996 M 7.9 and 2003 M 7.8 earthquakes. Lest we forget smaller earthquakes, like the 2007 M 7.2. So many earthquakes, so little time.
I include some earthquakes along this plate boundary system that are also interesting as they reveal how the plate boundary changes along strike, and how the margins of the plate boundary (e.g. the western and eastern termini) behave.
The M 6.6 earthquake is the result of north-northwest compression from the subduction of the Pacific plate underneath the North America plate to the north.
The majority of the Aleutian Islands are volcanic arc islands formed as a result of the subduction of the Pacific plate beneath the North America plate. As the oceanic crust subducts, the water in the rock tends is released into the overlying mantle, leading to magma formation. This magma is less dense and rises to form volcanoes that comprise this magmatic arc.
This and other earthquakes have occurred in the region of the subduction zone west of where the Adak fracture zone is aligned. Further to the east is the Amlia fracture zone. The Amlia fracture zone is a left lateral strike slip oriented fracture zone, which displaces crust of unequal age, beneath the megathrust. The difference in age results in a variety of factors that may contribute to differences in fault stress across the fracture zone (buoyancy, thermal properties, etc). For example, older crust is colder and denser, so it sinks lower into the mantle and exerts a different tectonic force upon the overriding plate.
To the west, there is another subduction zone along the Kuril and Kamchatka volcanic arcs. These subduction zones form deep sea trenches (the deepest parts of the ocean are in subduction zone trenches). Between these 2 subduction zones is another linear trough, but this does not denote the location of a subduction zone. The plate boundary between the Kamchatka and Aleutian trenches is the Bering Kresla shear zone (BKSZ). Below I present some earthquake reports that help explain the western terminus of the Aleutian subduction zone.
This earthquake sequence is unrelated to the earthquakes in northern Alaska earlier this week. Here is my report for that sequence.
There was also a sequence (that is still experiencing aftershocks) in the Gulf of Alaska. Here is my main report (there were updates) for this Gulf of Alaska earthquake.Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), in addition to some relevant historic earthquakes.
Mechanisms for historic earthquakes that come from publications other than the USGS fault plane solutions include the 1957 M 8.7 (Brown et al., 2013), the 1965 M 8.7 (Stauyder, 1968), and the 1965 M 7.6 earthquakes (Abe, 1972).
Magnetic Anomalies
I include some inset figures.
Other Report Pages
Some Background about the North America – Pacific plate boundary
Youtube Source IRIS
mp4 file for downloading.
Credits:
Some Relevant Discussion and Figures
Geologic Fundamentals
Compressional:
Extensional:
Alaska | Kamchatka | Kurile Earthquake Reports
General Overview
Earthquake Reports
Social Media
Not felt, but easily recorded by our seismograph on Gonzales Hill.
Details on this Alaska earthquake: https://t.co/FxCCohDiK3 pic.twitter.com/JbjmfYkOjU
References:
°
≥Earthquake Report: northern Alaska
My inbox has had a lower frequency of USGS ENS notifications since Kilauea has settled down somewhat. However, today, the aftershocks just keep rolling in. Those who are on the north slope are getting rattled for sure. I have had to reproduce my seismicity maps several times as the epicenters keep getting updated (thanks USGS). The two largest earthquakes are now actually aligned with the west northwest strike of the earthquake.
There are no active faults mapped in the region of today’s earthquakes. There is a series of thrust faults that form the mountains in this area (e.g. the Sadlerochit Mountains). To the north is a Quaternary active fold (the Marsh Creek anticline), however, this structure is too far away to be related to today’s activity.
The interesting thing is that today’s series of earthquakes are strike-slip earthquakes. It is possible that one of these thrust faults has been reactivated as a strike-slip fault (but they are probably dipping too shallow to do this). So, i suspect that these earthquakes are either on an un-mapped active fault or are distributed throughout the region on a variety of different faults (seems more likely, but I would defer to those who are studying the tectonics on the North Slope to be more informed about this). These earthquakes remind me of the 2002 dextral (right-lateral) strike-slip Denali fault earthquake. More on the Denali Earthquake can be found here too.
I include a second poster below that has more details about the regional geology. On this map I include faults and folds from the Alaska Quaternary Active Faults and Folds database (Keohler et al., 2013).
Based upon the seismicity, I interpret these earthquakes (at least the ones with mechanisms) as east striking right-lateral strike-slip earthquakes. The historic earthquakes are not as easy to interpret, so I include both nodal plane solutions as being possible. However, if they are related in some way to today’s seismicity, they are probably also right-lateral strike-slip earthquakes.Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
Geologic Fundamentals
Compressional:
Extensional:
Alaska | Kamchatka | Kurile Earthquake Reports
General Overview
Earthquake Reports
Arctic
General Overview
Earthquake Reports
Social Media
CNSN seismic data:https://t.co/9tr7colMOx pic.twitter.com/6j3VToVNSA
No reports (yet) of this earthquake being felt in Canada – but well-recorded – see shaking at Inuvik, NT (480 km distant):https://t.co/5siks24kpL pic.twitter.com/S4bczTVIlm
More info:https://t.co/qtsreelS0Ohttps://t.co/J7zVk7ZSBj pic.twitter.com/BmM3SGvu5f
References:
Earthquake Report: Gulf of Alaska UPDATE #2
I am not sure it worked as some issues cannot be dealt with simply with this visualization.
For example, the locations for these earthquakes may not be resolute enough [yet] to figure out the orientation of the faults at work here. The back projection data are perhaps the strongest evidence for an east-west fault. However, we still have the contradictory sense of motion along the fracture zones at the meso scale… (as revealed in the EMAG2 magnetic anomaly data).
As a reminder, if the M 7.9 earthquake fault is E-W oriented, it would be left-lateral. The offset magnetic anomalies show right-lateral offset across these fracture zones. This was perhaps the main reason why I thought that the main fault was not E-W, but N-S. After a day’s worth of aftershocks, the seismicity may reveal some north-south trends. But, as a drama student in 7th grade (1977), my drama teacher (Ms. Naichbor, rest in peace) asked our class to go stand up on stage. We all stood in a line and she mentioned that this is social behavior, that people tend to stand in lines (and to avoid doing this while on stage). Later, when in college, professors often commented about how people tend to seek linear trends in data (lines). I actually see 3-4 N-S trends and ~2 E-W trends in the seismicity data.
So, that being said, here is the animation I put together. I used the USGS query tool to get earthquakes from 1/22 until now, M ≥ 1.5. I include a couple inset maps presented in my interpretive posters. The music is copyright free. The animations run through twice.
Here is a screenshot of the 14 MB video embedded below. I encourage you to view it in full screen mode (or download it).
Social Media
Alaska | Kamchatka | Kurile
General Overview
Earthquake Reports
References: