I don’t always have the time to write a proper Earthquake Report. However, I prepare interpretive posters for these events. https://earthquake.usgs.gov/earthquakes/eventpage/us6000f53e/executive
Because of this, I present Earthquake Report Lite. (but it is more than just water, like the adult beverage that claims otherwise). I will try to describe the figures included in the poster, but sometimes I will simply post the poster here.
Below is my interpretive poster for this earthquake
I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 with magnitudes M ≥ 3.0 in one version.
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.
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.
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
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.
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.
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@USGSpic.twitter.com/BmtXhhUvWF
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
General Overview of how to interact with these summaries
Click on the earthquake “magnitude and location” label (e.g. “M 6.9 Fiji”) to go to the Earthquake Report website for any given earthquake. Click on the map to open a high resolution pdf version of the interpretive poster. More information about the poster is found on the Earthquake Report website.
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 ≥ 7.5 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
Background on the Earthquake Report posters
I placed a moment tensor / focal mechanism legend on the posters. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the maps. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>
Magnetic Anomalies
In the maps below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
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.
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.
Plotted with a century’s earthquakes with magnitudes M ≥ 6.5
Plotted with a century’s earthquakes with magnitudes M ≥ 3.5
There were two observations of a small amplitude (small wave height) tsunami recorded on tide gages in the region. Below are those observations.
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).
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.
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).
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.
The USGS updated their MMI contours to reflect their fault model. Below is my updated poster. I also added green dashed lines for the fracture zones related to today’s M 7.9 earthquake (on the magnetic anomaly inset map).
These are the observations as reported by the NTWC this morning (at 4:15 AM my local time).
Large Scale Interpretive Map (from update report)
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).
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.
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.
Here is the same poster but with the magnetic anomalies included (transparent).
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.
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 same map without historic seismicity.
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.
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 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).
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.
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.
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).
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.
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).
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.
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.
Hilo, Hawaii
Kawaihae, Hawaii
Temblor Reports:
Click on the graphic to see a pdf version of the article.
Click on the html link (date) to visit the Temblor site.
2018.05.05Pele, the Hawai’i Goddess of Fire, Lightning, Wind, and Volcanoes
2018.05.06Pele, la Diosa Hawaiana del Fuego, los Relámpagos, el Viento y los Volcanes de Hawái
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.
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).
Here is the map with a month’s seismicity plotted.
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.
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.
Here is the map with a month’s seismicity plotted.
Here is the map with a centuries seismicity plotted for earthquakes M ≥ 6.6.
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 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.
Here is the map with a centuries seismicity plotted with M ≥ 7.5.
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.
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.
Here is the map with a month’s seismicity plotted.
Here is an updated local scale (large scale) map showing the earthquake fault mechanisms for the current sequence. I label them with yellow numbers according to the sequence timing. I outlined the general areas that have had earthquakes into two zones (phases). Phase I includes the earthquakes up until today and Phase II includes the earthquakes from today. There is some overlap, but only for a few earthquakes. In general, it appears that the earthquakes have slipped in two areas of the Flores fault (or maybe two shallower thrust faults).
Here is the interpretive posted from the M 6.4 7/28 earthquake, with historic seismicity and earthquake mechanisms.
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).
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.
Here is the map with a month’s seismicity plotted, along with USGS earthquakes M ≥ 6.0.
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.
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).
Here is the map with a century’s seismicity plotted, along with USGS earthquakes M ≥ 7.0.
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].
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.
Here is the map with a centuries seismicity plotted.
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2018.09.06Violent shaking triggers massive landslides in Sapporo Japan earthquake
Today, there was a large earthquake associated with the subduction zone that forms the Kermadec Trench.
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.
Here is the map with a month’s seismicity plotted.
Here is the map with a centuries seismicity plotted.
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.
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.
There have been tsunami waves recorded on a tide gage over 300 km to the south of the epicenter, at a site called Mumuju. Below is a map and a plot of water surface elevations from this source.
Here is the map with a month’s seismicity plotted.
Here is the map with a centuries worth of seismicity plotted.
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.
My 2018.10.01 BC Newshour Interview
InSAR Analysis
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.
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.
I prepared a map using the NASA-JPL InSAR data. They post all their data online here. I used the tiff image as it is georeferenced. However, some may prefer to use the kmz file in Google Earth.
I include the faults mapped by Wilkinson and Hall (2017), the PGA contours from the USGS model results. More on Peak Ground Acceleration (PGA) can be found here. I also include the spatial extent of the largest landslides that I mapped using post-earthquake satellite imagery provided by Digital Globe using their open source imagery program.
M 7.5 Landslide Model vs. Observation Comparison
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.
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.
Here is the landslide probability map (Jessee et al., 2018). Below the poster I include the text from the USGS website that describes how this model is prepared.
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.
Here is the liquefaction probability (susceptibility) map (Zhu et al., 2017). Note that the regions of low slopes in the valleys and coastal plains are the areas with a high chance of experiencing liquefaction. Areas of slopes >5° are excluded from this analysis.
Note that the large landslides (yellow polygons) are not in regions of high probability for liquefaction.
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.
Temblor Reports:
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2018.09.28The Palu-Koro fault ruptures in a M=7.5 quake in Sulawesi, Indonesia, triggering a tsunami and likely more shocks
2018.10.03Tsunami in Sulawesi, Indonesia, triggered by earthquake, landslide, or both
2018.10.16Coseismic Landslides in Sulawesi, Indonesia
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.
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.
Here is the map with a century’s seismicity plotted.
Temblor Reports:
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2018.10.10M 7.5 Earthquake in New Britain, Papua New Guinea
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.
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.
Here is the map with a century’s seismicity plotted.
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.
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.
Here is the map with a century’s seismicity plotted.
Here is the tide gage data from Katakolo, which is only 65 km from the M 6.8 epicenter.
Temblor Reports:
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2018.10.26Greek earthquake in a region of high seismic hazard
There was a M = 6.8 earthquake along a transform fault connecting segments of the Mid Atlantic Ridge recently.
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.
Here is the map with a century’s seismicity plotted.
Here is the large scale map showing earthquake mechanisms for historic earthquakes in the region. Note how they mostly behave well (are almost perfectly aligned with the Jan Mayen fracture zone). There are a few exceptions, including an extensional earthquake possibly associated with extension on the MAR (2010.06.03 M = 5.6). Also, 2 earthquakes (2003.06.19 and 2005.07.25) are show oblique slip (not pure strike-slip as they have an amount of compressional motion) near the intersection of the fracture zone and the MAR.
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.
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.
There was a sequence of earthquakes along the subduction zone near New Caledonia and the Loyalty Islands.
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.
Here is the map with a month’s seismicity plotted.
Here is the map with a century’s seismicity plotted.
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 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.
Here is the map with a month’s seismicity plotted, including the age of the crust.
Here is the map with a century’s seismicity plotted, with earthquakes M ≥ 6.0, including the age of the crust.
UPDATE #1
Here is the map with a month’s seismicity plotted.
Here is the map with a century’s seismicity plotted, with earthquakes M ≥ 6.0.
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 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:
Here is the map with a century’s seismicity plotted.
Geologic Fundamentals
For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).
Here is another way to look at these beach balls.
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
There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
Strike Slip:
Compressional:
Extensional:
This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.
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.)
Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.
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.
The Mid Atlantic Ridge is an extensional plate boundary called an oceanic spreading ridge. Oceanic crust is formed along these types of plate boundaries.
Transform faults are faults that move side-by-side (i.e. strike-slip faults) that offset spreading ridges. Learn more about different types of faults in the geologic fundamentals section below.
The Atlantic Ocean is known for the spreading center, Mid Atlantic Ridge (MAR), which was probably born in the mid Cretaceous Period, about 130 million years ago. We use the age of the oceanic crust at the eastern and western margins of the Atlantic Ocean as a basis for this interpretation.
The Mid Atlantic Ridge also splits apart the island of Iceland, which also overlies a volcanic hot spot. I have always wanted to visit Iceland to see the rocks get older as I might travel east or west from the middle of Iceland.
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.
Below is my interpretive poster for this earthquake
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 ≥ 4.5 in one version.
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 also include the IPGP focal mechanism as that was available before the USGS moment tensor was available (I included it in my initial poster).
I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>
Magnetic Anomalies
In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Mid Atlantic Ridge).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
We can see the roughly north-south trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed.
Age of Oceanic Lithosphere
In the map below, I include a transparent overlay of the age of the oceanic crust data from Agegrid V 3 (Müller et al., 2008).
Because oceanic crust is formed at oceanic spreading ridges, the age of oceanic crust is youngest at these spreading ridges. The youngest crust is red and older crust is yellow (see legend at the top of this poster).
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
In the upper right corner is a plate tectonic map from Le Breton (et al., 2012). This map shows the configuration of the Mid Atlantic Ridge (MAR) and shows their interpretation about how this spreading center is divided into segments separated by transform faults. I placed a red star in the general location of the M = 6.8 earthquake.
In the upper left corner is a map showing the isochrons (lines of each age for the crust) as summarized by Gaina et al. (2017). Isochrons are displayed with color relative to age.
In the lower right corner is a larger scale map zoomed into the Jan Mayen fracture zone at the MAR. I placed existing USGS fault mechanisms (blue = moment tensor, orange = focal mechanism) for earthquakes with magnitudes M ≥ 5.5.
In the lower left corner is a map from the Temblor.net app. This map shows the seismic hazard from the GEAR model (Bird et al., 2012). Seismicity is shown as colored circles. The red dot is the M = 6.8 epicenter, which lies in a region that is forecast to have an earthquake of magnitude M = 6.25-6.5 in someone’s lifetime.
Here is the map with a month’s seismicity plotted.
Here is the map with a century’s seismicity plotted.
Here is the large scale map showing earthquake mechanisms for historic earthquakes in the region. Note how they mostly behave well (are almost perfectly aligned with the Jan Mayen fracture zone). There are a few exceptions, including an extensional earthquake possibly associated with extension on the MAR (2010.06.03 M = 5.6). Also, 2 earthquakes (2003.06.19 and 2005.07.25) are show oblique slip (not pure strike-slip as they have an amount of compressional motion) near the intersection of the fracture zone and the MAR.
Other Report Pages
Some Relevant Discussion and Figures
Here is a map that shows the ace of the oceanic lithosphere for the entire Earth.
Here is the tectonic map from Le Breton et al. (2012). Depth to the seafloor is shown in color. Note the spreading rates in red. Note how the MAR is offset by the Jan Mayen fracture zone, as well as the smaller unnamed fracture zones.
Principal tectonic features of the NE Atlantic Ocean on a bathymetric and topographic map (ETOPO1). Compressional structures (folds and reverse faults) on the NE Atlantic Continental Margin are from Doré et al. [2008], Johnson et al. [2005], Hamann et al. [2005], Price et al. [1997] and Tuitt et al. [2010]. Present-day spreading rates along Reykjavik, Kolbeinsey and Mohns Ridges are from Mosar et al. [2002]. Continent-ocean boundaries of Europe and Greenland are from Gaina et al. [2009] and Olesen et al. [2007]. Black thick lines indicate seismic profiles of Figure 3. Abbreviations (north to south): GFZ, Greenland Fracture Zone; SFZ, Senja Fracture Zone; JMFZ, Jan Mayen Fracture Zone (west and east); JMMC, Jan Mayen Microcontinent; HHA, Helland Hansen Arch; OL, Ormen Lange Dome; FR, Fugløy Ridge; GIR, Greenland-Iceland Ridge; IFR: Iceland-Faeroe Ridge; MGR, Munkagrunnar Ridge; WTR, Wyville Thomson Ridge; YR, Ymir Ridge; NHBFC, North Hatton Bank Fold Complex; MHBFC, Mid-Hatton Bank Fold Complex; CGFZ, Charlie Gibbs Fracture Zone. Map
projection is Universal Transverse Mercator (UTM, WGS 1984, zone 27N).
This is a fantastic figure that shows the isochrons on either side of the MAR in this region (Le Breton et al., 2012). Isochrons are lines of equal age, based on magnetic anomaly mapping and numerical ages from rock samples collected from the oceanic crust.The geomagnetic time scale is shown at the right. “Chrons” are numbered with their numerical ages in millions of years (Ma). These chron numbers are also on the map, showing the chron number for each isochron. For some reason I want to watch the film Tron right now.
Map of magnetic anomalies, NE Atlantic Ocean. Background image is recent model EMAG2 of crustal magnetic anomalies [Maus et al., 2009]. Ages of magnetic anomalies are from Cande and Kent [1995]. Map projection is Universal Transverse Mercator (UTM, WGS 1984, zone 27N).
This map shows their reconstruction of the fracture zones, MAR, and the Iceland Hot Spot for the Tertiary to present (Le Breton et al., 2012).
Positions relative to stationary Greenland plate of Europe, Jan Mayen Microcontinent (JMMC) and Iceland Mantle Plume at intervals of 10 Myr, according to stationary hot spot model of Lawver and Müller [1994] and moving hot spot model of Mihalffy et al. [2008]. Timing is (a) late Paleocene, 55.9 Ma; (b) late Eocene, 36.6 Ma ; (c) early Miocene, 19.6 Ma; and (d) present. (more info is in the original figure caption)
Here is the Gaina et al. (2017 a) isochron map for this region of the north Atlantic Ocean. Below are also some great summary figures that show a series of geophysical data from their work in the region (Gaina et al., 2017 b).
Magnetic anomaly and fracture zone identifications and interpreted isochrons.
On the left is a free air gravity map (Gaina et al., 2017 b). This is a gravity map after the “free-air” correction has been made (that corrects for the elevation that the gravity data were observed).
On the right is the isostatic gravity anomaly map. This is a gravity map that shows the results of correcting the gravity data for the variable density of materials in the earth’s crust and mantle.
(a) Free-air gravity (DTU10: Andersen 2010); (b) isostatic gravity anomaly (this was computed using the Airy–Heiskanen model, where the compensation is accomplished by variations in thickness of the constant density layers: the root is calculated using the ETOPO1 topography and bathymetry: Haase et al., this volume, in press);
On the left is the magnetic anomaly map (Gaina et al., 2017 b)
On the right is the sediment thickness map.
magnetic anomaly (Nasuti & Olesen 2014; Gaina et al., this volume, in review); and (d) sediment thickness (Funck et al. 2014). Distribution of volcanic edifices as in Figure 1. Dark grey lines indicate the active and extinct plate boundaries
This is a really cool map that shows how the MAR extends further into the Arctic Ocean. Color represents depth to the seafloor (Mjelde et al., 2008).
Location map of the North Atlantic and Arctic. ETOPO-2 shaded relief bathymetry and topography are based on data from Sandwell & Smith (1997). (more detail is found in the original figure caption)
Geologic Fundamentals
For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).
Here is another way to look at these beach balls.
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
There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
Strike Slip:
Compressional:
Extensional:
This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.
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.)
Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.
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.
Using a low pass filter highlights the Surface waves sweeping across the UK. You can clearly see that the lowest frequency components (widest spacing between wiggles) arrive before higher frequency (closer spaced wiggles) components. Scientists call this effect dispersion. pic.twitter.com/eRcZ37TtsQ
Bird, P., Jackson, D. D., Kagan, Y. Y., Kreemer, C., and Stein, R. S., 2015. GEAR1: A global earthquake activity rate model constructed from geodetic strain rates and smoothed seismicity, Bull. Seismol. Soc. Am., v. 105, no. 5, p. 2538–2554, DOI: 10.1785/0120150058
Gaina, C., Nasuti, A., Kimbell, G.S., and Blishchke, A., 2017 a. Break-up and seafloor spreading domains in the NE Atlantic in Peron-Pinvidic, G., Hopper, J. R., Stoker, M. S., Gaina, C., Doornenbal, J. C., Funck, T. & Arting, U. E. (eds) 2017. The NE Atlantic Region: A Reappraisal of Crustal Structure, Tectonostratigraphy and Magmatic Evolution. Geological Society, London, Special Publications, 447, 393–417. https://doi.org/10.1144/SP447.12
Gaina, C., Blischke, A., Geissler, W.H., Kimbell, G.S., and Erlendsson, O., 2017 b. Seamounts and oceanic igneous features in the NE Atlantic: a link between plate motions and mantle dynamics in the NE Atlantic in Peron-Pinvidic, G., Hopper, J. R., Stoker, M. S., Gaina, C., Doornenbal, J. C., Funck, T. & Arting, U. E. (eds) 2017. The NE Atlantic Region: A Reappraisal of Crustal Structure, Tectonostratigraphy and Magmatic Evolution. Geological Society, London, Special Publications, 447, 393–417. https://doi.org/10.1144/SP447.12
Le Breton, E., P. R. Cobbold, O. Dauteuil, and G. Lewis (2012), Variations in amount and direction of seafloor spreading along the northeast Atlantic Ocean and resulting deformation of the continental margin of northwest Europe, Tectonics, 31, TC5006, doi:10.1029/2011TC003087.
If there was an earthquake in the middle of the ocean, and nobody felt it… would it still be an earthquake? Maybe we need to get the Karl Popper out for a good read…
There was a good sized shaker less than an hour ago (at the time I started writing this, when it was first estimated to be M 6.8). Here is the USGS website for the M 6.6 earthquake that appears to be associated with the Chan fracture zone.
This may be related to a recent earthquake on the Romanche fracture zone, but it is probably too far away. Here is my report for this M 7.1 earthquake from 2016.08.29.
Below is my interpretive poster for this earthquake
I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS epicenters from 1900-2016 for magnitudes M ≥ 6.0.
I also include the USGS moment tensor for today’s earthquake.
I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. Based upon the series of earthquakes and the mapped faults, I interpret this M 5.1 earthquake as a left-lateral strike-slip earthquake related to slip associated with the Gorda plate.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include some inset figures in the poster.
In the upper left corner is a map that shows the age of the oceanic crust in Million Years B.P. (before present). The MAR is highlighted because it is of about zero age, so is shown as red (Müller et al., 2008). I place a blue star in the general location of this M 6.6 earthquake (as for the other inset maps).
In the lower left corner is a map that shows a revised interpretation of the timing and orientations of the initial breakup and formation of the MAR and the Atlantic Ocean from Torsvik et al. (2009)
t = 0 Earth’s magnetic field is the same as it is today. We term this “normal” polarization. Rocks (and sediments) formed at this time can have magnetic minerals and grains align with the magnetic field orientation at that time.
Colored oceanic crust (in this diagram) represents crust formed when the magnetic field is oriented like it is today.
t = 1 Tens of thousands to hundreds of thousands years later, as the plates diverge from the spreading ridge, decompression melting leads to accretion of new oceanic crust at the ridge. At this time, the magnetic field has reversed (north pole becomes south pole, dogs and cats living together (from movie “Ghostbusters”)), so the oceanic crust rock is magnetized with a field oriented opposite from the way it is today. We term this crust to have “reverse” polarization. Then, the magnetic field flips orientation again, becoming normal again. The diagram shows at this time, the crust forming at the ridge is normally polarized.
t = 2 Tens of thousands to hundreds of thousands years later, the plates continue to diverge from the spreading ridge forming new crust, and there are additional cycles of flipping magnetic fields. The result is a pattern, symmetrical about the spreading ridge, of oceanic crust that has alternating bands of crust with different polarities.
In the upper center is a map that shows the two fracture zones in this region (Romanche and Chain) along with focal mechanisms for earthquakes along these fracture zones. The fracture zones, which offset the Mid Atlantic Ridge (MAR), are transform plate boundaries. Considering the orientation of these fracture zones and the MAR, these earthquakes are right-lateral strike-slip earthquakes. Today’s M 6.6 earthquake is no different. Note how this earthquake is nearly perfect strike slip.
In the upper right corner is a larger scale map that shows the details of the sea floor in this region.The majority of the seismicity occurs along the fracture zones, which offset the MAR. Today’s earthquake is most likely associated with strike-slip faulting along the Chan fracture zone.
Here is the Bonatti et al. (2001) figure from the interpretive poster above. I include the figure caption as a blockquote below.
A: Multibeam topography of Romanche region, showing north-south profiles where sampling was carried out. Black dots and red numbers indicate estimated age (in million years) of lithosphere south of Romanche Transform, assuming spreading half-rate of 17 mm/yr within present-day ridge and transform geometry. White dots indicate epicenters of teleseismically recorded 1970–1995 events (magnitude . 4). FZ is fracture zone. B: Topography and petrology at eastern intersection of Romanche Fracture Zone with Mid-Atlantic Ridge. Data were obtained during expeditions S-16, S-19, and G-96 (Bonatti et al., 1994, 1996). C: Location of A along Mid-Atlantic Ridge.
Here is the Müller et al. (2008) figure from the interpretive poster above.
Here is the Torsvik et al. (2009) figure from the interpretive poster above. I include the figure caption as a blockquote below.
General structural map of the South Atlantic Ocean draped on topographic/bathymetric map from GTOPO 30. Boundaries between the four segments (Equatorial, Central, South and Falkland) are shown by dotted lines (RFZ, Romanche Fracture Zone; FFZ, Florianopolis Fracture Zone; AFFZ, Agulhas– Falkland Fracture Zone). Aptian salt basins (orange), LIPs (P, Parana; E, Etendeka; Karroo, Sierra Leone Rise and Agulhas), Seaward Dipping Reflectors (SDRs, white) and active hotspots (F, Fernando; C, Cameroon; Tr, Trinidade; Sh, St Helena; T, Tristan; V, Vema; B, Bouvet (Meteor) are also shown. Of these hotspots, only Tristan (responsible for the Parana-Etendeka LIP and Rio Grande Rise–Walvis Ridge) is classically considered as a deep plume in the literature (see Torsvik et al. 2006). However, Bouvet (Meteor) possibly responsible for Agulhas, and Maud Rise (East Antarctica) and Madagascar Ridge volcanism could possibly also have a deep plume origin (Section 6). PG, Ponta Grossa Dyke System; PA, Paraguay Dyke system; FI, Falkland Islands.
I created an animation that shows the seismicity for this region of the MAR from 1900-2016.08.29. The animation includes earthquakes of magnitudes M ≥ 6.0. Here is the kml that I created from the USGS website. Below is a map of all the earthquakes plotted, with the animation below.
Torsvik, T.H., Tousse, S., Labaila, C., and Smethurst, M.A., 2009. A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin in Geophysical Journal INternational, v. 177, p. 1315-1333.
Between my trip to southern Oregon for a talk on Cascadia subduction zone earthquake and tsunami hazards and preparing for a talk on the tectonic contribution to sea-level rise, there were two earthquakes. The first one, which I will write about later, was a M 4.5 in northern California. The second earthquake was a M 7.1 along the Romanche fracture zone (RFZ), a transform plate boundary that offsets the Mid Atlantic Ridge (MAR). Here is the USGS webpage for this M 7.1 earthquake.
There have been a number of M 7 earthquakes in this region in the past century and I have reported on some elsewhere along the MAR. Below are some Earthquake Reports for some of these earthquakes.
Below is my interpretive poster for this M 7.1 earthquake from yesterday morning (my time).
Below is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. Based upon the location of this earthquake along the RFZ, I interpret this to have been a right lateral strike-slip earthquake.
I have included some inset figures.
In the upper right corner, I include a map that shows the detailed bathymetry (the shape of the sea floor) for this region from Bonatti et al. (2001). In the lower panel (A) one may observe how the RFZ offsets the MAR. Ages of the oceanic lithosphere is plotted in red letters with 0 Ma (mega annum) at the MAR on the right and 60 Ma just to the west of the MAR on the left. We can calculate the slip-rate on the RFZ by using these age data and the distance that the MAR has been offset. Note the scale in the lower right corner.
To the left of the Bonatti et al. (2001) map is a map that shows the age of the oceanic crust in Million Years B.P. (before present). The MAR is highlighted because it is of about zero age, so is shown as red (Müller et al., 2008).
In the lower left corner is a map that shows a revised interpretation of the timing and orientations of the initial breakup and formation of the MAR and the Atlantic Ocean from Torsvik et al. (2009)
Here is the Bonatti et al. (2001) figure from the interpretive poster above. I include the figure caption as a blockquote below.
A: Multibeam topography of Romanche region, showing north-south profiles where sampling was carried out. Black dots and red numbers indicate estimated age (in million years) of lithosphere south of Romanche Transform, assuming spreading half-rate of 17 mm/yr within present-day ridge and transform geometry. White dots indicate epicenters of teleseismically recorded 1970–1995 events (magnitude . 4). FZ is fracture zone. B: Topography and petrology at eastern intersection of Romanche Fracture Zone with Mid-Atlantic Ridge. Data were obtained during expeditions S-16, S-19, and G-96 (Bonatti et al., 1994, 1996). C: Location of A along Mid-Atlantic Ridge.
Here is the Müller et al. (2008) figure from the interpretive poster above.
Here is the Torsvik et al. (2009) figure from the interpretive poster above. I include the figure caption as a blockquote below.
General structural map of the South Atlantic Ocean draped on topographic/bathymetric map from GTOPO 30. Boundaries between the four segments (Equatorial, Central, South and Falkland) are shown by dotted lines (RFZ, Romanche Fracture Zone; FFZ, Florianopolis Fracture Zone; AFFZ, Agulhas– Falkland Fracture Zone). Aptian salt basins (orange), LIPs (P, Parana; E, Etendeka; Karroo, Sierra Leone Rise and Agulhas), Seaward Dipping Reflectors (SDRs, white) and active hotspots (F, Fernando; C, Cameroon; Tr, Trinidade; Sh, St Helena; T, Tristan; V, Vema; B, Bouvet (Meteor) are also shown. Of these hotspots, only Tristan (responsible for the Parana-Etendeka LIP and Rio Grande Rise–Walvis Ridge) is classically considered as a deep plume in the literature (see Torsvik et al. 2006). However, Bouvet (Meteor) possibly responsible for Agulhas, and Maud Rise (East Antarctica) and Madagascar Ridge volcanism could possibly also have a deep plume origin (Section 6). PG, Ponta Grossa Dyke System; PA, Paraguay Dyke system; FI, Falkland Islands.
I created an animation that shows the seismicity for this region of the MAR from 1900-2016.08.29. The animation includes earthquakes of magnitudes M ≥ 6.0. Here is the kml that I created from the USGS website. Below is a map of all the earthquakes plotted, with the animation below.
Here are a couple Earthquake Report interpretive posters for earthquakes from the past couple of years.
Here is a map that shows the 2015.06.17 Mw 7.0 earthquake, as well as the 2015.05.24 M = 6.3 earthquake (which has a compressional moment tensor). This occurred along an unnamed fracture zone.
Here is a map that shows the fracture zones and recent (2015) seismicity to the north of the M 6.3 earthquake.
Torsvik, T.H., Tousse, S., Labaila, C., and Smethurst, M.A., 2009. A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin in Geophysical Journal INternational, v. 177, p. 1315-1333.
There was a large thrust earthquake along the Scotia plate today. Here is the USGS website for this M 7.4 earthquake. This is a very interesting region of the world with a very cool history.
There is an east verging (dips to the west) subduction zone where the South America plate subducts beneath the Sandwich plate (mmmmm, sandwiches). The USGS convergence rate shown on Google Earth is 7.8 cm/yr (for reference, this is about twice that of the Cascadia subduction zone). There is a volcanic arc associated with this subduction zone (the Islands west of the Scotia subduction zone). To the east, there is a spreading ridge that separates the Sandwich and Scotia plates. On the northern and southern boundaries of the Scotia plate are left-lateral strike-slip plate boundary transform faults. In the past several years, there have been earthquakes in most of these regions. I list some of these below.
Recent seismicity accounted for in earthjay.com Earthquake Reports
2016.05.28 M 7.2 Scotia subduction zone USGS site (happened while I was at sea, so I did not make a report for this EQ)
Here is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. This is an earthquake that occurred along a northwest striking thrust or reverse fault. The fault may be dipping in the northeast or southwest direction. My guess, based upon the orientation of the subduction zone, that this fault is dipping in the southwest direction. However, it could be antithetic to the megathrust (a back thrust), so we cannot be sure without more information or analyses.
I include some inset figures and maps.
In the lower left corner, there are two figures from Civile et al. (2012). The upper panel is a map of the region that shows different geographical names and locations labeled upon a basemap that displays water depth (bathymetry) as different shades of blue (darker blue = deeper). The lower panel uses a basemap with generalized elevation as gray scale, but with historic seismicity plotted as dots with colors representing depth.
To the right of the moment tensor legend is a plot of water surface elevation observations from a tide gage as reported by NOAA (Stu Weinstein).
In the lower right corner there is a map that shows the tectonics of this region (Nerlich et al., 2013). There are colored lines with red in the middle that turns into yellow and then green. These colors represent the age of the lithospere (red = youngest). The red regions are generally associated with spreading ridges, where oceanic lithosphere is created (I love making statements about creation in an earthquake report).
To the left of that is the fault plane solution from the USGS (see link to the USGS site above to find out more about this solution). The USGS uses seismic wave data and estimates of the fault location to iteratively estimate the magnitude and spatial distribution of slip along the fault. Color represents the magnitude and there are vectors that represent the direction (also magnitude) of the slip during this earthquake. This is just an estimate.
In the upper right corner is a figure from Nerlich et al. (2013) that shows their interpretation of the tectonic evolution of this region. I show this figure below and include their figure caption as a blockquote.
To the left of that is a map that shows an estimate of plate age as calculated/modeled by Nerlich et al. (2013). Red colors represent the youngest lithosphere and green colors represent older lithosphere.
Here are some of the figures included in my interpretive poster above.
Here is the physiographic map and the map that shows historic seismicity. Note how the earthquakes get deeper from east to west along the Scotia subduction zone (as the South America plate subducts westward beneath the Sandwich plate, the megathrust fault gets deeper. The Civile et al. (2012) paper is particularly relevant to the strike-slip faults on the southern boundary of the Scotia plate. I include their figure caption below as a blockquote.
(top panel) Physiographic map of the Scotia Arc, with the main geological provinces discussed in the text. Bathymetric contours from satellite-derived data (Smith and Sandwell, 1997). Box shows the present-day plate tectonic sketch for the Scotia Sea and surrounding regions. NSR, North Scotia Ridge; SSR, South Scotia Ridge; ESR, East Scotia Ridge; SFZ, Shackleton Fracture Zone; BD, Bruce Deep; SSP, South Shetland plate; sSP, South Sandwich plate. (bottom panel) Distribution of earthquakes in the Scotia Arc region. Epicenters and focal depths are obtained from EHB Bulletin (Engdahl et al., 1998). EHB shallow (red), intermediate (orange and yellow), and deep (blue), earthquakes are shown as circles. Smaller black circles represent earthquakes located by ISC (http://www.isc.ac.uk).
The tectonic setting for this region from Nerlich et al. (2013). I include their figure caption below as a blockquote.
View of Scotia Sea consisting of the Scotia and Sandwich plates, located in between the Antarctic Peninsula and South America (see also insert map, where the red dot indicates the center of the top view map). The region is framed by transform boundaries in the north (North Scotia Ridge (NSR)), south (South Scotia Ridge (SSR)), west (Shackelton Fracture Zone (SFZ)), and the South Sandwich subduction zone (SSSZ) in the east. Other features are Shag Rocks (SR), South Georgia (SG), and the South Orkney Microcontinent (SOM). Flowlines displaying motion paths of different continental fragments are shown in green. Active spreading (East Scotia Ridge (ESR)) exists in the East Scotia Sea. Extinct spreading ridges are found in theWest Scotia Sea (West Scotia Ridge (WSR)), Central Scotia Sea (Central Scotia Ridge (CSR) which remains controversial; see Subsection 2.2) and in the Protector (Pro), Dove (Dov), and Discovery Basins (Dis), respectively, which are bounded by presumably — as some discussion on their origin persists — South American continental fragments (Terror Rise (TR), Pirie Bank (PB), Discovery Bank (DB)). Extinct ridges are also found on the boundary between the Antarctic plate and former Phoenix plate as well as in the Powell Basin (Pow). Location of the dredge sample with Pacific mantle type signature (Pearce et al., 2001) [see discussion] is marked by a triangle.
Here is the kinematic tectonic reconstruction from Nerlich et al. (2013). The panel begins in the upper left at 50 million years ago. I include their figure caption below as a blockquote.
Tectonic reconstructions corresponding to 50 (a), 41 (b), 32 (c), 21 (d), 7 (e) and 0 Ma (f); labels as in Fig. 1; present-day bathymetry is shown (i.e. no age-masking). White/grey arrows indicate active/inactive spreading. 50 Ma ago, South America and Antarctica are connected by a coherent bridge of continental fragments. At 41 Ma, separation between South America and Antarctica and subduction behind Discovery Bank and South Georgia has started, as suggested by Barker (2001). Protector Basin has opened. 32 Ma ago, active spreading occurs along the West Scotia Ridge but has ceased in Dove Basin. At 21 Ma, further subduction behind Discovery Bank leads to opening of Discovery Basin. Moreover, subduction behind South Georgia is about to cause back-arc spreading in the Central Scotia Sea. Powell Basin is already fully open, placing the South Orkney Microcontinent to its present position relative to Antarctica. Seafloor spreading lasts along the West Scotia Ridge. 7 Ma ago spreading occurs along the West and East Scotia Ridges and ceases along the Central Scotia Ridge.
Nerlich et al. (2013) used asthenospheric flow estimates, their kinematic history model, and a dynamic topography deconvoloution method (using 4DPlates; Clark et al., 2012) to model the age of the oceanic lithosphere in this region. I include their figure caption below as a blockquote.
Synthetic age-grid based on the reconstruction model as shown in Fig. 2.
