Earthquake Report: Pacoima!

Early this morning, there was a M 3.5 earthquake that was widely felt across southern California. It is a good reminder for people to prepare for the potential of larger earthquakes in this region. The last earthquake of significant magnitude was the 2014.03.28 M = 5.1 La Habra earthquake. Here is my first earthquake report for that earthquake and here is an update with more interpretations. This earthquake happened a day after the 50th anniversary of the 1964 Good Friday Earthquake. Here is the USGS web page for today’s M = 3.5 earthquake.
Today’s earthquake did not rupture a previously mapped fault, but is close to the right-lateral strike-slip fault system to the north, the San Gabriel fault. This SGF has a slip rate of about 1-5 mm/yr. The USGS has a web page summarizing information about the SGF here. Today’s earthquake occurred near the 1971 Sylmar earthquake, which was a compressional earthquake that caused significant damage. The Sylmar quake led to the passage of the Alquist-Priolo Law, which until recently was not funded.
Here is a map that shows the epicenter as an orange dot (representing depth, which is really shallow). I plot the shaking intensity contours, using the Modified Mercalli Intensity Scale (MMI), with a legend in the upper left corner. The MMI is based upon people’s observations of ground shaking and damage. It is a qualitative scale, but is really useful because it is based on real observations that often have a much more dense coverage than that provided by seismometers. I also place the USGS focal mechanism for this earthquake on the map. Based upon the nearby mapped faults, I interpret this earthquake as a right-lateral strike-slip earthquake. There is a map of the regional faulting associated with the San Gabriel fault system (Cromwell, 1952). More on the SGF can be found here and here (Beyer et al., 2009).
I placed a moment tensor / focal mechanism legend in the upper right corner of the map. 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.


Here is that Cromwell (1952) map alone.


This map shows how widely felt this M 3.5 earthquake was. These results are based upon the USGS “Did You Feel It?” website. The colors match the MMI contours in my map above.


Here is the USGS shakemap for comparison. This map is based upon modeling of ground motions using Ground Motion Prediction Equations (GMPEs, or attenuation relations). GMPEs are empirical relations between earthquake magnitude, distance to the fault, type of earthquake, type of crust, type of earth that the seismic waves are propagating through, etc.


Here is a comparison between the modeled ground motions and the observed ground motions (the above two maps). The orange line represents the GMPE model output and the blue dots are the data from real person’s observations as reported to USGS. There is a moderate fit to the modeled data, albeit imperfect. This earthquake magnitude is small, so there are more earthquakes that provide the empirical relations for this size of an earthquake. The misfit for some of the blue data points may be attributed to site conditions (where the obervers were located). Perhaps the people who made those reports live in a sedimentary basin or on a ridge top, both of which can amplify the ground motions.


In April 2015, there was some activity along the Newport Inglewood fault, another right lateral strike slip fault system in the Los Angeles Basin. Here is an earthquake report for that series of earthquakes. Below is a map from that earthquake series.


Here is a map that shows several of the significant earthquakes that I have mentioned above. I originally posted this on this earthquake report from April 2015.

    References:

  • Beyer, L.A., McCulloh, T.H., Denison, R.E., Morin, R.W., Enrico, R.J., Barron, J.A., and Fleck, R.J., 2009, Post-Miocene right separation on the San Gabriel and Vasquez Creek Faults, with supporting chronostratigraphy, western San Gabriel Mountains, California: U.S. Geological Survey Professional Paper 1759, 44 p.
  • Crowell, J.C., 1952, Probable large lateral displacement on San Gabriel fault, southern California: Am. Assoc. Petroleum Geologists Bull., vol. 35, pp. 2026-2035.

Earthquake Report: Bismarck Sea!

Last night we had an earthquake in the Bismarck Sea, a region that is formed by an oceanic spreading ridge and bordered to the north and to the south by subduction zones. This M = 5.3 earthquake is small in magnitude compared to the earthquakes that occur along the New Britain trench to the south. Here is the USGS web page for this earthquake.
I placed a moment tensor / focal mechanism legend in the upper right corner of the map. 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.
Here is a map that shows the regional tectonics and the epicenters from the three ~M = 5.x earthquakes from yesterday. I plot the moment tensor from the largest of these, the M = 5.3 earthquake. This M = 5.3 earthquake lies along a transform plate boundary (a strike slip fault system that connects two spreading ridges) that is a configuration that would lead to left lateral motion. Therefore, I interpret this earthquake to have been a left-lateral strike slip earthquake.


Here is a map modified from Hamilton (1979), also inset into the above map. This shows the relative motion between the Solomon Sea plate and the South Bismarck plate. There are some right-lateral transform faults in the South Bismarck plate (the upper plate), but today’s moment tensor and hypocentral depth support the hypothesis that this was on the megathrust.


This is a cross section that goes along with the above map. This shows the configuration of the megathrust. I got both of the graphics from Oregon State University, CEOAS here.


In March 2015, there was a swarm of earthquakes related to the subduction zone that forms the New Britain trench. Here is my report for that earthquake swarm. Below is a map that I prepared for that earthquake report. In the map below, I also plot the slab contours, the depth to the subduction zone fault interface (Hayes et al., 2012). These contours are based upon an interpretation of the location of the fault based largely upon seismicity. When one looks at the seismicity in cross-sectional view, it is clear that the hypocenters do not align to a curvilinear planar surface, which is how we generally view faults in our simplistic minds. Subduction zone fault systems are comprised of shear zones that may be tens to hundreds of meters thick. Also, there may be additional sub-parallel fault systems that accommodate some strain. Therefore, these contours are simply approximations of the position of the megathrust. Since nobody has ever been there to directly observe the location of the fault, these are probably the best estimate of its location.


Following that swarm, I queried the USGS earthquake database for earthquakes with moment tensors plotted for the period from 2000-2015. Here is a report where I discuss these earthquakes. Below is a map that presents these earthquakes with their moment tensors.


Here is a map that shows how these recent earthquakes represent different plate boundaries in this region. I put this together as a result of the March, 2015 earthquake swarm.

Earthquake near Petrolia!

There was a M = 3.0 earthquake near Petrolia, CA today. Here is the USGS website for this earthquake. On Oct. 28, 2015, there was a small earthquake near Bayside, CA.
Here is a map where I plot the focal mechanism for today’s earthquake, along with the moment tensor from the Mendocino fault earthquake from earlier.
I placed a moment tensor / focal mechanism legend in the lower left corner of the map. 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.


Both the Mendocino fault and the San Andreas fault systems are right-lateral strike-slip fault systems. Considering this, I prefer to interpret this M = 3.0 earthquake to be a north-northwest striking right-lateral strike-slip earthquake. Currently, the fault that forms the northeastern boundary of the King Range is mapped as a backthrust to the Vizcaino block. The King Range has the highest uplift rates in the continental U.S., probably due to the northward propagation of the San Andreas fault. The San Andreas fault appears to die out north of Shelter Cove. There is no evidence that it trends offshore of Shelter Cove (as plotted on other maps; I extend the Mendocino fault eastwards based upon moment tensor/focal mechanisms from earthquakes in this region). Jeff Beeson, a Ph.D. Candidate at Oregon State University, is currently working on the offshore evidence of the San Andreas fault in this region. If is possible that the SAF continues on land, but we need to find this evidence that to date has been elusive.
I will post more later tonight. Check back later.

Amlia fracture zone earthquake: Update #1

While in my slumber, the USGS prepared a moment tensor and another aftershock occurred. Below is an updated map to my initial report here.
I plot this moment tensor, which shows a compressional mechanism. I placed a moment tensor / focal mechanism legend in the upper right corner of the map. 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.
The Amlia fracture zone is a strike-slip fault system in the downgoing Pacific plate. This earthquake swarm is aligned with the AFZ, though the M 5.9 mainshock has a compressional mechanism. These earthquakes are probably along the megathrust fault interface, but that is speculation since I was not there to observe them within the Earth.
The inset map in the upper left corner is from Dr. Peter Haeussler, USGS, Alaska Science Center, showing the historic earthquakes along this subduction zone.


Here is a map that shows the regional extent of the 1964 earthquake. Regions of coseismic uplift/subsidence are delineated by blue/red polygons.


This shows a cross section of a subduction zone through the two main parts of the earthquake cycle. The interseismic part (inbetween earthquakes) and the coseismic part (during earthquakes). This was developed by George Plafker and published in his 1972 paper on the Good Friday Earthquake.


Here is a map from Krutikov et al. 2008 (Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179 Copyright 2008 by the American Geophysical Union. 10.1029/179GM07)
Note that there are blocks that are rotating to accomodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.


Here is a video that discusses the 1964 earthquake.
Youtube Source IRIS
WMV file for downloading.

Animation & graphics by Jenda Johnson, geologist
Directed by Robert F. Butler, University of Portland
U.S. Geological Survey consultants: Robert C. Witter, Alaska Science Center Peter J. Haeussler, Alaska Science Center
Narrated by Roger Groom, Mount Tabor Middle School
Maps from Google Earth. Video from US Army Corps of Engineers. Tsunami animation from National Oceanic & Atmospheric Administration (NOAA). Photographs from US Geological Survey.

Earthquakes along the Amlia fracture zone!

There were just a few earthquakes in the mid 5 magnitude range. The first one was largest, so we would consider the others aftershocks. They appear to align north-south, so may be related to the Amlia fracture zone. The Amlia fracture zone is a strike-slip fault system in the downgoing Pacific plate.

    These 2015.11.02 earthquakes include:

  • 08:15 M 5.9
  • 08:24 M 5.2
  • 08:31 M 5.3

Here is a map that shows these three earthquakes as red circles. The three earthquakes temporally plot north to south (i.e. the M = 5.9 is the northernmost and the M = 5.3 is the southernmost).


Here is a map that shows historic earthquake slip regions as pink polygons (Peter Haeussler, USGS). Dr. Haeussler also plotted the magnetic anomalies (grey regions), the arc volcanoes (black diamonds), and the plate motion vectors (mm/yr, NAP vs PP). Today’s M 6.9 earthquake occurred in the eastern part of the 1957 M = 8.6 earthquake region.


Below are two videos that show animations of the seismicity from 1960 until May 2015. Here is a map showing the epicenters in the following animations. Check out my post about the M 6.7 earthquake along the Alaska Peninsula from May 2015. I include more information about the regional tectonics on that page (and provide links to other sources too).


The first one leaves the epicenters on the screen for the entire animation. Here is a link to the file to save to your computer.


This animation has a moving time window (~1 year), so that 1 year after the earthquake, it is removed from the map. Here is a link to the file to save to your computer.

This region has been active lately, with a M 5.6 and related swarm in September of 2015. Here is a map from that earthquake swarm.


In July of 2015, there was another earthquake swarm to the east, near the Fox Islands. Here is my report about that swarm. Here is a map in which I compilte the seismicity for this region from 1996 through July 2015.

Here is a map showing the shaking intensity contours for the M = 5.9. These contours show regions of equal ground shaking using the Modified Mercalli Shaking Intensity scale.


Here is the modeled estimate of ground shaking. This is based upon attenuation models of ground shaking, as developed by regressions of thousands of earthquake seismologic records. This also uses the MMI scale.


Here is the PAGER estimate of damages to people and their belongings. This is primarily based upon the same model that is used to prepare the above map. Once more seismologic records are produced and analyzed, there may be updates to this PAGER report. I do not suspect that this earthquake will have an update.


What is a better estimate of ground shaking is the “Did You Feel It?” map, which is made from actual observations from people.

Mendocino fault earthquake!

We just had a M = 4.3 earthquake along the Mendocino fault zone. Here is the USGS website for this earthquake.
There were some earthquakes of similar magnitude along the MF recently in January 2014 and April, 2015.
This earthquake appears to have occurred along the Mendocino fault, a right-lateral (dextral) transform plate boundary. This plate boundary connects the Gorda ridge and Juan de Fuca rise spreading centers with their counterparts in the Gulf of California, with the San Andreas strike-slip fault system. Transform plate boundaries are defined that they are strike-slip and that they connect spreading ridges. In this sense of the definition, the Mendocino fault and the San Andreas fault are part of the same system.
Here is a map that shows the epicenter as a red circle. I also plot the moment tensor for this earthquake, along with the epicenter and focal mechanism from a recent earthquake in the Gorda plate on 2015.10.29. I interpret this M = 4.5 earthquake to be a right-lateral strike-slip earthquake.


In the above map is a plot showing how the ground motions attenuate (lessen) with distance from the earthquake, in the lower left corner. The orange line is an estimate of the intensity of ground motions based on a numerical model. This numerical model is based on a regression of hundreds of earthquakes (distance vs. magnitude/intensity). These regressions form the basis for Ground Motion Prediction Equations (GMPEs). The blue dots are the actual observations made by real people. These model based estimates of ground shaking intensity are used, especially for larger earthquakes, to determine what damage might be expected. The results of the DYFI survey are plotted in the inset map in the lower right corner of the above map.
I placed a moment tensor / focal mechanism legend in the upper right corner of the map. 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.
For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2004). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).

    There are several sources of seismicity in northern California, The Cascadia subduction zone, the Gorda plate, the Mendocino fault, the San Andreas fault, the Blanco fracture zone, and within the North America plate. Below are some pages that discuss earthquakes with these different sources.

  • 2015.01.28 M 5.7 Mendocino fault
  • 2014.03.10 M 6.8 Gorda plate
  • 2015.05.26 M 4.3 Gorda plate
  • 2015.06.10 M 5.9 Blanco fracture zone
  • 2015.04.12 M 4.2 Blanco fracture zone

Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the January 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004). The 1980, 1992, 1994, 2005, and 2010 earthquakes are plotted and labeled. I did not mention the 2010 earthquake, but it most likely was just like 1980 and 2005, a left-lateral strike-slip earthquake on a northeast striking fault.


In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.


The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. These two aftershocks align on what may be the eastern extension of the Mendocino fault.
There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.
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. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
Strike Slip:

Compressional:

Extensional:

This figure shows what a transform plate boundary fault is. Looking down from outer space, the crust on either side of the fault moves side-by-side. When one is standing on the ground, on one side of the fault, looking across the fault as it moves… If the crust on the other side of the fault moves to the right, the fault is a “right lateral” strike slip fault. The Mendocino and San Andreas faults are right-lateral (dextral) strike-slip faults.


Here is an IRIS animation showing a transform plate boundary fault as it relates to spreading ridges.

    References:

  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.