Earthquake along the South New Hebrides trench

We just had a swarm of shallow seismic activity along the arc above the South New Hebrides trench. The largest magnitude was a Mw 6.0. There was a very deep (~220km) Mw 6.8 just to the north about a week ago.
Here is a global scale map showing the general location of these earthquakes.

This map shows the moment tensors for these two earthquakes. The Mw 6.0 may either be a NW striking right-lateral or NE striking left-lateral earthquake. The Mw 6.8 shows north-south compression.

Here is a primer for us to help us interpret these moment tensors. This figure, from the USGS, actually is for focal mechanisms. However, the graphical representation of these two symbols is the same.

The plate convergence in this part of the arc is almost purely normal (the convergence is perpendicular to the trench). Because of this, there is little to no strain partitioning, so there is probably not a forearc sliver fault system (like the Sumatra and Mentawai faults in Sumatra). Here is what a forearc sliver fault looks like in schematic. This does not necessarily help us interpret the moment tensor, but if there were a forearc sliver, resulting from oblique convergence, we would probably interpret the fault as nw striking.

This is a plot of how oblique convergence is along the New Hebrides trench (McCaffrey, 1996). North is to the left. This shows that there is oblique convergence further north (the plot shows about 75 mm/yr obliquity).

This shows the Modified Mercalli Intensity contours for the Me 6.0.

Here is a USGS poster for the tectonics in this region.


  • McCaffrey, E., 1996. Estimates of modern arc-parallel strain rates in fore arcs, Geology, v. 24, no. 1, p. 27-30, 1996.

Stress Changes from Strike Slip Faults

One may ask what are the relations between the Mendocino fault (MF) and the Cascadia subduction zone (CSZ). Earthquakes on both fault systems are responding to the same plate motions. As the Gorda plate moves to the east, relative to the North America plate and the Pacific plate, stress is stored along the Mendocino fault and the Cascadia subduction zone fault. I posted some information about the Mw 5.7 earthquake here and more about the aftershocks here. I also have posted some material about Cascadia here.
Here is a map showing the MF and CSZ.

What is the relation between the Mw 5.7 earthquake and the CSZ? Do we expect a CSZ earthquake due to the MF swarm?
In general, there would be an increase in stress along the CSZ fault following the Mw 5.7 MF earthquake. The changes in coulomb stress along receiver faults is very small compared to the total stress budget on these faults. Generally, earthquakes release stress in the range of Mega Pascals and the stress changes imparted from other earthquakes are in the range of kPa. So, earthquake faults must be near the stress limit of earthquake rupture if they are to be triggered by a nearby earthquake.
Here is a figure from Lin and Stein (2004). The figure on the left shows the stress change imparted by a right lateral strike slip fault upon a thrust fault.

Here I have rotated the figure to match the configuration of the MF as it relates to the CSZ. This is simply an analogy and not perfect. Someone would actually need to do the coulomb stress analyses to have a better idea what the stress changes are due to the Mw 5.7 earthquake swarm.


  • Lin, J. and Stein, R.S., 2004. Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults, Journal of Geophysical Research, v. 109, B02303, doi:10.1029/2003JB002607, 2004.

Mendocino fault Aftershocks: Update

The aftershocks continue following the Mw 5.7 mainshock earthquake swarm along the Mendocino fault. Yesterday I plotted my interpretation for the location of the fault that this swarm may have ruptured upon. Some epicenters have been relocated and appear to better align with the fault that I placed on my map.
Here is the map from this morning, updated at 9:22 AM local time. We just had a Mw 4.3 aftershock. If you felt it (or not!), please go here to report your observations. I have not edited the fault lines that I had plotted yesterday. You can compare this map with the map below, which I made around 9 PM last night.

Here is the map from last night, for comparison. Note how some epicenters have aligned more closely to the faults that I drew on the map yesterday. At first, I thought there might be an off-fault triggered swarm along a conjugate fault that strikes north-south. There are three epicenters on the eastern region of this swarm. However, since the locations have been updated (see above map), there is now only one of these off-fault epicenters remaining. So, it was my imagination.

    Here are the USGS earthquake web pages for the larger magnitude earthquakes of this swarm that have focal mechanism or moment tensor fault solutions.

  • Mw 4.3 foreshock
  • Ml 3.2 foreshock
  • Mw 5.7 mainshock
  • Ml 3.1 aftershock
  • Mw 4.3 aftershock
    The Northern California Seismic System (NCSS) operated by UC Berkeley and USGS generated a series of probabilities for aftershocks related to the Mw 5.7 earthquake. This was updated this morning for version 3.

  • STRONG AFTERSHOCKS (Magnitude 5 and larger) –
    At this time (18 hours after the mainshock) the probability of a strong and possibly damaging aftershock IN THE NEXT 7 DAYS is approximately 19 PERCENT
    Most likely, the recent mainshock will be the largest in the sequence. However, there is a small chance (APPROXIMATELY 5 TO 10 PERCENT) of an earthquake equal to or larger than this mainshock in the next 7 days.
  • WEAK AFTERSHOCKS (Magnitude 3 to 5) –
    In addition, approximately 5 to 25 SMALL AFTERSHOCKS are expected in the same 7-DAY PERIOD and may be felt locally.
  • This probability report is based on the statistics of aftershocks typical for California. This is not an exact prediction, but only a rough guide to expected aftershock activity. This probability report may be revised as more information becomes available.

Mendocino fault earthquake 1/28/15 1:08 PM local time

We just had a good sized shaker, probably on the Mendocino fault system. Today’s earthquake occurred on 1/28/2015 at 1:08 local time. The mainshock, an earthquake of magnitude Mw = 5.7, has an epicenter approximately 20 km due west of the mouth of the Mattole River. Please check back here soon as I am updating this page rapidly. I have now plotted one of the foreshocks in my interpretation figure below, a magnitude 3.2 earthquake that plots west of the Mw 5.7 mainshock. The M 3.2 happened on 1/25/15.
We do NOT expect a tsunami from this earthquake. The magnitude is small and the type of earthquake is strike slip.
According to the NCSS, there is a high likelihood of aftershocks. Read their statement here.
Here is a map showing the mainshock (the largest orange dot). The USGS active faults are also shown. There is also a big red dot, but that is plotted incorrectly.

Here is a more localized map, showing the shape of the seafloor as it relates to the deformation of the Mendocino Transform fault plate boundary. The Mendocino fault is a right-lateral strike-slip fault that separates the Gorda plate to the north from the Pacific plate to the south.

Here is a map with some interpretations plotted on it. I have labeled the three plate boundary faults along with their sense of motion. I have also placed the moment tensor of the Mw 5.7 main shock on this map. I have drawn two red lines of possible faults that may be responsible for for the earthquakes that ruptured today.

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:



Here is a photo of the HSU Dept of Geology Seismograph.

Here is the regional map with the Did You Feel It shaking intensity data plotted. Please go to the USGS website to register your observations here. Your observations help us learn about the local geology.

Here is the moment tensor for this earthquake. This is interpreted to be a strike slip earthquake based on this moment tensor.

This is the automated map showing expected Modified Mercalli Shaking Intensities for this earthquake. This is simply based on a model, while the DYFI map is actually based upon people’s observations (like yours!).

This DYFI map shows that people in Chico, California felt this earthquake! The earthquake was also felt in Oregon! Updated at 4:45 local time.

Here is the “PAGER” page that shows a modeled estimate of loss of human lives and their possessions (like buildings). There is actually a 24% likelihood that there may be between 1 and 10 fatalities!

This is a map showing the plate configuration of our region. Today’s earthquake probably occurred along the Mendocino fault system. There is more information about the Cascadia subduction zone here. From Nelson et al., 2006.

Here is a primer that helps people learn how to interpret focal mechanisms and moment tensors. Moment tensors are calculated differently from focal mechanisms, but the interpretation of their graphical solution is similar. This is from the USGS.

For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

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.

This shows a cross section and map view of the Cascadia subduction zone. There are other transform faults further north (e.g. the Blanco fracture zone), which are similar to the Mendocino fault. This cross section is at the latitude of the southern Willamette Valley.

Here is a map from Rollins and Stein (2010) that shows historic earthquakes in the Mendocino triple junction region. The earthquakes in 1994 and 1983 were also in the Mendocino fault. The 1983 swarm occurred in the same region as today’s Mw 5.7 swarm.

Here is a large scale map of the 1983 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles. Note how the aftershocks trend slightly southeast in this region. Today’s swarm does the same (and the moment tensor also shows a slightly southeast strike). Note how the interpreted fault dips slightly to the north, which is the result of north-south compression from the relative northward motion of the Pacific plate.

Here is a large scale map of the 1994 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles.

Here is a plot of focal mechanisms from the Dengler et al. (1995) paper in California Geology.

Here is a photo of the Rio Dell Bluffs taken by Mary Lou Willits today. According to her, the cliffs that are colored grey were newly exposed today (compared to the yellowish cliffs to the right).


  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • 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, no. B12306, p. 19 pp.
  • Nelson, A.R., Kelsey, H.M., Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone. Quaternary Research 65, 354-365.

Cascadia’s 315th Birthday

On this morning, 315 years ago, the Cascadia subduction zone fault ruptured as a margin wide earthquake. I here commemorate this birthday with some figures that are in two USGS open source professional papers. The Atwater et al. (2005) paper discusses how we came to the conclusion that this last full margin earthquake happened on January 26, 1700 at about 9 PM (there may have been other large magnitude earthquakes in Cascadia in the 19th century). The Goldfinger et al. (2012) paper discusses how we have concluded that the records from terrestrial paleoseismology are correlatable and how we think that the margin may have ruptured in the past (rupture patch sizes and timing). The reference list is extensive and this is but a tiny snapshot of what we have learned about Cascadia subduction zone earthquakes.
Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). 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).

This map (McCrory et al., 2006) shows the secular (ongoing modern) rates of motion for the Juan de Fuca and Gorda plates relative to the North America plate (Wilson, 1998; McCrory, 2000). Red triangles denote active arc volcanoes.

Here is a view of the subduction zone showing the landscape and the plate configuration within the Earth. The cross section is located near the southern Willamette Valley. This is schematic and does not completely match the real geography. Note how the downgoing plate melts and the rising magma leads to volcanism of the Cascade volcanoes (a volcanic arc).

This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.

Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.

This figure shows the regions that participate in this interseismic and coseismic deformation at Cascadia. Atwater et al., 2005. Black dots on the map show sites where evidence for coseismic subsidence has been found in coastal marshes, lakes, and estuaries.

This shows how the CSZ is deforming vertically today (Wang et al., 2003). The panel on the right shows the vertical motion in mm/yr.

This figure, also from Wang et al. (2003), shows their estimate of how the coseismic vertical motion may happen. Contours are in meters.

Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.

Here is a photo of the ghost forest, created from coseismic subsidence during the Jan. 26, 1700 Cascadia subduction zone earthquake. Atwater et al., 2005.

Here is a photo I took in Alaska, where there was a subduction zone earthquake in 1964. These tree snags were living trees prior to the earthquake and remain to remind us of the earthquake hazards along subduction zones.

This shows how a tsunami deposit may be preserved in the sediment stratigraphy following a subduction zone earthquake, like in Cascadia. Atwater et al., 2005. If there is a source of sediment to be transported by a tsunami, it will come along for the ride and possibly be deposited upon the pre-earthquake ground surface. Following the earthquake, tidal sediment is deposited above the tsunami transported sediment. Sometimes plants that were growing prior to the earthquake get entombed within the tsunami deposit.

Here is a new animation of the tsunami that was triggered during the 1700 AD CSZ earthquake. This is just a model and has considerable uncertainty associated with it. From the US NWS Pacific Tsunami Warning Center (PTWC).

This is the timeline of prehistoric earthquakes as preserved in sediment stratigraphy in Grays Harbor and Willapa Bay, Washington. Atwater et al., 2005. This timeline is based upon numerous radiocarbon age determinations for materials that died close to the time of the prehistoric earthquakes inferred from the sediment stratigraphy at locations along the Grays Harbor, Willapa Bay, and Columbia River estuary paleoseismic sites.

Offshore, Goldfinger and others (from the 1960’s into the 21st Century, see references in Goldfinger et al., 2012) collected cores in the deep sea. These cores contain submarine landslide deposits (called turbidites). These turbidites are thought to have been deposited as a result of strong ground shaking from large magnitude earthquakes. Goldfinger et al. (2012) compile their research in the USGS professional paper. This map shows where the cores are located.

Here is an example of how these “seismoturbidites” have been correlated. The correlations are the basis for the interpretation that these submarine landslides were triggered by Cascadia subduction zone earthquakes. This correlation figure demonstrates how well these turbidites have been correlated. Goldfinger et al., 2012

This map shows the various possible prehistoric earthquake rupture regions (patches) for the past 10,000 years. Goldfinger et al., 2012. These rupture scenarios have been adopted by the USGS hazards team that determines the seismic hazards for the USA.

I have a paper that also discusses the paleoseismology and sedimentary settings in Cascadia (and Sumatra). Patton et al., 2013.
Here is my abstract:
Turbidite deposition along slope and trench settings is evaluated for the Cascadia and Sumatra–Andaman subduction zones. Source proximity, basin effects, turbidity current flow path, temporal and spatial earthquake rupture, hydrodynamics, and topography all likely play roles in the deposition of the turbidites as evidenced by the vertical structure of the final deposits. Channel systems tend to promote low-frequency components of the content of the current over longer distances, while more proximal slope basins and base-of-slope apron fan settings result in a turbidite structure that is likely influenced by local physiography and other factors. Cascadia’s margin is dominated by glacial cycle constructed pathways which promote turbidity current flows for large distances. Sumatra margin pathways do not inherit these antecedent sedimentary systems, so turbidity currents are more localized.

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gràcia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2012 a. Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper # 1661F. U.S. Geological Survey, Reston, VA, 184 pp.
  • McCrory, P.A., 2000, Upper plate contraction north of the migrating Mendocino triple junction, northern California: Implications for partitioning of strain: Tectonics, v. 19, p. 11441160.
  • McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006, Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey
  • Nelson, A.R., Kelsey, H.M., Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone. Quaternary Research 65, 354-365.
  • Patton, J. R., Goldfinger, C., Morey, A. E., Romsos, C., Black, B., Djadjadihardja, Y., and Udrekh: Seismoturbidite record as preserved at core sites at the Cascadia and Sumatra–Andaman subduction zones, Nat. Hazards Earth Syst. Sci., 13, 833-867, doi:10.5194/nhess-13-833-2013, 2013.
  • Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003, A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.