# 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., Kelsey, H.M., and Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, doi:10.1016/j.yqres.2006.02.009, p. 354-365.
• 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.

# 10/23/2015 Inaugural Cascadia GeoSciences Present

You’re invited to come network with old friends and colleagues, meet new ones, enjoy a locally-made beverage, maybe learn something and share your local knowledge with others.

A Research Presentation by Todd B. Williams and Dr. Jason R. Patton
Unraveling tectonic and eustatic factors of sea level rise in northern California, Humboldt Bay.

WHEN: Friday October 23rd 5:30-8 pm
WHERE: Arcata D Street Neighborhood Center
1301 D St, Arcata, CA 95521
(see map below)
Hope to see you there!

Future Presents will be posted online here.

# IPCC 5th Assessment Report Forthcoming

Coming out later this month here http://www.climatechange2013.org/report/

i pasted some text from the group I fact sheet that demonstrates why the IPCC assessment reports are the most comprehensive and peer reviewed science ever. 9,200 references! ~55,000 comments! no single paper published anywhere at any time has been so comprehensive. single papers, which may disagree with minor aspects of the ARs, pale in comparison (but continue to be used by blog writers to attempt to discredit the AR). single authors also attempt to raise questions, but frequently they are only experts in a minor or related field (eg Don Easterbrook, world class glaciologist, but not a climatologist).

you have questions? WG1AR5_Questions.pdf

you want to read the fact sheet? WG1AR5_Questions.pdf

The Report
1 Scoping Meeting to outline 14 Chapters Over 1000 nominations from 63 countries
209 Lead Authors and 50 Review Editors from 39 countries Over 600 Contributing Authors from 32 countries Over 2 million gigabytes of numerical data from climate model simulations Over 9200 scientific publications cited

The First Order Draft Expert Review
Nearly 1500 individuals registered 21,400 comments from 659 Expert Reviewers from
47 countries

The Second Order Draft Expert and Government Review
Over 1500 individuals registered 31,422 comments from 800 Expert Reviewers from
46 countries and 26 Governments

The Final Government Distribution
1855 comments from 32 Governments on the Final Draft Summary for Policymakers

Total Reviews
54,677 comments 1089 Expert Reviewers from 55 countries 38 Governments