{"id":1329,"date":"2014-03-09T21:53:16","date_gmt":"2014-03-10T05:53:16","guid":{"rendered":"http:\/\/earthjay.com\/?p=1329"},"modified":"2023-03-10T07:00:41","modified_gmt":"2023-03-10T07:00:41","slug":"m6-9-gorda-plate-earthquake","status":"publish","type":"post","link":"https:\/\/earthjay.com\/?p=1329","title":{"rendered":"M 6.8 Gorda plate earthquake 2014\/03\/10"},"content":{"rendered":"

Updated map below. See additional more updated information about Gorda plate earthquakes in the Earthquake Report for the 10 January 2010 M 6.5 earthquake here<\/a>.<\/p>\n

That was exciting. I counted over 40 seconds of motion in Manila. This is a wake up call for all of us, including me.
\nCheck back here for updates. I am posting some basic material to begin with. We had another earthquake in this region last year. Here<\/a> is a page i put together for that earthquake.
\nDon’t forget to get to the USGS online to fill out the “Did You Feel It?”<\/a> form. The information provided helps geologists and seismologists learn about the ground shaking response for earthquakes in this region. They also can apply this information elsewhere (in some ways).<\/p>\n

https:\/\/earthquake.usgs.gov\/earthquakes\/eventpage\/nc72182046\/executive<\/a><\/p>\n

The Cascadia subduction zone is formed where the Gorda and Juan de Fuca plates subduct northeastward beneath the North America plate. Here is a figure that Alan Nelson put together. I have updated it with material from Jason Chaytor’s 2004 paper.
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This is most likely an earthquake in the underlying Gorda plate. The Gorda is losing the battle between the JdF plate to the north and the Pacific plate to the south, both of which are colder, older, and more dense (basically, they form a vise that is squeezing Gorda so much that it deforms internally). This internal deformation results in the formation of left lateral strike slip faults in the southern GP that form on preexisting faults (originally formed at the Gorda rise, where the Gorda plate crust is created).<\/p>\n

Here is a map of the epicenter, about 70 km west of eureka: \"\"<\/a>
\nThis is the moment tensor, which shows it is either a northeast striking left-lateral strike slip earthquake or a southeast striking right lateral strike slip earthquake. Given what we know about the regional tectonics, I would interpret this to be a left lateral earthquake. It plots just southwest of the 1980 M 7.2 Trinidad earthquake. There was a focal mechanism earlier that matched this moment tensor and then a later focal mechanism that was incorrect. I have removed both of them as the moment tensor is a more reliable measure of the sense of motion on the fault.<\/p>\n

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Here is a map showing historic seismicity. The largest circle to the northeast of the epicenter in orange is the 1980 Trinidad earthquake. \"\"<\/a><\/p>\n

Here is a map from Rollins and Stein (2010) showing the faults and tectonics of the Gorda plate. Today’s M 6.9 is probably somewhere in the right step of the dashed fault labeled “B.” \"\"<\/a><\/p>\n

Here is the seismograph from Jamie Shuttmutt (downloaded from here<\/a>) \"\"
\nHere is the seismograph from HSU dept. of Geology as taken by Jamie Shuttmutt \"\"<\/p>\n

Here is the seismograph from the UC Berkeley Jacoby Creek seismometer, posted by Lori Dengler<\/a>: \"\"
\nHere is a map showing the Modified Mercalli Shaking Intensity for the region. The contours and color over land have the same color scale. Note the increased shaking in the Humboldt Bay region. This is probably the result of the underlying material here (sediments vs bedrock). \"\"<\/a><\/p>\n

Here is the shakemap, with colors designating the Modified Mercalli Shaking Intensity (scale bar in the legend): \"\"<\/a><\/p>\n

This is the map of shaking intensity which changes as more people fill out their “Did You Feel It” (link above): \"\"<\/a><\/p>\n

Here is the map from the Southern California Earthquake Center (SCEC). There are several aftershocks in red. There was a M3.3 foreshock. The aftershocks appear to align with the northeast striking faults in this region of the Gorda plate. These are likely triggered earthquakes on different faults than the mainshock (so may not be considered aftershocks, but triggered seismicity). \"\"
\nHere is a primer for the different types of earthquake faults: \"\"
\nThese are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.<\/p>\n

\"Mw<\/a>
\nHere is the pager information (shows the potential exposure, economic or human, to the ground shaking). \"\"<\/a><\/p>\n

UPDATE 2023.03.09<\/font><\/H2><\/p>\n

The Updated Earthquake Report<\/font><\/H2><\/p>\n

This earthquake is in a tectonically complicated region of the western United States, the Mendocino triple junction. Here, three plate boundary fault systems meet (the definition of a triple junction): the San Andreas fault from the south, the Cascadia subduction zone from the north, and the Mendocino fault from the west. These plate boundary fault systems all overlap like fingers do when we fold our hands together. <\/p>\n

The Cascadia subduction zone is a convergent (moving together) plate boundary where the Gorda and Juan de Fuca plates dive into the Earth beneath the North America plate. The fault formed here is called the megathrust subduction zone fault. Earthquakes on subduction zone faults generate the largest magnitude earthquakes of all fault types and also generate tsunami that can impact the local area and also travel across the ocean to impact places elsewhere. The most recent known Cascadia megathrust subduction zone fault earthquake was in January 1700<\/a>.<\/p>\n

The San Andreas and Mendocino fault systems are strike-slip (plates move side by side) fault systems. Many are familiar with the 1906 San Francisco Earthquake<\/a>.<\/p>\n

While the largest source of annual seismicity are intraplate Gorda plate earthquakes, the two largest contributors to seismic hazards in California are the Cascadia subduction zone (CSZ) and the San Andreas fault (SAF) systems. These sources overlap in the region of the Mendocino triple junction (MTJ) and may interact in ways we are only beginning to understand as evidenced by the 2016 M7.8 Kaik\u014dura earthquake in New Zealand (Clark et al., 2017 Litchfield et al., 2018), which occurred along a similar subduction\/transform boundary, and included co-seismic rupture of more than 20 faults.<\/p>\n

The M 6.8 earthquake was a left-lateral strike-slip earthquake within the Gorda plate (an intra plate earthquake). <\/p>\n

One thing people almost always ask is about whether or not there is a higher chance that there will be a Cascadia subduction zone earthquake. This is currently impossible to tell. However, we can make some estimates of how forces within the Earth might have changed after a given earthquake. There was a Gorda plate earthquake sequence in 2018 that allowed us to consider these changes in the crust to see if the megathrust was brought more close to rupture. Here is the report from that Gorda plate earthquake sequence<\/a>.<\/p>\n

I plot aftershocks and triggered earthquakes from both the 2014 (in green) and the 2010 (in blue) earthquake sequences. Each sequence includes primary rupture on northeast striking strike-slip faults within the Gorda plate. I place dashed lines roughly where these faults are.<\/p>\n

I also interpret additional faults that appear to have slipped during these sequences. There is one fault for the 2010 sequence and two faults for the 2014 sequence. Those of you who are familiar with the 2012 Wharton Basin earthquakes would have these examples in your mind when you look at the interpretive poster below.<\/p>\n

I never noticed these before but after working on similar seismicity trends following the 1992 Cape Mendocino and 2022 False Cape (Ferndale) earthquake sequences. We are learning more and more with each earthquake sequence. Stay tuned for more on these fault interactions. <\/p>\n

Below is my interpretive poster for this earthquake<\/font><\/H2><\/p>\n