{"id":10604,"date":"2022-12-23T04:38:55","date_gmt":"2022-12-23T04:38:55","guid":{"rendered":"https:\/\/earthjay.com\/?p=10604"},"modified":"2023-10-22T17:25:47","modified_gmt":"2023-10-22T17:25:47","slug":"earthquake-report-m-6-4-gorda-plate","status":"publish","type":"post","link":"https:\/\/earthjay.com\/?p=10604","title":{"rendered":"Earthquake Report: M 6.4 Gorda plate"},"content":{"rendered":"

Initial Narrative<\/font><\/H2><\/p>\n

Well, it has been a very busy week. I had gotten back from the American Geophysical Union Fall Meeting in Chicago late Saturday night. I had one day to hang out with my cats before I was to head down to Santa Cruz to meet with the city there to discuss installing a tide gage. Santa Cruz lacks a gage yet receives large tsunami inundations.<\/p>\n

So, I drove down and got there about 10pm Monday evening. I was up for an hour or two and went to sleep. <\/p>\n

At shortly after 2:30am I got a text message about a M 6.4 earthquake near Ferndale. I immediately got up and texted my colleague Cynthia Pridmore. We are tasked to prepare Earthquake Quick Reports that we (California Geological Survey, CGS) provide to the California Governor’s Office of Emergency Services (Cal OES). These reports provide technical information that helps them provide resources to local first responders during times following natural hazards impacts. <\/p>\n

https:\/\/earthquake.usgs.gov\/earthquakes\/eventpage\/nc73821036\/executive<\/a><\/p>\n

These reports are reviewed by the head of the Seismic Hazards Program (Tim Dawson) and by the State Geologist prior to being provided to the leadership in our organization and parent organizations. Reports for larger earthquakes and tsunami sometimes end up on the Governor’s desk.<\/p>\n

We got our report submitted within about 45 minutes and we prepared for a long couple of days. We at CGS met at 8am to discuss our field response activities.<\/p>\n

CGS and the U.S. Geological Survey (USGS) work closely together to document field evidence from earthquakes and tsunami. Kate Thomas (CGS) and Luke Blair (USGS) have a database ready to go within about 15 minutes after an earthquake. This database is used on mobile devices to collect observational information that include photos and other information. We use the ESRI Field Maps app for this purpose. <\/p>\n

We decided to send CGS staff from the Eureka office out to collect information. I was to drive back to Humboldt and then join the field teams the following day.<\/p>\n

Something that also happens following significant or damaging earthquakes is the activation of the California Earthquake Clearinghouse<\/a>. Pridmore (CGS) is the chair of the EQCH and works with our partners (USGS, EERI, etc.) to decide when to activate the EQCH. <\/p>\n

Data from these CGS\/USGS field observations, along with data from other field teams, are posted onto the EQCH page for this event. Here is where those data are made available for this M 6.4 Ferndale Earthquake<\/a>. The dataset of field observations are posted on that page are found by clicking on the “Resources” tab, also linked here<\/a>.<\/p>\n

When I returned to my home, the power was still out. We (CGS) had a scheduled meeting at 6pm and the EQCH meeting at 7pm. So, I went to the Eureka National Weather Service (NWS) Office on Woodley Island. They have electric power backup and satellite internet access. I work closely with the NWS and Cal OES and have been granted access to set up my workstation there during natural hazard emergencies like earthquake and tsunami. This was we can all better coordinate our actions without the burden of having power or internet outages at our residences. We are thankful for these relationships between CGS, the NWS (Ryan Aylward, Troy Nicolini) and Cal OES Eureka (Todd Becker).<\/p>\n

So, I got up very early to work with my co-workers to continue the field investigations. There was little geological evidence from the earthquake. We identified some landslides and cracks in road fill. We did not locate any evidence for liquefaction, even though the USGS liquefaction susceptibility data suggested a high chance for that phenomena.<\/p>\n

The 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 6.4 earthquake was a strike-slip earthquake within the downgoing Gorda plate (an intra plate earthquake). The earthquake started offshore and then the fault slipped to the east. <\/p>\n

There is modest evidence that this earthquake generated focused seismic waves in the direction of fault slip (this is called directivity). In addition, the area of the lower Eel River Valley is a sedimentary basin. Sedimentary basins are known for amplifying ground shaking and trapping seismic waves, further increasing the ground shaking. The lower Eel River Valley is formed by tectonic folding caused by the northward migration of the Mendocino triple junction (read my contributions in the 2022 Pacific Cell Friends of the Pleistocene guidebook<\/a> for more information about the structure of the Eel River and Van Duzen River valleys and surrounding regions.<\/p>\n

So the seismic waves could have been trapped in the sedimentary basin formed within the Eel River Valley. However, there is an even older sedimentary basin here in which the Eel\/Van Duzen river sediments are deposited within. These older sedimentary rocks have different seismic velocity properties that could also affect how seismic waves are transmitted here. There is a terrane bounding fault that separates these older rocks (Cretaceous Franciscan Formation) to the south from the younger rocks (Quaternary-Tertiary Wildcat Group) to the north.<\/p>\n

Also, any of the large crustal fault systems (e.g., the Russ fault, the Little Salmon or Table Bluff faults, etc.) could guide seismic waves (a.k.a. act as wave guides), directing them in orientations relative to the fault systems. <\/p>\n

My leading hypothesis is that the younger (latest Pleistocene to Holocene) river sediments that form the younger sedimentary basin and the crustal faults are both responsible for modifying the seismic wave transmission from this 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 will update this report further in the future, as we collect additional information.<\/p>\n

One last thing for now. Bob McPherson formed a research group that we call Team Gorda. Team Gorda, supported by Connie Stewart at Cal Poly Humboldt, is using recently constructed fiber cables as a seismic instrument (called distributed acoustic seismic, DAS) to learn more about the underlying tectonic structures in the region. This fiber cable acts as thousands of little seismometers. Jeff McGuire and his team just installed the interrogator in our office at the Arcata City Hall. Horst from the Berkeley Seismic Lab is also working with Bob to install seismometers along the fiber cable so that we can calibrate the DAS observations. <\/p>\n

We ran our first DAS experiment earlier this year and plan on doing more experiments far into the future, including fiber cables that are installed from here into the Pacific Ocean (on their way to Asia).<\/p>\n

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