Lori Dengler | Recent North Coast earthquakes may be related – Times-Standard

Written by Lori Dengler and Bob McPherson

It has been 10 days since the 7.0 magnitude earthquake and what has been learned? A small army of scientists was in the field looking for landslides and liquefaction and talking to residents about more than 100 seismometers to better pinpoint the location of aftershocks. Many discussions are taking place here and with distant colleagues.

These are busy days for those of us who follow North Coast tectonics. Each earthquake is an opportunity to learn more about the forces driving our tectonics, how this earthquake is related to the previous one, and the potential for future earthquakes. For my colleagues, this is also a time to think about the bigger picture and, in some cases, clarify thoughts that have been troubling the back of the brain for years.

I've asked my longtime friend and colleague Bob McPherson to co-author my column today. I met Bob in 1978 when I first came to Humboldt as a temporary teacher. Bob graduated from the Department of Geology four years ago and was working as a seismologist for TERA deploying, maintaining and interpreting data from a deployed seismic network to study the seismic safety of the Humboldt Bay Nuclear Power Plant.

Bob knew more about North Coast earthquakes than anyone else, and three years later, when the Geology Department added a master's degree program and I became a permanent member of the faculty, I eagerly agreed to be his thesis advisor. His understanding of the complexity of our faults and earthquake sources was also greater than mine, and my main contribution to his thesis project was editing the text.

Bob and I collaborated a lot on post-earthquake reconnaissance. In 1992, we co-authored a paper with our colleague Humboldt Professor Emeritus Gary Carver on the sources of our regional earthquakes that remains the only comprehensive treatment to this day. It was Bob who pointed out the mystery of the 1954 M6.5 earthquake, and it prompted many of us to reconsider this event and realize that even deeply buried seismic faults produce frequent earthquakes. The December 2022 M6.4 near Ferndale appears to have been ruptured by the same fault as the 1975 Mid-5th earthquake.

There's a lot we know about the December 5th earthquake. The Mendocino Fault extends west about 170 miles from Cape Mendocino. It represents the plate boundary between the Gorda plate in the north and the Pacific plate in the south. This is what we call a transform plate boundary, like the San Andreas Fault system, where the relative motion between the two plates is horizontal, like cars moving on a British motorway. It is a very active fault, producing more than 200 earthquakes of magnitude 4 or greater since 1990 including a M7.1 in 1994 and a 6.6 in 2016. Measured by the rate of moderate to strong earthquakes, it is the most active fault in California .

Aftershocks are nature's way of eliminating irregularities in fault slip. As I write, nearly 600 small earthquakes have been recorded along and near the December 5th rupture zone. Most were too small to be felt, but 14 reached the M4 range, large enough to be felt in the Cape Mendocino and southern Humboldt area. With each passing day, the number of aftershocks decreases, although the USGS gives a slight chance that larger aftershocks may occur.

We know that the December 5 earthquake is not an aftershock of the December 2022M6.4 earthquake just off Ferndale Beach. This earthquake produced a series of powerful aftershocks including a magnitude 5.4 which some Rio del residents reported being felt with greater force than the main tremor. But by the end of 2023, seismologists agreed that seismic activity had returned to baseline levels in the region, and since then, seismic activity has been relatively quiet.

Every major earthquake leaves a lasting impression, not only on the minds of the people who experienced it, but also on the Earth itself. The 2022 earthquake was not on the Mendocino fault but less than 10 miles away. The fault rupture in 2022 was not as large and slip was only half that of this year's quake, but it still moved a lot of real estate and profoundly affected the stress state around the fault, including adjacent faults like the Mendocino Fault.

Seismologists call this stress transfer. Pushing or pulling in one area can increase or decrease pressure in adjacent areas. An example of stress transfer can be seen on a billiard table when the energy of one ball is transferred to the other balls. This process occurs much more slowly in the ground and it may take months, years, or decades before a different part of the fault or an adjacent part responds.

Bob believes the story of stress transfer and the December 5 earthquake goes back at least to December 20, 2021. Just after noon, an earthquake ruptured a zone of the Mendocino Fault near the coast. Eleven seconds later, a second earthquake occurred on a completely different fault about 20 miles below Cape Mendocino. Bob and I were part of a group that re-evaluated these earthquakes (link below), and determined that they had a magnitude of 6 and that the second quake was caused by seismic waves generated by the first quake. It was an example of stress transfer over a relatively short earthquake fuse.

The second earthquake in 2021 was caused by a fault directed in a northeast direction and rupture of the fault pushed north. One year later, the Earth responded with the 2022 Ferndale earthquake. This earthquake started offshore and caused a 12-mile-long ENE fault heading toward Fortuna. The combination of orientation, thrust, and regional geology placed the Rio Dell at the center of the target, producing some of the strongest ground motions ever recorded in a California earthquake. Recent damage estimates put losses at approximately $90 million.

The 2022 earthquake also changed the stress field in the vicinity, especially in the direction of the Mendocino fault line. Extending the 2022 rupture back into this fault places it roughly where the December 5 M7.0 earthquake began. It is a reasonable hypothesis to imagine that 2021 helped trigger the 2022 earthquake which in turn created an additional impetus to trigger the 2024 earthquake.

The December 5 earthquake caused damage. Homes in the Mattul Valley shook violently, and damage was reported in several areas of southern Humboldt. But the peak acceleration was only a third as strong as it was in 2022, and although the tremors lasted longer, they did not cause as much damage.

This interrelationship between earthquakes is not new. The 1992 M7.2 Cape Mendocino earthquake caused two large earthquakes offshore in the following 18 hours, and there were similar sequences in the 1980s. But the earthquakes of 2021, 2022, and 2024 may illustrate this complex balance between faults in the vicinity of the Mendocino Triple Junction more clearly than ever before.

Is this the end of the story? Absolutely not. The more than six-foot slide on December 5 also changed the pressure field, loading and unloading other faults in the area. It may be weeks or years before the next big earthquake occurs, but it is definitely coming.

Note: North Coast Earthquakes Papers: “North Coast Earthquake Sequences of 2021 and 2022 and Fault Complexity in the Area Around the Mendocino Triple Junction”: https://kamome.humboldt.edu/sites/default/files/Hellweg%202024. pdf, and sources of earthquakes on the North Coast: https://kamome.humboldt.edu/sites/default/files/Sources%20of%20North%20Coast%20Seismicity.pdf

Lori Dengler is professor emeritus of geology at Cal Poly Humboldt and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of the “Living on Shaky Ground” preparedness magazine? Leave a message at 707-826-6019 or email [email protected].

Originally Posted: December 14, 2024 at 11:42 AM PT

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