Lori Dengler | Recent studies highlight earthquake threats on North Coast – Times-Standard

I'm often asked how I come up with topics to write about each week. It's easy. Between Mother Nature and the many brilliant, hard-working, and creative scientists in my field, the problem is usually one of narrowing down multiple possibilities. I'm having a hard time choosing this week, so I'll try to give a quick overview of a couple of papers that caught my eye because they're relevant to where we live.

In August, the journal Earthquake Research Letters published an opinion article on supershear earthquakes. This article, written by Ahmed Al-Banna and six co-authors, argues that supershear earthquakes are more common than previously thought, and that current design codes both in California and around the world do not adequately address the powerful vibrations they can produce.

I've written about supershear earthquakes before (Not My Fault, May 23, 2021). Like shock waves from supersonic flight, supershear earthquakes result when the speed of fault rupture exceeds the speed of sound waves in the Earth. The result is a “sonic boom,” so to speak, a concentrated cone of seismic waves that concentrates the force of the shaking, especially in the direction in which the rupture propagates.

We have known that the strength of an earthquake's shaking is not uniform with respect to the distance from the epicenter for more than half a century. The type of fault movement concentrates much of the vibration perpendicular to the fault slip, especially in strike-slip earthquakes. This was one factor that made the December 2022 earthquake appear much stronger in Rio del than in Ferndale.

The possibility of supershear rupture was hypothesized in the 1970s, but is difficult to measure directly. In the field, it takes a dense network of high-resolution seismic instruments near a fault to detect the first arrivals of the secondary or S wave. With newer analysis techniques and awareness of what to look for, it appears that supershear rupture may be the rule rather than the exception for large earthquakes.

Examples of recent supershear earthquakes include the 1999 M7.6 earthquake in Izmit, Turkey, the 2018 M7.5 Palu earthquake, the Indonesia earthquake I wrote about in 2021, the 2023 M7.8 Turkey-Syria earthquake, and this year's M7.7 earthquake in Burma (Myanmar). A re-examination of vintage events adds San Francisco of 1906 to this list. Experiments conducted by Aris Rozakis, one of the co-authors of the latest study, confirmed the supershear rupture process in the laboratory.

Thanks to new technology, the recent Mendocino Fault earthquake appears to have ruptured at breakneck speed. For the past five years, a consortium of scientists led by the USGS has studied fiber optics as a means of detecting earthquakes. By a stroke of great luck, a trial array was run on the new section of cable along Old Arcata Road to Myrtle, east of 101, and captured images of the December 5, 2024 earthquake. The optical signal provides very precise timing and allows a detailed picture of the rupture process as it began about 40 miles offshore, slowed near the triple junction area, and then jumped to super-shear velocities when I hit the coast. Fortunately, the area is sparsely populated, and the damage was minor.

Al-Banna and his team argue that more work is needed to fully understand the risks of supershear ruptures, and that California, a state rife with slip-induced defects, would be remiss if it did not include them in building design codes and zoning considerations. More instrumentation in fault zones, modeling, simulation and investigation of structural response to focused S-wave vibration is essential for real-world applications.

The second post also has a clear West Coast perspective. “Unraveling the Dance of Earthquakes: Evidence for Partial Synchrony of the North San Andreas Fault and the Great Cascadia Thrust,” was published two weeks ago in Geosphere, a publication of the Geological Society of America that examines evidence for a link between earthquakes in the Cascadia subduction zone and the San Andreas Fault. The team, led by Chris Goldfinger of Oregon State, examined marine samples and came to the conclusion that we could be in for a double whammy of great earthquakes.

The crux of the story is the sourness. Turbidities are a type of submerged landslide that are ubiquitous in coastal marine environments. Rivers continually transport sediment to the continental shelf where it is redistributed by currents and accumulates at the heads of submarine canyons. When it becomes gravitationally unstable, a turbidity current is formed. A mass of sediment and water slides down the valley and spreads out on the abyssal level below. Turbidites are gravitationally sorted with larger grains at the bottom, grading upward in a distinct sequence. Evidence of ancient turbidity currents is preserved in many places on the north coast where raised rocks show layers of avalanches that occurred millions of years ago.

In the absence of any external forcing mechanism, one would expect turbidite ages to vary from region to region in a fairly random pattern. John Adams of the Geological Survey of Canada was the first to note that the turbidities were not random in age but occurred at the same time in widely spaced areas off the coast of Washington and southern British Columbia. He suggested that great earthquakes could be the cause.

Many people have looked at turbidite ages since then, and none more comprehensively than Chris Goldfinger, who has made marine mapping and retrieval of turbidite cores a cornerstone of his academic work. His work validated the synchronization pattern, where turbidities occur in pulses and slide over a vast marine area at the same time. Large earthquakes that produce powerful tremors over several hundred miles are an obvious triggering mechanism.

Goldfinger's research is not limited to the Cascadia region. His group has worked in earthquake zones around the world studying turbidities in Indonesia and Chile. He also ventured south of the Mendocino trijunction into the realm of the San Andreas Fault. He's not the first to suggest the possibility of a tectonic link between Cascadia and San Andreas. The two large fault systems essentially contact at the Mendocino Triple Junction, and significant slip on either fault will change the stress pattern on the other. But the stratigraphic data provided by this study are as close to conclusive proof as we are likely to get – until an actual “double” event occurs.

What is the evidence? It's all about the basic details and distribution of features from core to core. Instead of finding just one clear turbidity event with the typical rough material at the bottom smoothing up, they found two events very close together, the last event disturbing the previous one. This limits the timing of the two to a short time window. There are three instances in the past 1,500 years where these individual binaries have occurred, including the more recent Cascadia earthquake on January 26, 1700, where the two ruptures appear to have been only minutes to hours apart.

When a large earthquake occurs, aftershocks are inevitable and emergency responders take this into account, monitoring aftershocks closely so they can evacuate unstable areas quickly. But the Cascadia-San Andreas duo is a very different beast indeed. Either one alone would require a massive response from areas outside the earthquake zone, and our current planning focuses on one area at a time with one area providing mutual aid to the other. Goldfinger says it's time to include the double event in our planning, and act now to mitigate all possible risks ahead of time.

Which brings me to ShakeOut. This Thursday, California and much of the world will participate in “The Great Shaking,” a time to practice what to do when the ground begins to shake. Most schools and many businesses and other organizations will practice “Drop, Cover and Hold” drills at 10:16 a.m. on October 16. I hope this year we can expand our thinking beyond drills – reducing risks at home and in the workplace, encouraging our decision-makers to prioritize public and personal safety, and incorporating resiliency into all community planning processes. Please visit https://rctwg.humboldt.edu/great-shakeout for more ideas on how to get involved.

Note: Free OLLI lecture on the 1954 Fickell Hill earthquake (Monday, October 13 from noon to 1:30 p.m.). Go to https://www.humboldt.edu/olli/events/1954-north-coast-earthquake-enigmablue-lake-earthquake for the Zoom link.

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: October 11, 2025 at 12:02 PM PDT

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