Laurie Dengler | Shaky start to 2023 – Times Standard

New Year’s Day 2023: I usually put together the annual recap of the 2022 earthquakes, but Mother Nature has other ideas. I had just finished updating the Humboldt Earthquake hotline 707-826-6020 when my phone announced, “EARTHZAKE EARTHZAKE DROP COVER AND HOLD ON.” The words were barely spoken when the first jolt hit, it wasn’t very strong but I would have noticed even if my phone hadn’t alerted me. After a few more seconds the vibration got a little stronger but not enough to bring out the skewed images. It finished in less than five seconds.

It took a minute or two before the initial size and location showed up on my phone. First 5.1, then 4.9, and finally 5.4. No, these were not three different earthquakes and there were no faults. The first predestination was estimates made by computers looking at limited station data. We want size and location information quickly and computer estimates are the best way to get the raw values.

Computers are getting much better at picking out seismic arrivals, but these early fast spots are often off to a few tenths of a unit of size. It’s more problematic on the north coast where we don’t have marine tools and site restriction requires a trained eye. If you are a visitor to the USGS Latest Earthquakes page, scroll down to the “Origin” box on their earthquake page and look at the “Review Status”. The seismologist takes a few minutes to fetch the data and review the computer analysis. It is not unusual to make a few adjustments.

Large earthquakes or multiple overlapping earthquakes can be more challenging. In December 2021, sizing and a final site took two weeks to be approved. It turns out we had two earthquakes – a 5.7 followed by a 6.2 18 miles 11 seconds apart. Some seismologists still argue that the magnitudes need a little adjustment.

It didn’t take me long to realize that the January 1st M5.4 was different from the 20t M6.4 in several ways. It was almost twice as deep and pointed in a different direction. The 6.4 rip was about 8 to 10 miles long and the movement was a left strike slip, the rocks on the north or Fortuna side of the fault staggered to the west and the rocks on the Rio Del and Ferndale side moved to the east, sliding horizontally like cars on an American highway.

M5.4 was 10 miles south of the M6.4 fault plane and on a different fault. This fault was also slidable but was oriented in a northwest-southeast direction. The movement was similar to the British Highway with boulders on the east side being pushed towards Dung Lake and the west side being pushed in the direction of Ferndale. We call this right lateral slip.

Are I Lost The tectonics of our North Coast confuse a lot of people, even geoscientists unfamiliar with the wild world of the Mendocino Triple Junction. I can’t tell you how many journalists have asked me to identify the fault that caused the earthquakes. Not just one mistake – we have so many that only mistakes large enough to crack the surface have names. Some of the more informed journalists like to pull out a USGS fault map and point to one of the named faults that appears to be close to the epicenter. No – that won’t work. Very few of our historical flaws are on those set on the surface. Both December 20 were 6.4 and New Year’s Day was 5.4 in the Gorda plate, well below the interface that separates it from the North American plate and all the imperfections we can see.

I love puzzles. Let’s say I finish a 500 piece puzzle and then decide to build another different puzzle on top of it. Top puzzle pieces vary in size and shape. When I’m done, I can’t see the detailed structure of the bottom part. I cannot use the upper puzzle to predict what lies below.

The Mendocino three-joint area is more complex than the two puzzles. Most of the action takes place in the lower puzzle and stresses have mixed and squeezed pieces of the puzzle together. Like puzzles, if you click on one piece it can affect a neighbor. This is what is happening in the southern Humboldt region right now.

If a different earthquake happens, can it still be an aftershock? Yes, if it is on a closely related fault and at the time of the high earthquake after the first quake. We have about 300 aftershocks so far, about 220 near the fault of the main rift, 44 in a shotgun pattern around the eastern end of the rupture and a few others, like January 1, north or south of the main rupture area. If 5.4 happens two years from now after earthquake activity has returned to background levels, I would call it a separate earthquake.

I received MyShake Alerts for both 6.4 and 5.4 and was so happy to get them. I know many of you were frustrated that the alerts didn’t give you much, if any, alert. How does MyShake work and why did I receive a 1 second notification last Sunday?

MyShake is a California earthquake alert system. Oregon and Washington have a similar program called ShakeAlert. For large earthquakes, you may also receive Wireless Emergency Alerts (WEA) via your phone. All of these alerts are based on the same principle: detecting earthquake rupture in the first few seconds after it has already started. There’s no prediction going on here, and when enough seismic stations record those early signs, it can be very accurate.

After at least four stations have “picked” a signal (the seismology of when the seismic wave arrives), an algorithm takes over the estimation of the epicenter and strength of the earthquake, and the prediction of possible shaking in the area. If the magnitude is at least 4.5 (5 for WEA alerts) and the expected vibration reaches the threshold of Intensity III (moderate vibration), alerts are sent to everyone in vibration III and strongest via mobile phones and, in some cases, computers. On December 20, it took 5.5 seconds to send the first alert after 6.4. The M5.4 earthquake on New Year’s Day lasted 8 seconds.

Herein lies the difficulty in early warning of an earthquake. Seismic waves are very fast and within 7 to 8 seconds the primary wave or P wave can travel about 35 miles. This means that everyone within 35 miles of the epicenter will not receive an alert because it has not been issued yet. On January 1st, the alert reached my phone less than a second before I could feel it. I live in McKinleyville, 36 miles from the epicenter on January 1st.

Could MyShake do better? Yes, and it’s getting better. The density of seismic networks is the most important factor. We have now had several stations on the North Coast for over a decade and more are being installed all the time. Algorithms are tested and improved with every real event. But until we have detection systems out there, many of our earthquake alerts will arrive around the same time the shaking occurs. And there is no way to receive a warning for 20-30 seconds unless you are somewhat far from the epicenter.

Do not confuse these earthquake early warning alerts with your county’s emergency notification system. Almost all California counties have a system in place to notify you in the event of hazardous conditions. It is used to notify you of a tsunami warning in our area and other conditions for your action. A Humboldt Alert was issued on January 3 to tell us about storm conditions and to reduce nonessential driving. These county notifications do not broadcast early warnings about earthquakes.

I don’t know when this sequence will end. The seismicity was relatively low in the area followed by the earthquake in the past day. But I doubt if it’s over yet.

Laurie Dengler is Professor Emeritus of Geology at Cal Poly Humboldt, and an expert on tsunami and earthquake risk. Questions or comments about this column, or want a free copy of Living on Shaky Ground readiness magazine? Leave a message at 707-826-6019 or send an email to [email protected].

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