Laurie Dingler | Monday’s version 6.2 reminds us of the need to improve alertness – Times Standard

I had just started a holiday baking project when I noticed a tone from my phone. It took a second or two to get it out – a voice says, “Quake drop cap and wait.” MyShake app was on my phone. A second later, an alert went off on my husband’s phone. Then we heard the windows rattling and the house started swaying.

This was the first time for me, the first time an earthquake early warning app gave me a notification and then vibrated soon after. The MyShake California Statewide Earthquake Notification System was released in October 2019. It first ran on the North Coast on March 18, 2020. This earthquake was 5.2 on Cape Mendocino and was felt lightly. But I didn’t get an alert because I was outside the automated felt area generated by the algorithm. Four days later, my phone announced a 6.2-magnitude earthquake and told me to expect shaking. I got out from under my desk and waited and waited. But no shaking followed – that earthquake was only 4.8 and I was already out of the feeling zone.

Last Monday, the system worked, and several thousand people received a notification a few seconds before feeling it. In my house, the shaking was mild and went away quickly. The first volume estimate was 5.2 with a location near Cape Mendocino. Five minutes later, I received a statement from the National Tsunami Warning Center in Alaska that a magnitude 6.2 and that a tsunami was not expected.

It’s not at all uncommon for estimates of size and location to be off, especially here on the North Coast. The site is a triangulation process and ideally seismologists use data from the instruments that surround the source. We don’t have ocean floor instruments and that means there are no good constraints for marine earthquakes. Accessing additional records and improving data processing reduces this uncertainty but means that adjustments in location, depth, and size can persist for several days afterward.

As I write, the USGS has located the earthquake off the shore of Cape Mendocino, 23 miles west of Petrolia, 45 miles southwest of Eureka, and 54 miles from my home in McKinleyville. The depth was recorded at nine miles below the surface and the fault movement was mainly horizontal. At first glance, this earthquake appears to be located on the Mendocino fault, the plate boundary between the Pacific Plate and the Gorda Plate I wrote about two weeks ago.

But a closer look reveals a more complex story. This earthquake was within the Tri-Mendocino Confluence Zone, the complex deformation zone that marks the confluence of the Pacific, North American, and Gorda plates. It is also where three major fault systems connect: the San Andreas, the Mendocino fault, and the Cascadia subduction zone.

Much of the tension is concentrated in the triple junction area. The result is many faults and folds, high lift rates and some of the deformed rock units I have ever seen. This region and its adjacent marine area are the most seismically active in the 58 contiguous states. The current discussion among seismologists indicates that it was not at fault for the Mendocino and is likely to be at fault within the Gorda plate. And we cannot rule out the involvement of the northern end of the San Andreas fault so far.

The best news about the earthquake is that no injuries were reported, although it was widely felt. More than 4,300 people responded to the USGS Did You Feel It site from as far south as Monterey, north to Coos Bay and inland to the Lake Tahoe area. We also have a very good impact score. There have been numerous reports of objects falling from shelves in the Cape Mendocino and Eyl River Valley areas. Bear River Casino’s roof tiles fell and broken bottles littered the grocery store aisles. Several small landslides occurred and some roads were temporarily closed.

The biggest damage was the partial collapse of the Loleta Creamery wall. The Creamery is an old brick building that has been deserted since the Loleta Cheese Factory’s bankruptcy proceedings began more than five years ago. It was identified by the county as a dangerous structure due to its lack of reinforcement and general damage. Fortunately, no one was in the building at the time of the earthquake.

Here’s a quick rundown of the questions people ask me.

• How did they know so quickly that there is no danger of a tsunami? The earthquake was too small to pose a tsunami threat and was the wrong kind of fault movement. We need a vertical deformation of the sea floor to generate a tsunami, and it usually takes 7 degrees or greater. But if you were in the tsunami zone, the feeling of the earthquake should have prompted you to start moving away from the shore to higher ground. Most of our local radio stations did a good job of spreading the “No Tsunami Hazard” message once it was posted.

• Why did people have such different experiences of earthquake in locations that seemed close? There are many factors that contribute to the individual experience and distance from the epicenter is just one of them. Local geology plays a role – people who live near the bay on softer sediments tend to feel it more strongly than those who live on bedrock. Home construction also contributes – the upper floors of timber-framed houses tend to make the most of the impact. Even two people in the same building can have different experiences – if you are sitting or lying down, you will feel stronger than when you are walking or being active.

• Did this earthquake relieve tension and increase the possibility of a larger earthquake? An earthquake released tension – that’s what an earthquake is. But it is unlikely to play any role in the regional seismic threat. While 6.2 may seem large, it is small compared to the regional stress accumulation. There is a network of GPS instruments on the beach that can detect small changes in pressure and have been able to sense small changes from past earthquakes. The December 20 earthquake seems to have had no effect. Bottom line – we are still prepared for another strong earthquake and the next one could be bigger or closer to populated areas.

The most important take-home message is that we live in an earthquake country and we use this reminder to review your preparedness. When the ground begins to shake, please drop – cover – steady.

Laurie Dingler is Professor Emeritus of Geology at Humboldt State University and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of Preparedness magazine “Living on Shaken Earth”? Leave a message at 707-826-6019 or email [email protected].

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