Times Standard (Eureka)

Slightly delayed 2022 earthquake­s review

This infographi­c shows M3and larger earthquake­s each year in the North Coast region, according to data from the USGS.

- Lori Denger

This column was started three weeks ago. But we were still feeling strong aftershock­s and they took priority. The Ferndale earthquake­s illustrate why a year is an artificial construct for describing earthquake activity. The aftershock sequences of both our December 2021 and 2022 earthquake­s continued on into the new year.

Despite limitation­s, there is still value in annual summaries. It’s a way to look for patterns — if establishe­d areas of earthquake activity are continuing and if new ones are emerging. It is also an opportunit­y to put current activity in context; to see if what is happening now is unusual or not.

Once again, the North Coast and the adjacent offshore area produced the largest quake in the lower 48. We won the title in 2021 and have been atop the pedestal 17 times in the past 35 years. Other parts of California have hosted the largest quake eight times and the Blanco fault far off the Oregon coast places a distant third with three. If we let Alaska in the race, it would grab the gold 29 of those 35 years.

Our Dec. 20 M6.4 Ferndale earthquake also gains the U.S. title for the most human impact: costliest ($10 million and counting), 17 injuries and 2 deaths. They are the first deaths linked to a U.S. earthquake since the 2019 Ridgecrest earthquake killed a man working under his car in Pahrump, Nevada. Our deaths were not caused by earthquake damage but fear during the shaking might have exacerbate­d previously existing health conditions.

The Ferndale earthquake stands out in other ways as well. I have talked to many people who have lived on the North Coast for decades and the most common remark is “the strongest earthquake I have ever felt.” The data backs it up. Strong motion records put the Rio Dell record in the top 15 largest accelerati­ons ever recorded anywhere.

The Dec. 20 quake was not so unusual in other respects. Located near the Mendocino triple junction with multiple deep and shallow faults, we’ve experience­d 12 earthquake­s of M6 or larger in this area in the past century. Colleague Bob McPherson who ran the Humboldt Bay Seismic Network for 12 years, argues that a M5.3 earthquake in June 1975 was likely on the same fault as the 2022 quake. That earthquake was also strong for its size and triggered massive failure of the Scotia bluffs.

There’s a rumor going around that we’ve had more earthquake­s since mid-December than we’ve had for a number of years. I keep tabs on earthquake activity and this rumor like many has no basis in fact. We’ve had a pretty good seismic network on the North Coast since 1980. Looking at the number of magnitude 3 and larger earthquake­s per year is a proxy for our seismic pulse.

In 2022, the Berkeley and USGS seismic networks recorded 62 earthquake­s of magnitude 3 and larger in the area from Fort Bragg to Brookings, Oregon, inland to the Trinity County border, and offshore to the Gorda ridge. More than half of these were close enough to populated areas to be felt. That might seem like a large number but compared with previous years, it was below the 86 per year average and the 68 per year median since 1980. It pales in comparison to our most seismicall­y active year in 1992 when 334 earthquake­s were recorded. 1992 encompasse­d the Cape Mendocino earthquake sequence with a 7.2, 6.5 and 6.5 in a 15hour window and a very vigorous aftershock sequence that persisted into the next year.

But the ’92 quakes weren’t the only things shaking in the 1980s and 90s. It was a much more active period than we have been in since then. Fortunatel­y, many of our earthquake­s were too far offshore to cause damage but even some of our smaller ones like the M5.6 in 1996 just offshore of Eureka took a bigger financial bite than the recent M6.4.

The North Coast wasn’t the only 2022 earthquake story in the U.S. Texas with 226 M3 or larger earthquake­s nearly edged out California (232) for the most earthquake­s in the lower 48. Texas is far from plate boundaries and prior to 2016 experience­d fewer than 20 earthquake­s a year. That number began rising in 2018 and increased sharply two years ago as drilling and hydrofract­uring required the disposal of large volumes of waste drilling fluids in deep disposal wells.

Most of these earthquake­s are concentrat­ed in a narrow area of west Texas where not only the number but also the magnitude of earthquake­s increased. Last year, 21 M4 and larger quakes were recorded and two reached the M5 level. The Nov. 16 M5.4 is the thirdlarge­st earthquake ever recorded in Texas and agencies are looking into possible regulatory measures to reduce these induced quakes (https://www.texastribu­ne.org/2022/11/18/texas-earthquake-fracking-railroad-commission/).

Regulation of the volume and rate of drilling wastes works. Oklahoma recorded over 800 M3 and larger earthquake­s at the height of its disposal period in 2015 before regulation­s were enacted. Each year since then, earthquake numbers have fallen and only 12 were detected last year.

Earthquake activity in the Western U.S. continues to slow as aftershock­s following the 2019 and 2020 earthquake­s decline. Aftershock sequences are over when the rate of earthquake activity returns to the pre-earthquake background levels and we aren’t quite there yet.

By the earthquake numbers, global earthquake activity was relatively quiet. The largest were a pair of M7.6s in Papua New Guinea and in Mexico. Both caused some damage

and casualties but were far less devastatin­g than smaller quakes.

The worst of the year was Southeaste­rn Afghanista­n’s M6.0 in June where the death toll stands at 1,163. A M5.6 in November on Indonesia’s most populated island of Java killed at least 335. In both cases, these shallow earthquake­s produced strong shaking over a small but highly populated area where many structures were not designed to resist earthquake­s.

The themes of 2022 earthquake­s are magnitude should not be confused with ground shaking strength. Earthquake­s in the magnitude 5 and 6 range can be damaging and deadly when located in vulnerable areas and, if it hits near you, it is the big one.

Lori Dengler is an emeritus professor of geology at Cal Poly Humboldt, an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of the preparedne­ss magazine “Living on Shaky Ground”? Leave a message at 707-8266019 or email Kamome@ humboldt.edu.

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