Laurie Dingler | 2021 earthquakes at the bottom 48 – Times Standard

The year is an artificial unit of time when talking about earthquakes. Several earthquakes in the past year are part of a sequence that started earlier, and we’re already seeing 2022 activity rooted in 2021. With that in mind, here’s a quick overview.

The focus today is on 48 contiguous states and adjacent territorial waters. The USGS lists 890 earthquakes of magnitude 3 and greater, which is the level of magnitude people would likely have felt if people lived nearby. Ninety-two were in the M4 range, nine were M5s and two were M6s.

Many of you felt the biggest earthquake of the year on December 20th in the Mendocino Tri-junction area west of Petrolia. M6.2 has been reported by more than 4,300 people. It is not uncommon for the North Coast to experience the largest earthquake of the year. Since 1990, the North Coast region has claimed this title 16 times, and is the only region in the mainland United States to claim it with double digits.

Sequence 6.2 is not yet finished. Since December 20, 200 smaller earthquakes have been recorded in the Triple Junction area inside and outside Cape Mendocino. An aftershock of M4.6 was widely felt on January 5, the second largest aftershock to date. Aftershocks can last for weeks or months and there is still a small but real chance of another M5 or 6 breaking out of this sequence.

The second largest earthquake of 2021 was midway between Lake Tahoe and Mono Lake. The July 8, M6.0 earthquake in Antelope Valley left a much greater impression than our North Coast 6.2 earthquake. More than 25,000 people have submitted reports on the USGS Did You Feel It website from as far away as San Diego, Portland and Salt Lake City.

The Antelope Valley earthquake has its roots in 2019. It is part of the elevated activity in California’s eastern shear that began in 2019 with the M7.1 Ridgecrest earthquake and continued into May 2020 with M6.5 in western Nevada (Monte Cristo) and 5.8 near Lone Pine in the following month. Aftershock activity continued into 2021 in both the Ridgecrest and Monte Cristo area. The 4.8 aftershock on January 4 near Mina, Nevada indicated that the Monte Cristo sequence was still far from over.

The good news about these earthquakes is that they did not cause much damage. A wall collapsed in the Loleta Creamery’s unbricked building on December 20, and earthquakes in the North Coast and Antelope Valley both caused landslides, damaged roofs, and caused objects to fall from shelves, but no injuries were reported from either.

California was the largest earthquake producer in 2021, with 389 m3 earthquakes reported. This is about the same for 2020, but much lower than the more than 1,200 in 2019. It reflects the 2019 Ridgecrest’s rise and strong streak. In the decade prior to 2019, the country averaged about 200 cubic meters per year. Continued seismic activity in the eastern part of the state keeps our numbers high.

Texas took second place with a 196-cubic-meter earthquake in 2021. That’s more than double the 88 earthquakes in 2020 and well above the 1-2 average earthquake in the decades before 2010. I’ve written about increases in Texas and Kansas earthquakes three before weeks, but until I counted the yearly totals, I didn’t realize how important the jump was. These earthquakes are likely caused by the injection of waste fluids from drilling operations into deep wells.

Texas has recognized the problem of induced earthquakes for more than a decade. A combination of earthquakes in the Dallas-Fort Worth area between 2012 and 2015 led to regulations regarding waste injection in that area. Last year’s activity was concentrated in West Texas, where nearly 75% of the state’s activity was concentrated along an area 50 miles south of the New Mexico border. Not only did the number increase, but the size also increased. In 2021, 13 earthquakes reached the M4 scale, some strong enough to be felt in El Paso and SE New Mexico.

Nevada, 69, and Idaho, 67, nearly tied for third. Either way, the earthquakes correlate with a continuous aftershock sequence of earthquakes of magnitude 6.5 in 2020. Aftershock activity has slowed this year – Nevada ranked first in earthquakes on the mainland in 2020 with 545 M3s and Idaho had nearly 300 .

There have been no major surprises in American activism. For the sixth year in a row, seismic activity decreased in Oklahoma. From 2014 to 2016, Oklahoma was the most seismic region in the lower 48 with a peak of 863 m3 earthquakes in 2015. Like Texas, this seismic activity is associated with waste fluid injection. Regulations on injection rates and volumes imposed in 2015 and reduced Oklahoma oil production since 2019 likely accounted for just 26 earthquakes last year, the lowest annual number since the fluid injection era began.

One of the year’s busiest sequences didn’t make it to the Contiguous 48 category. Between November 19 and December 14, 160 earthquakes of magnitude 3 or greater were recorded on the Blanco Fault 220 to 300 miles west of the Oregon coast placing it outside the territorial limits of the United States. The sequence produced 18 earthquakes with a magnitude of 5 and two with a magnitude of 5.8. But even these larger earthquakes were only mildly felt on the coast due to the great distance. Oregon ranked fifth with all but four of the thirty-six earthquakes on a portion of the Blanco Fault within 200 miles of the coast.

What does all this bode for 2022? The aftershocks will continue, and there will be surprises, as we approach one year to the next massively devastating earthquake in the United States.

Note: Almost all of this information can be obtained from https://earthquake.usgs.gov USGS – Explanations and errors are mine.

Laurie Dingler is Professor Emeritus of Geology at Humboldt State University, and an expert on 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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