Antelope Valley Earthquake - 07/08/21

This quake really shook things up in the Sacramento area:

An earthquake jolted Central California on Thursday afternoon, shaking up residents of the Sierra Nevada foothills and the San Joaquin and Sacramento valleys.

The shaking struck at 3:49 p.m., revised to a magnitude 6.0, and was centered in the Little Antelope Valley about four miles south of Coleville in Mono County, about 150 miles east of Sacramento.

Seismologists dubbed it the Antelope Valley earthquake, during a 5:30 p.m. briefing.

As of 7 a.m. Friday, more than 100 aftershocks have struck, including temblors as strong as 5.2 and 4.6, have shaken the area near the epicenter, according to the U.S. Geological Survey.

“It’s a region with known active faults,” USGS researcher Austin Elliott said, referring to the remote area on the back side of the Sierra. “This is a classic place geologists go to study.”

There have been about two-dozen earthquakes of this magnitude in the region in the past 50 years; Thursday’s was the largest in the vicinity since a 6.1 quake in 1994.

Dozens of people across California — in Sacramento to as far south as Los Angeles, north to the Oregon border, the Bay Area and beyond — reported feeling the quake to the U.S. Geological Survey. By 7 p.m., over 23,000 self-reported responses were given to the federal agency’s website.

I certainly felt the rolling waves pass through the house. My house is unusually sensitive to shaking:  In the past, I've felt distant offshore quakes at the Triple Plate Junction near Eureka that otherwise escaped notice in Sacramento. 

There was some confusion in the first few minutes. USGS reported a second quake near Stockton just seconds later, but quickly decided the second quake was a mirage - a reflection of the first quake.

Preliminary reports indicated two earthquakes had struck 25 seconds but 100 miles apart — but the U.S. Geological Survey revised the shaking and removed the report of a magnitude 4.8 quake in in San Joaquin County.

Elliott and other seismologists said the glitch is not uncommon as seismic waves cause reflections as they propagate. One of those reflections just happened to center in the community of Farmington, about five miles southeast of Stockton.

Elliott said the mistake happened because “our instrumentation is sparse (in areas) away from the large population centers,” and once officials reviewed the data, they updated it along with the magnitude of the actual quake.

The experimental ShakeAlert network apparently sounded off, and even though the information the system was working with was incomplete, people appreciated the warning:

The system initially got the earthquake’s magnitude wrong: called it a 4.8; later, it was upgraded to a 6.0.

The system thought the rupture happened about 31 miles south of where it actually did.  And at first it presumed it was detecting three different small earthquakes, rather than one large one.

Such details didn’t matter to some California residents, who welcomed the warning in time to take action.

“It worked. The whole point is to give you a little notice,” said a Lodi-based wine industry consultant who asked that his name not be used. He rushed from his office to the safety of a bathroom when the system’s app issued a startling emergency alert. “It’s really worth it.”