California earthquake felt over surprisingly large area

A 5.7 magnitude temblor Thursday night was the largest earthquake to shake California since 2008 and has generated curiosity from seismologists.

The temblor occurred in a rugged section of Northern California that has not been studied as throughly by scientists as Southern California and the Bay Area. There is less earthquake monitoring equipment there, and experts said they were surprised the quake was felt over such a large area. They plan to go up to the region to investigate further.

The magnitude 5.7 quake struck at around 8:47 p.m. Thursday, about 150 miles northeast of Sacramento; its epicenter was about 27 miles southwest of the town of Susanville and seven miles west northwest of Greenville.

The last quake similar to this magnitude, recorded at a magnitude 5.5, struck Chino Hills in Southern California in July 2008, said David Schwartz, an earthquake geologist for the Northern California USGS division in Menlo Park. The temblor caused little damage, but it was the most sizable quake to hit a metropolitan part of California since the much larger and destructive 1994 Northridge quake.

Thursday’s quake did occur in a zone where there are known active faults, Schwartz said, including a series of faults that extend through the northern end of Lake Tahoe all the way to Oregon. But 5.7 is the strongest magnitude recorded in the area. This mountainous eastern Sierra Nevada region, known for its lakes, rivers and national forests, has had about seven magnitude 4 earthquakes since the 1930s, Schwartz said.

Seismologists are still studying the intensity of Thursday’s ground shaking, and scientists said it will be a few days before they can identify with greater certainty which fault the temblor actually occurred on.

The region’s seismic activity is not as well studied compared to urban areas like Los Angeles or San Francisco, Schwartz said. There are more seismographs in place for these more populated, high-seismic areas, he said.

Since Thursday’s quake, officials have moved some of these instruments to the rugged region to monitor the aftershocks.

Within minutes of the first quake, more than 7,000 people reported feeling it, from across state borders into Oregon and Nevada and as far south as the San Francisco area, according to the U.S. Geological Survey website. Officials in Susanville and Sacramento said the quake set off a number of home and car alarms and rattled windows. A Chico resident told The Times he felt a slow roll that lasted about 30 seconds.

The quake itself was not a huge surprise for Schwartz’s USGS division, but “what was interesting was it was felt along an unusual distance,� he said. “Earthquakes in different parts of the state are felt over different distances. We just haven’t had that many examples of earthquakes in this part of the state, really, for comparison.�

“There are more interesting questions now than we have answers for, at present,� he said.

More than four dozen aftershocks, ranging up to a magnitude 4.9 in a zone of about 4 miles by 5 miles, have been recorded since the first quake, according to the USGS.

Schwartz said these aftershocks look to be “fairly standard.� Within the next week, there is a 20% chance an earthquake larger than magnitude 5 will strike the area and a 5% to 10% chance a quake of a magnitude greater than 5.7, according to a USGS probability report released Friday morning.

There have been no reports of injures in the areas closest to the epicenter, Plumas County Sheriff’s officials said. About 600 residents lost power for a brief period, and a water tank was ruptured due to the earthquake, affecting up to 1500 customers.

At least three homes in the area had moderate damage — collapsed chimneys and plaster cracking, authorities said. No structural damages have been reported .

“A 5.7 is still a moderate size earthquake, and earthquakes of that magnitude can occur really anywhere throughout the state,� Schwartz said. “But it’s large enough to generate interest and provide us some real info on how things work. We plan to keep looking at the sequence.�

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