Plumas County earthquakes reveal new, active faults under Lake Almanor

To geology expert Todd Greene, the earthquake that struck the Lake Almanor area Thursday afternoon represents something new that needs to be studied further.

The 5.5-magnitude quake that rumbled Thursday afternoon beneath the lake did not happen on any of the known underground faults in the area, said Greene, a geology professor in Chico State University's Department of Earth and Environmental Sciences.

"This is an area where where there are known faults to exist. The faults, however, have not been known to be active, or at least active within the last 15,000 years," Greene said.

"Having these earthquakes occur not only close to these faults, but underneath Lake Almanor is very new information. Not only is it telling us that there are other faults underneath Lake Almanor that we didn't know about, but it's telling us that these faults are active," he said.

While area residents reported they felt the shaking and businesses reported minor damage, Plumas County officials reported no serious damage from the quake. Residents reported feeling the earthquake as far away as Stockton.

Since the quake hit at 4:19 p.m. Thursday, there have been more than 50 aftershocks, including one that registered 5.2 magnitude early Friday morning, according to the Southern California Earthquake Data Center. Eight of the aftershocks were magnitude 3.0 or higher, according to the U.S. Geological Survey.

Greene said the lakebed should be mapped out to learn more about what faults lie beneath, especially since the Canyon Dam, which creates Lake Almanor, sits on a known fault just southeast of the epicenter of Thursday's quake.

Pacific Gas and Electric Company officials inspected the dam and the many other PG&E facilities in the area and found no problems, said Paul Moreno, a company spokesman.

The quake that rattled the area Thursday afternoon was in an area underneath the lake that does not include any faults included on USGS fault maps, Greene said.

Greene said Thursday's quake and aftershocks aren't necessarily a precursor to a larger seismic event.

But with Lassen Peak nearby — a 10,457-foot tall volcano that last erupted from 1914 to 1917 — officials with the USGS have been closely monitoring earthquake activity, said Jim Richardson, superintendent at Lassen Volcanic National Park.

The park includes many includes many volcanic feature, including boiling mud pits, boiling pools and steaming gas vents known as fumaroles.

The park has not seen an uptick in volcanic or seismic activity since the Lake Almanor earthquake to the south, Richardson said.

"But it's smart to be thinking of that connection because certainly earthquake activity is one of the primary signs of volcanic activity potentially increasing," Richardson said.

Evidence of looming volcanic activity would include an increase or decrease in gases being released from the many vents in the park. There would also be deformations in the earth, such as parts of the ground that would begin to uplift or sink, he said.

So far, though, the seismic activity has remained south of the park, Richardson said.

"The USGS does have a bunch of monitoring equipment in the park in very, very specifically chosen locations. And at least right now they're they're not indicating any changes or upticks in activity," Richardson said.

Reporter Damon Arthur welcomes story tips at 530-338-8834, by email at damon.arthur@redding.com and on Twitter at @damonarthur_RS. Help local journalism thrive by subscribing today!

This article originally appeared on Redding Record Searchlight: Plumas County earthquakes reveal new, active faults under Lake Almanor