Scientists: Sierra Nevada quakes not rare
June 6, 2013
NEVADA COUNTY, Calif. — The 5.7 magnitude earthquake felt throughout the region — and as far away as Oregon — last Thursday evening was the largest earthquake to hit California since 2008, when the Chino Hills earthquake shook Southern California.
The moderate earthquake, which struck about 11 kilometers below the surface near Greenville and about 100 miles north of Grass Valley, was followed by dozens of small aftershocks, prompting the United States Geological Survey to dispatch a team of equipped scientists to the area.
"The distribution of seismographs in that area is not as dense as, say, San Francisco or Los Angeles," said USGS earthquake geologist David Schwartz, adding population and seismic activity account for the allocation of resources.
A series of fault lines that run north/south from Lake Tahoe through Lake Almanor up into Oregon accounts for earthquakes that occur in the Sierra Nevada with moderate frequency, Schwartz said.
Schwartz warned that Thursday’s 5.7 earthquake could be what is referred to as a “foreshock”
— a moderate earthquake that typically precedes a more powerful earthquake.
The fault lines run underneath the eastern slope of the Sierra Nevada, where major geological plates are being slowly pulled apart, causing occasional seismic activity.
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While activity is centered on the eastern slope, a younger fault exists on the western side that sometimes causes disruption.
"I'm old enough to remember the Oroville earthquake in 1975," Schwartz said. The earthquake was also a 5.7 magnitude.
"Still, the western slope is one of the quietest parts of California, along with the Central Valley," Schwartz said.
The California Division of Safety of Dams had scheduled a routine inspection of three dams in proximity to the earthquake epicenter, and inspectors were able to help perform inspections in the aftermath of the quake.
"We were on it," said Bill Pennington, an engineer for the division. "For the whole system, I haven't seen anything of significance."
The system includes dams at Lake Almanor, Butt Valley and Indian Ole.
For Nevada County, Nevada Irrigation District inspectors took stock of all the dams within its purview to ensure safety.
"We've checked all the dams," said Nevada Irrigation District Hydroelectric Manager Bill Morrow. "They are all in good shape and (we) found nothing to be wary of in any of them."
In general, earthquakes do not result in dam failures, Schwartz said, qualifying his statement by saying the Van Norman Dam nearly failed during the San Fernando earthquake in 1971.
In some cases, intense seismic activity indicates possible volcanic activity.
Schwartz said scientists in the volcanic division of the USGS have begun to mobilize to study any potential impacts between the earthquake near Greenville and a possible eruption of Mount Lassen.
Mount Lassen is officially dormant but was also the most recently active volcano in California, as it erupted in May 1915, causing a column of ash that could be viewed from as far away as Eureka.
"The faults where the earthquake occurred are very distinct from the Mount Lassen area," Schwartz said. "However, it is a natural question, and our volcano experts are currently attempting to determine what possible effect, if any, the earthquake could have on Mount Lassen."
Schwartz warned that Thursday's 5.7 earthquake could be what is referred to as a "foreshock" — a moderate earthquake that typically precedes a more powerful earthquake.
"It could be that the earthquake released stress, and everything has settled," Schwartz said. "Or it could be we will see something larger down the road."