May 29, 2013

Significant quake in the Northern Sierra

Posted by Austin Elliott

Last Thursday my heart sank when, driving home along I-80, my phone exploded with texts and calls after an earthquake rolled through Davis. My friends had generally felt it, or at least had noticed and been confused by it, but alas, after five years of waiting around for one up here, I had been separated from it by four rubber dampers, metal shock absorbers, and 70 miles per hour. Even back home and in the greater Sacramento area the relatively distant quake had been a silent, subtle swaying or shifting, clearly evident to those sitting around quietly at home after dinner, but difficult to perceive in any more active environment.

The quake was a 5.7–a rather large one, the likes of which only occur around once or less per year in CA. It emanated from the remote northern Sierra Nevada, and was felt over a wide swath of Northern California, southern Oregon, and West-Central Nevada. The source parameters indicate oblique-normal motion (a nice illustrated primer on what that means), I suspect resulting from slip on the west-northwest striking right-lateral nodal plane, based on regional tectonics. The candidate faults that slipped include the Skinner Flat fault, the unfortunately named (really, I can’t help giggling) Butt Creek Fault, or an unmapped northern extension of the Indian Valley fault. All these structures are visible pointing towards the cluster of aftershocks in this Google Earth screen grab:

Google Earth scene of the May 23 M5.7 Plumas County earthquake and its aftershocks. Mount Lassen is the snowy peak in the background, and Lake Almanor is the site of the quakes. Northwest striking right-normal faults dominate the active structures, but are poorly organized in a complex and probably not thoroughly mapped geometry.

The faults in the area are coarsely mapped due to their low level of activity, discontinuous nature, and rugged forested terrain. In the geologic scheme of things, the tectonic setting of this area is immature or transitional, rapidly changing as the convergent subduction zone to the west migrates northward and right-lateral motion takes over like it already has farther to the south.

In the societal scheme of things, this was a relatively modest earthquake, widely but weakly felt, except in the epicentral region where remote mountain towns suffered a great deal of damage to chimneys, windows, decks, and other weak structures. Plumas County has claimed the earthquake caused $1 million of property damage.

Damage to a home near Lake Almanor after Thursday's 5.7 quake

You can watch the modest effects of the earthquake in the town of Greenville, very near the epicenter. Other than this, not much thrilling video footage of it exists:

There were some alarmist local news reports trying to scare up a connection to Lassen Volcano, which I won’t bother to link to here since the connection is fear-mongering speculation and there’ve been no signs of any change at the volcano. The only notable connection is that this quake happened a mere two days after the [98th] anniversary of Mt. Lassen’s last eruption. Neato connection, if you like those sorts of coincidences. As for me and the perpetual disappointment of missing earthquakes… I guess I’ll have to wait for the next one.