Saturday, October 3, 2009

The Keeler Earthquake Swarm




The U.S. Geological Survey (and my department seismometer) is reporting a vigorous swarm of small and moderate quakes about seven miles south of Keeler, California. The swarm has included two 5+ magnitude shocks, and at least 22 events between magnitude 3 and 5. Smaller events number in the hundreds.

Keeler is a small village along the former shoreline of Owens Lake at the south end of the Owens Valley. The pictures provide some context: the Owens Valley is the first of the Basin and Range Province grabens east of the Sierra Nevada, with the White/Inyo Mountains forming the east side. The top picture shows the Sierra crest, including the peaks in the vicinity of Mt. Whitney, the tallest mountain in the lower 48 states. The bouldery Alabama Hills and the town of Lone Pine are in the bottom of the valley, center-left, and the Inyo Mountains form the right-side skyline. With the valley floor at 4,000 feet and the surrounding mountains ranging up to 14,000 feet, the valley is twice as deep as the Grand Canyon. Although the Owens River flows through the valley (sort of), it is a faulted graben, not a river-cut valley.

The second photo (from the same vantage point on the Cottonwood Lakes road) shows Owens Lake, the south end of the Inyo Mountains, and the domes and craters of the Coso Mountains Volcanic Field on the right. Owens Lake is one of the pluvial ice age lakes that formed when glacial meltwater from Sierra glaciers drained into the desert. Unlike most of the dry lakes across the Basin and Range, Owens Lake actually had water until the 1920's. Los Angeles diverted the streams that flowed into the lake and sent the water south through a massive aqueduct to the growing city. Keeler was a depot for silver and lead being mined high in the Inyo Mountains at the Cerro Gordo Mine in the late 1800's. Ores were carried down to the town by aerial tramway, and then loaded on a steamship to cross the lake to railroad station.

The epicenters of the swarm would be in about the center of the second photo. Why are there quakes here? As noted before, the entire valley is a fault graben, so quakes are not at all unusual. One of California's largest historical earthquakes, the 1872 Lone Pine earthquake (mag. 7.8 or so) happened just north of here. Swarms have happened in the local region in the past, including some magnitude 5 events in the 1990's.

The Coso Volcanic Field lies just to the south. A collection of basalt flows and rhyolite domes, it was last active around 40,000 years ago, though a few cones may be younger.

UPDATE: not of the earthquake, but of the Coso Volcanic Field. It might be an actively forming metamorphic core complex! In 2 to 4 million years, the detachment surface may be exposed. I always enjoy learning something I didn't know before...

UPDATE #2: Loads of info on the quake swarm on the SCSN Pages here. The largest quakes are showing right lateral focal plane solutions, which is in keeping with the regional structural trends on the Owens Valley floor. This strongly suggests to me that the quakes are tectonic and not related to any kind of volcanic activity (but I'm not claiming expertise in the matter).

UPDATE #3: Reports of sand boils on the Owens Lakebed (see the comments) have been confirmed by the SCSN and USGS office in Pasadena (personal communications, no link).

UPDATE #4: See down in the comments for a correction on the history of Keeler from Roger Vargo, who is the sometimes caretaker at Cerro Gordo. Thanks for the correction, I was operating off the memory of a field trip from 25 years ago.