Seimic Network

Earthquakes occurring in the Long Valley area are recorded by a network of 18 seismic stations within the caldera and an additional 20 stations within a distance of 50 km beyond the caldera boundary. These stations comprise parts of both the Northern California Seismic Network (NCSN) operated by the U.S. Geological Survey in Menlo Park, California (stations names ending in M), and the Nevada seismic network operated by the University of Nevada, Reno (station names ending in R). The signals from the combined stations are shared by both networks. The signals telemetered to USGS headquarters in Menlo Park are processed along with data from the rest of the Northern California Seismic Network and archived on a mass storage device at the University of California, Berkeley - USGS data center (http://quake.geo.berkeley.edu)

Historical seismicity

A period of ongoing geologic unrest in the Long Valley area began in 1978, when a magnitude 5.4 earthquake struck 6 miles southeast of the caldera. This tremblor ended two decades of low quake activity in eastern California. The area has since experienced numerous swarms of earthquakes, especially in the southern part of the caldera and the adjacent Sierra Nevada (Bailey and Hill, 1990).

Most earthquakes occuring near Long Valley caldera have the broad band signature typical of tectonic or volcano-tectonics earthquakes with impulsive, high frequency P and S waves. With the Mammoth Mountain earthquake swarm in mid 1989, the seismic network begin to detect events with features typical of long-period volcanic earthquakes. These events may indicate the movement of magmatic fluids (Pitt and Hill, 1994).

A limited number of anomalous seismic events characterized by non-douple couple moment tensors and significant volumetric components have been observed during the 1997 episode of caldera deformation. These anomalous events may have been triggered by fault normal stress reduction due to high-pressure fluid injection or pressurization of fluid saturated faults due to magmatic heating (Dreger et al., 2000).

An update map of the caldera seismic activity is available on the Long Valley Observatory web page (http://lvo.wr.usgs.gov)