Long-period earthquakes in the Long Valley Caldera Region, eastern California

Most earthquakes occurring near Long Valley caldera since the onset of recurring swarm activity in 1980 have the broad-band signature typical of tectonic or volcano-tectonic earthquakes with impulsive, high-frequency P and S waves. With the Mammoth Mountain earthquake swarm in mid 1989, we began detecting occasional events with a marked deficiency in energy above 5 Hz, a feature typical of long-period (LP) volcanic earthquakes. These events occur beneath the southwest flank of Mammoth Mountain at focal depths ranging from 10 to 28 km, distinctly deeper than the 2- to 10-km depth range for tectonic earthquakes in the area. The LP events occur at intervals ranging from weeks to months. Individual occurrences typically consist of several events within 2 to 5 minutes where the largest event has never been first. Magnitudes range from 0.5 to 1.8. The mid-crustal focal depths of the LP events are similar to occurrences at a number of areas with Holocene volcanism in Japan and the western United States. They may indicate the movement of magmatic fluids but do not necessarily indicate an imminent volcanic eruption.