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

Abstract

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.