A. M. Pitt

Remotely Triggered Seismicity on the United States West Coast following the Mw 7.9 Denali Fault Earthquake

The Mw 7.9 Denali fault earthquake in central Alaska of 3 November 2002 triggered earthquakes across western North America at epicentral distances of up to at least 3660 km. We describe the spatial and temporal development of triggered activity in California and the Pacific Northwest, focusing on Mount Rainier, the Geysers geothermal field, the Long Valley caldera, and the Coso geothermal field. The onset of triggered seismicity at each of these areas began during the Love and Raleigh waves of the Mw 7.9 wave train, which had dominant periods of 15 to 40 sec, indicating that earthquakes were triggered locally by dynamic stress changes due to low-frequency surface wave arrivals. Swarms during the wave train continued for 4 min (Mount Rainier) to 40 min (the Geysers) after the surface wave arrivals and were characterized by spasmodic bursts of small (M 2.5) earthquakes. Dy- namic stresses within the surface wave train at the time of the first triggered earth- quakes ranged from 0.01 MPa (Coso) to 0.09 MPa (Mount Rainier). In addition to the swarms that began during the surface wave arrivals, Long Valley caldera and Mount Rainier experienced unusually large seismic swarms hours to days after the Denali fault earthquake. These swarms seem to represent a delayed response to the Denali fault earthquake. The occurrence of spatially and temporally distinct swarms of triggered seismicity at the same site suggests that earthquakes may be triggered by more than one physical process.

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.

Seismic evidence for magma in the vicinity of Mt. Katmai, Alaska

P-wave traveltime delays of as much as 0.9 sec are consistently observed at one seismic station from local and regional earthquakes 70 to 150 km deep. This station is on the southwest flank of Mt. Trident, the most recently active volcano within Katmai National Park, Alaska. Delays from local shallow earthquakes are typically less than 0.3 sec, suggesting that most of the major delay results from anomalous material at depths of more than a few kilometers. This station is located near the center of a bowlshaped low in the Bouguer gravity field that is approximately 15 km in diameter and more than 25 mgals deep. These anomalies suggest, but do not prove, the presence of considerable amounts of magma in the shallow part of the crust that could have been the source for all magma erupted in the vicinity of Mt. Katmai and Mt. Trident this century.

Deep fluid pathways beneath Mammoth Mountain, California, illuminated by migrating earthquake swarms

Earthquakes beneath a Californian volcano track fluid movement and outline a zone of partial melt in the deep and mid-crust. Although most volcanic seismicity is shallow (within several kilometers of the surface), some volcanoes exhibit deeper seismicity (10 to 30+ km) that may reflect active processes such as magma resupply and volatile transfer. One such volcano is Mammoth Mountain, California, which has also recently exhibited high rates of CO2 discharge at the surface. We perform high-resolution earthquake detection and relocation to reveal punctuated episodes of rapidly propagating seismicity at mid-crustal depths along a narrow fracture zone surrounding a body of partial melt. We infer that these earthquakes track dike intrusions or fluid pressure pulses associated with CO2 exsolution, suggesting that the deep plumbing system of Mammoth Mountain is an active conduit for fluid transport from the base of the crust to the surface.