Remotely Triggered Seismicity on the West Coast Following the M 7.9 Fault Earthquake

S. G. Prejean, D. P. Hill, E. E. Brodsky, S. E. Hough, M. J. S. Johnston, S. D. Malone, D. H. Oppenheimer, A. M. Pitt, K. B. Richards-Dinger

1. Abstract

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 3,660 to 4000 km. This is the most far-reaching and most thoroughly documented case of remotely triggered seismicity yet observed. We describe the spatial and temporal development of triggered activity in and the . We focus on activity at in central Washington, the Geysers geothermal field in northern

California, the Long Valley caldera area in , and the Coso geothermal field in southeastern California.

The onset of triggered seismicity at each of the areas we describe began during the Love and Raleigh waves of the M 7.9 wavetrain which had dominant periods of 15 to

40 seconds. These observations indicate that earthquakes were triggered locally by dynamic stress changes due to low frequency surface wave arrivals. Swarms during the wavetrain continued for ~ 4 minutes (Mt. Rainier) to ~ 40 mintues (the Geysers) after the surface wave arrivals, and were characterized by spasmodic bursts of small (M < 2) brittle-failure earthquakes. Dynamic stresses within the surface wavetrain at the time of the first triggered earthquakes range from 0.01 MPa (Coso) to 0.09 MPa (Rainier). The dynamic stress estimates associated with the onset of triggered activity at Coso are among the smallest observed. In addition to the swarms that began during the surface

1 wave arrivals, Long Valley caldera and Mount Rainier, both young volcanic systems,

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 seismicity may be triggered by more than one

physical process.

2. Introduction

The onset of remotely triggered seismicity at many volcanic and geothermal centers across the western United States immediately following the 1992 M 7.3 Landers

earthquake (Hill et al., 1993, Anderson et al., 1994, Hill et al., 1995) surprised many

Earth scientists and raised many intriguing questions about the physical processes

occurring in the Earth’s crust that initiate earthquakes. Static stress changes from the

Landers rupture, for example, dropped below tidal stresses at distances beyond 300 km

and were thus too small to explain the triggered seismicity observed at distances from

400 km as great at 1250 km (Hill et al., 1993). Since Landers, similar episodes of

remotely triggered seismicity have been documented in the western United States

following the 1999 M 7.1 Hector Mine, California earthquake (Gomberg et al., 2001), in

Katmai National Park following the 1999 M 7.0 Karluk Lake, Alaska earthquake and the

2000 M 6.9 Kodiak Island Alaska earthquake (Power et al., 2001, Moran et al., this issue), in Greece following the 2000 M 7.4 Izmit, Turkey earthquake (Brodsky et al.,

2000), and in the Geysers geothermal area in following several large earthquakes (Stark and Davis, 1996). These studies provide strong evidence that remotely triggered earthquakes are initiated dynamically during the passage of seismic

2 waves from a distant large earthquak