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

Abstract

The M w 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 M w 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 ϳ4 min (Mount Rainier) to ϳ40 min (the Geysers) after the surface wave arrivals and were characterized by spasmodic bursts of small (M Յ 2.5) earthquakes. Dynamic stresses within the surface wave train at the time of the first triggered earthquakes 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.

Triggered Seismic Events along the Eastern Denali Fault in Northwest Canada Following the 2012 Mw 7.8 Haida Gwaii, 2013 Mw 7.5 Craig, and Two Mw>8.5 Teleseismic Earthquakes

“…S6c), near the Klutlan Glacier terminus, where there is also a historical record of shallow (< 10 km depth) seismicity and regional events flagged as glacial events by Alaska Earthquake Center analysts (West, 2014). Because of the poor data quality, it is difficult to distinguish whether the sources are icequakes similar to those observed in Antarctica triggered by the M w 8.8 Chile earthquake (Peng et al, 2014) or shallow earthquakes like those triggered near active volcanoes or geothermal regions (Prejean et al, 2004;West et al, 2005). Deducing which type of events actually occurred-icequakes or shallow earthquakes-would require more stations in the region.…”

Long‐range triggered earthquakes that continue after the wave train passes

“…The identification of these sequences as triggered earth-quakes is discussed elsewhere [Hill et al, 1993;Prejean et al, 2004]. All sequences with !100 cataloged earthquakes within 40 km and 5 days are included.…”

“…For instance, as surface waves passed through geothermal sites like Yellowstone and Long Valley after earthquakes like the 1992 M w 7.3 Landers and 2002 M w 7.9 Denali events, local earthquakes flared up [Hill et al, 1993;Stark and Davis, 1996;Prejean et al, 2004]. However, the increased seismicity did not end with the end of the surface waves.…”

Remotely Triggered Earthquakes Following Moderate Mainshocks (or, Why California Is Not Falling into the Ocean)

“…Following recent large earthquakes, triggered seismicity was observed to occur preferentially (although not exclusively; e.g., Bodin and Gomberg, 1994) in regions such as Long Valley Caldera, The Geysers, and the Salton Sea region (e.g., Gomberg and Davis, 1996;Stark and Davis, 1996;Prejean et al, 2005). Triggering has also been observed at geothermal and volcanic sites elsewhere around the world (e.g., Power et al, 2001), leading some to conclude that triggered earthquakes do not occur in other seismotectonic settings (Scholz, 2003).…”