On November 3, 2002 a Mw 7.9 earthquake ruptured part of the Susitna Glacier, Denali, and Totshunda faults in Alaska providing a unique opportunity to look for intermediate-term (days to months) responses of Alaskan volcanoes to shaking from a large regional earthquake. The Alaska Volcano Observatory (AVO) monitors 24 volcanoes with seismograph networks. We examined seismograms of at least one station at each volcano, generally the closest (typically 5 km from the vent) unless noise, or other factors made the data unusable. Data were digitally filtered between 0.8 and 5 Hz to enhance the signal-to-noise ratio. The seismicity at Mt Wrangell and Mt. Veniaminof volcanoes typically consists of low-frequency earthquakes. Data for the period four weeks before to four weeks after the Mw7.9 earthquake were then plotted at a standard scale used for AVO routine monitoring.
Mt. Wrangell, the closest volcano to the epicenter, had a background rate of about 15 events per day. Data from station WANC could not be measured for 3 days after the Mw 7.9 earthquake because the large number and size of aftershocks precluded identification of local seismicity. For the following 30 days, however, its seismicity rate dropped by a factor of two. Mt. Veniaminof volcano, which has had recent mild eruptions and a rate of approximately 10 seismic events per day on station VNNF, suffered a drop in seismicity by a factor of two after the earthquake; this may have lasted for 15 days. We infer that these changes, albeit subtle, may be related to the Denali fault earthquake. It is known that Mt. Wrangell increased its heat output after the Mw 9.2 Alaska earthquake of 1964 and again after the Ms 7.1 St.Elias earthquake of 1979. The other 22 volcanoes showed no changes in seismicity that can be attributable to the Mw 7.9 earthquake.
We conclude that intermediate-term seismicity drops occurred at Mt. Wrangell and Mt. Veniaminof volcanoes, in strong contrast to cases of short-term triggered seismicity increases observed at volcanic systems such as Katmai, Mount Rainier, Yellowstone, Mammoth Mountain, and The Geysers, Coso and Cerro Prieto (northern Mexico) geothermal fields. This suggests that fundamentally different mechanisms may be acting to modify seismicity at volcanoes.