Intermediate - Term Declines in Seismicity at Mt . Wrangell

The Mw 7.9 Denali fault earthquake ruptured segments of the Susitna Glacier, Denali, and Totschunda faults in central Alaska, providing a unique opportunity to look for intermediate-term (weeks to months) responses of active volcanoes to shaking from a large earthquake. The Alaska Volcano Observatory (AVO) monitors 24 volcanoes with seismograph networks. We examined one station per volcano. Digitally-filtered data for the period four weeks before to four weeks after the mainshock were plotted at a standard scale. Mt. Wrangell, the closest volcano to the epicenter (247 km), had a background rate of 16 events/day. For the following 30 days, however, its seismicity rate dropped by 50%. Mt. Veniaminof (1400 km from the epicenter) had a rate of 8 seismic events/day, but suffered a drop in seismicity by 80% after the maishock; this may have lasted for 15 days. The seismicity at both volcanoes is dominated by long-period seismic events. With the exception of Martin and Novarupta volcanoes, the other 20 volcanoes showed no changes in seismicity attributable to the Denali fault earthquake. We conclude that the changes in seismicity observed are real, and are related to the Denali fault earthquake. These seismicity drops are in strong contrast to cases of short-term triggered seismicity increases observed at other volcanic systems such as Martin-Novarupta, Mt. Rainier, Yellowstone, Mammoth Mountain, and The Geysers, Coso and Cerro Prieto (Mexico) geothermal fields. This suggests that fundamentally different mechanisms may be acting to modify seismicity at volcanoes.