On November 3, 2002 a Mw 7.9 earthquake ruptured segments of the Susitna Glacier,
Denali, and Totschunda Faults in Alaska providing a unique opportunity to look for
responses of active volcanic centers to shaking from a large regional earthquake. The
Alaska Volcano Observatory (AVO) monitors 24 volcanoes with seismograph networks.
We examined one station for each volcano, typically within 5 km from the vent, unless
noise, or other factors rendered the data unusable. Data were digitally filtered between
0.8 and 5 Hz to enhance local earthquakes. At Mt. Wrangell and Mt. Veniaminof the
seismicity is dominated by earthquakes of the low-frequency type. Data for the period
six weeks before to six weeks after the Mw 7.9 earthquake were then plotted at a standard
scale used for AVO routine monitoring. Wrangell volcano, located less than a rupture
length from the epicenter, had a background rate of 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 40 days, however, its seismicity rate dropped by a factor of two. It is known
that Wrangell increased its heat output after the Mw 9.2 Alaska earthquake of 1964 and
again after the Mw 7.5 St.Elias earthquake of 1979. Mt. Veniaminof is located more than
four rupture lengths from the epicenter; it has had recent mild eruptions, and a rate of
about 10 seismic events per day on station VNNF. The volcano suffered a drop in
seismicity by a factor of two after the earthquake; this may have lasted for 15 days. The
detection thresholds of the seismograph networks at Mt. Wrangell and Mt. Veniaminof
are low and the stations operated normally during the time of observation, thus we infer
that the drops in seismicity are a response of the volcanoes to the Mw 7.9 earthquake.
Most of 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. This is in strong contrast to the cases of
short-term seismicity increases observed at other volcanic systems such as Katmai,
Mount 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.