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Earthquake Triggering at Alaskan Volcanoes Following the 3 November 2002 Denali Fault Earthquake

¹ U.S. Geological Survey—Cascades Volcano Observatory
1300 SE Cardinal Ct., Bldg. 10
Vancouver, Washington 98683
smoran{at}usgs.gov
 (S.C.M.)

² U.S. Geological Survey—Alaska Volcano Observatory
4200 University Dr.
Anchorage, Alaska 99508
jpower{at}usgs.gov
jca{at}usgs.gov
 (J.A.P., J.C.-A.)

³ Alaska Volcano Observatory
Geophysical Institute, University of Alaska Fairbanks
903 Koyukuk Dr.
Fairbanks, Alaska, 99775-7320
stihler{at}giseis.alaska.edu
jjsanchez{at}giseis.alaska.edu
 (S.D.S., J.A.S.)

The 3 November 2002 M_W 7.9 Denali fault earthquake provided an excellent opportunity to investigate triggered earthquakes at Alaskan volcanoes. The Alaska Volcano Observatory operates short-period seismic networks on 24 historically active volcanoes in Alaska, 247–2159 km distant from the mainshock epicenter. We searched for evidence of triggered seismicity by examining the unfiltered waveforms for all stations in each volcano network for 1 hr after the M_W 7.9 arrival time at each network and for significant increases in located earthquakes in the hours after the mainshock. We found compelling evidence for triggering only at the Katmai volcanic cluster (KVC, 720–755 km southwest of the epicenter), where small earthquakes with distinct P and S arrivals appeared within the mainshock coda at one station and a small increase in located earthquakes occurred for several hours after the mainshock. Peak dynamic stresses of 0.1 MPa at Augustine Volcano (560 km southwest of the epicenter) are significantly lower than those recorded in Yellowstone and Utah (>3000 km southeast of the epicenter), suggesting that strong directivity effects were at least partly responsible for the lack of triggering at Alaskan volcanoes. We describe other incidents of earthquake-induced triggering in the KVC, and outline a qualitative magnitude/distance-dependent triggering threshold. We argue that triggering results from the perturbation of magmatic-hydrothermal systems in the KVC and suggest that the comparative lack of triggering at other Alaskan volcanoes could be a result of differences in the nature of magmatic-hydrothermal systems.

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