Abstract
As surface waves from the 26 December, 2004 earthquake in Sumatra swept across Alaska they triggered an 11-minute swarm of 14 local earthquakes near Mt. Wrangell volcano, 11,000 km from the epicenter. Earthquakes occurred at intervals of 20-30 s in phase with the largest positive vertical ground displacements during the Rayleigh surface waves. This correlation is possible because of the combination of unusually long surface waves and seismic stations in close proximity to the local earthquakes. This phase of Rayleigh wave motion is dominated by horizontal extensional stresses reaching 25 kPa. These observations imply that local events were triggered by simple shear failure on normal faults.

Data
Data for this event sequence is available as SAC trace files.
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Stress field
The correlation of event triggering with a specific phase of Rayleigh wave motion permits the determination of the specific stress field responsible for initiating local earthquakes.
details of stress calculations

Publications
West, M., J. Sánchez, and S. McNutt (2005) Periodically Triggered Seismicity at Mount Wrangell, Alaska, After the Sumatra Earthquake. Science 20 May 2005: 1144-1146.

West, M., J. Sanchez, S. McNutt (2005), Periodically-triggered seismicity at Mt. Wrangell volcano following the Sumatra earthquake. Seismological Society of America, April 26-29, Lake Tahoe, NV. (mini-poster)

Mt. Wrangell summit crater (at right)
This view is to the west looking toward Gennallen. The north crater, a few hundred yards in diameter, is one of three craters on Mt. Wrangell. Steam is normal from this crater and can often be seen from Glennallen. A seismic station, WANC, on the left shoulder of the crater (just out of site) was one of four stations in the area which recorded the small earthquakes following the 26 December 2004 Sumatra earthquake. These stations are mainatained by the Alaska Volcano Observatory - a joint program of the United States Geological Survey (USGS), the Geophysical Institute of the University of Alaska Fairbanks (UAFGI), and the State of Alaska Division of Geological and Geophysical Surveys (ADGGS).
Photo credit: Guy Tytgat, Alaska Volcano Observatory at the University of Alaska Fairbanks