HR: 0800h
AN: V51D-0781    [Abstracts]
TI: Observational Evidence for a Rapidly Moving Seismic Source at Mount St. Helens
AU: * Prejean, S
EM: sprejean@usgs.gov
AF: USGS Alaska Science Center, Alaska Volcano Observatory, Anchorage, AK 99508, United States
AU: Snieder, R
EM: rsnieder@mines.edu
AF: Center for Wave Phenomena, Department of Geophysics, Colorado School of Mines, Golden, CO 80401, United States
AU: Johnson, J B
EM: jeff.johnson@ees.nmt.edu
AF: Department of Earth and Environmental Sciences, New Mexico Tech, Socorro, NM 87801, United States
AB: We analyzed highly repeatable waveforms recorded at Mount St. Helens on October 9, 2004. The earthquake clones, or "drumbeats", from this time interval marked the onset of lava extrusion in the ongoing eruption. Hourly stacks of these waveforms correlate extremely well, with the correlation coefficient decreasing as the time separation between these stacks increases. We used coda wave interferometry [Snieder and Vrijlandt, 2005] to relate the correlation coefficient between hourly stacks recorded at different times to the migration of the source that excites these waveforms. Coda wave interferometry uses the changes in the seismic coda to diagnose changes in the medium, or in the excitation of these waves. We applied our analysis to different frequency bands and to different time windows within the coda. Results suggest that the source mechanism is constant over the day-long time interval studied while the Green's function is changing due to changes in the source location or physical properties of the surrounding crust. The consistency of the inferred source migration, as measured at distinct stations, argues that changing source location, instead of changes in velocity structure along the ray path, causes the observed change in the waveforms. The estimated source migration is about 55 m/day. This value is about a factor 10 larger than estimates of the vertical displacement of the plug in Mount St. Helens. This implies that if the seismic events are caused by brittle failure on the edges of the extruding plug, the seismicity moves with respect to the plug. Snieder, R. and M. Vrijlandt, M., Constraining Relative Source Locations with Coda Wave Interferometry: Theory and Application to Earthquake Doublets in the Hayward Fault, California, J. Geophys. Res., 110, B04301, 10.1029/2004JB003317, 2005.
DE: 7215 Earthquake source observations (1240)
DE: 7280 Volcano seismology (8419)
DE: 8414 Eruption mechanisms and flow emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting