HR: 10:35h
AN: V52B-02 INVITED     [Abstracts]
TI: Overview of seismicity associated with the 2004-2005 eruption of Mount St. Helens
AU: * Moran, S C
EM: smoran@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Vancouver, WA 98683 United States
AU: Malone, S D
EM: steve@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195 United States
AU: Qamar, A I
EM: tony@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195 United States
AU: Thelen, W
EM: wethelen@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195 United States
AU: Waite, G
EM: gwaite@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Horton, S
EM: horton@ceri.memphis.edu
AF: University of Memphis - CERI, 3876 Central Ave., Memphis, TN 38152 United States
AU: Lahusen, R G
EM: rlahusen@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Vancouver, WA 98683 United States
AU: Major, J J
EM: jjmajor@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Vancouver, WA 98683 United States
AB: Following 18 years of quiescence, an earthquake swarm on September 23, 2004, heralded the reawakening of Mount St. Helens (MSH). Seismicity during the 18-year quiescence consisted of shallow (< 3 km) and deep (3-10 km) volcano-tectonic (VT) earthquakes, with the deeper earthquakes occurring mostly during several extended periods of elevated seismicity that likely reflected intrusions of new magma. The swarm on September 23 intensified in a step-wise manner over the first week, and by the first phreatic explosion on October 1 earthquakes had increased significantly in size (Mmax = 3.5) and rate (1-2 per minute). The earthquakes also became shallower (< 1 km) and transitioned from VT to hybrid in character. Following a final phreatic explosion on October 5 earthquake magnitudes and rates decreased significantly. One of the remarkable aspects of MSH seismicity has been the occurrence of repetitive (in waveform) and regular (in time) shallow "drumbeat"" hybrid earthquakes that began in mid-October as lava dome extrusion commenced. With a few exceptions drumbeats occurred roughly every 40-60 seconds through April 2005 before progressively increasing to over 1000 seconds by mid-August. They occurred in association with extrusion of a fault-gouge-covered lava dome that emerged at a relatively constant rate, a linkage that strongly suggests a stick-slip origin for the drumbeats. The increase in inter-event times does not appear to directly correlate with decreasing extrusion rate, suggesting that dome extrusion has become increasingly aseismic over time. Another remarkable aspect is the occurrence of shallow "big"" earthquakes (M 2.5-3.5) starting in mid-November in association with the breakup of the first erupted dome. To date there have been over 250 of these events that have occurred in four temporally-defined groups, most associated with periods when extruded domes were breaking apart. Roughly half have impulsive arrivals, and many of these have dominantly down first motions consistent either with very shallow reverse faulting or a non-double-couple source (such as failure along a curvilinear fault). Broadband recordings show that very long-period (VLP) events with 10-30s periods accompany many of the ""big"" events as well as many of the drumbeat events. The impulsive big events may reflect large-scale lurches of the new dome, perhaps beginning as slow-slip events as indicated by VLP occurrence. Big events with emergent arrivals have recently been found via time-lapse photography to be associated with slumps of the dome, and thus appear to be directly related to failures within the dome following extrusion.
DE: 7280 Volcano seismology (8419)
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8419 Volcano monitoring (7280)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005