HR: 1340h
AN: V53D-1600    [Abstracts]
TI: Automated Earthquake Detection During 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: 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: Buurman, H M
EM: s0236087@sms.ed.ac.uk
AF: University of Edinburgh, Old College, Edinburgh, EH8 9YL United Kingdom
AB: Seismicity associated with the 2004-2005 eruption of Mount St. Helens (MSH) has featured well over 600,000 individual earthquakes. To date only a small fraction have been located by an analyst, and accurate locations for all events will require time-intensive analyst review as well as cross-correlation and relative relocation analyses. In the interim we employed amplitude-based algorithms to detect events on individual stations and track basic event parameters such as inter-event times and peak amplitudes throughout the course of the eruption. The algorithms had to be fine-tuned to accommodate occasional noise glitches and differences in sensor types, and automated picks were reviewed to ensure that the algorithm was correctly discriminating between events and noise. We compiled parameters for several far-field (> 3 km) and near-field (< 1 km) stations that were in operation for variable lengths of time during the eruption. Events detected at far-field stations were dominated by so-called ""drumbeat"" earthquakes that have accompanied the lava-dome-building eruption since mid-October. Through April 2005 these events were an amazingly regular 40-60 seconds apart, and event sizes were also regular. From April 12-19 inter-event times more than doubled and increased in event size, coinciding with a change in the locus of active dome extrusion. From late April through the beginning of September inter-event times increased to over 1000 seconds and became more irregular while event size gradually declined. This coincided with extrusion and break up of the last two lava domes emplaced to date, and the decline in seismicity may reflect a change in system dynamics such as decreasing normal stress or friction along conduit margins. The near-field stations showed no regular inter-event time patterns, as they were sensitive to many small events in addition to the drumbeat events. Plots of peak amplitudes show complexities not seen in the drumbeat earthquakes, such as systematic fluctuations in event amplitudes over periods of several days. Although location of these events is difficult, their small size and apparent independence of drumbeats suggest that they occur at the surface or within the dome itself and may thus reflect mechanics of dome motion after 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