HR: 1340h
AN: V53D-1602    [Abstracts]
TI: Slip Energy Budgets of the Mount St. Helens Earthquakes
AU: * Harrington, R M
EM: rebecca@moho.ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Drive East 3806 Geology Building, Los Angeles, CA 90095-1567 United States
AU: Brodsky, E E
EM: brodsky@ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Drive East 3806 Geology Building, Los Angeles, CA 90095-1567 United States
AU: Spieler, O
EM: spieler@lmu.de
AF: Earth and Environmental Sciences, LMU Muenchen, Theresienstrasse 41/III, Munich, 80333 Germany
AB: The hybrid-appearing drumbeat earthquakes of the 2004 Mount St. Helens eruption have a number of features that indicate that they are merely stick-slip failure events. An analysis of particle wave motion indicates consistent Rayleigh - P wave arrival times. Additionally, there is an absence of observable strictly high-frequency VT events indicating that the path may be complex enough to make prolonged ringing on all events. A lack of systematic trends in the ratios of long-period to short-period velocity amplitudes implies that there are not independent high-frequency and long-period sources. Finally, the overall slip budget (discussed below) concurs with geodetic data. Using the M_L magnitudes as a proxy for moment provides a connection between the earthquakes and the overall slip budget. There are on the order of 100,000 earthquakes in the first three months of the whaleback extrusion with an average seismic moment of 1.6 × 10¹² Nm. Using the measured maximum hypocentral depths of approximately 2000 m, and assuming M_L = M_W and the usual M_W-M₀ relationship, we compute that these events result in approximately 500 m seismic slip. This is comparable to the 470 m measured geodetically by the Cascades Volcano Observatory as of February 1^st. As observable gouge is generated from these events, they provide a nearly unique opportunity to constrain the energy budget of an earthquake. From the seismic waveforms, the typical radiated energy of an event is 3 × 10⁶ J. Based on laboratory measurements and CVO field observations, we calculate that the total gouge generated over the sequence has roughly 1 × 10¹² m² surface area, and so accounts for 1 × 10⁷ J/event. According to these preliminary values, gouge generation is at least as significant a factor in the energy budget of earthquakes as the radiated energy.
DE: 7280 Volcano seismology (8419)
DE: 7299 General or miscellaneous
DE: 8419 Volcano monitoring (7280)
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005