S43F-04
Source Mechanism of Tiny Long-Period Events at Mount St. Helens in July 2005
Thursday, 17 December 2015: 14:25
307 (Moscone South)
Robin S Matoza, University of California, Santa Barbara, Department of Earth Science and Earth Research Institute, Santa Barbara, CA, United States, Bernard A Chouet, USGS Alaska Science Center, Anchorage, AK, United States, Phillip B Dawson, USGS Western Regional Offices Menlo Park, Menlo Park, CA, United States, Peter M Shearer, University of California San Diego, La Jolla, CA, United States, Matthew M Haney, Alaska Volcano Observatory Fairbanks, Fairbanks, AK, United States, Gregory P Waite, Michigan Technological University, Geological and Mining Engineering and Sciences, Houghton, MI, United States, Seth C Moran, USGS Central Region Offices Denver, Denver, CO, United States and T. Dylan Mikesell, Boise State University, Department of Geosciences, Boise, ID, United States
Abstract:
Long-period (LP, 0.5-5 Hz) seismicity is a recognized signature of unrest and eruption at volcanoes worldwide. The characteristic seismicity during the sustained dome-building phase of the 2004–2008 eruption of Mount St. Helens (MSH), USA was cyclic LP “drumbeating”. However, accompanying the LP drumbeating was a near-continuous, randomly occurring series of tiny LP seismic events (LP “subevents”), which may hold important additional information on the mechanism of seismogenesis at restless volcanoes. We employ template matching, phase-weighted stacking, and full-waveform inversion to image the source mechanism of one multiplet of these LP subevents at MSH in July 2005. We apply network-based template matching to 8 days of continuous velocity waveform data from 29 June to 7 July 2005. We stack waveforms for high-quality triggers at each station and component, using a combination of linear and phase-weighted stacking to produce clean stacks for use in waveform inversion. The derived source mechanism points to the volumetric oscillation (~10 m3) of a subhorizontal crack located at shallow depth (~30 m) in an area to the south of Crater Glacier in the southern portion of the breached MSH crater. A possible excitation mechanism is the sudden condensation of metastable steam from a shallow pressurized hydrothermal system as it encounters cool meteoric water in the outer parts of the edifice.
