Source mechanism of small long-period events at Mount St. Helens in July 2005 using template matching, phase-weighted stacking, and full-waveform inversion
Document Type
Article
Publication Date
8-14-2015
Department
Department of Geological and Mining Engineering and Sciences
Abstract
Long-period (LP, 0.5-5 Hz) seismicity, observed at volcanoes worldwide, is a recognized signature of unrest and eruption. Cyclic LP "drumbeating" was the characteristic seismicity accompanying the sustained dome-building phase of the 2004-2008 eruption of Mount St. Helens (MSH), WA. However, together with 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. The signal-to-noise ratios of the individual events are too low to produce reliable waveform inversion results, but the events are repetitive and can be stacked. We apply network-based template matching to 8 days of continuous velocity waveform data from 29 June to 7 July 2005 using a master event to detect 822 network triggers. We stack waveforms for 359 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, perhaps supplied from snow melt.
Publication Title
Journal of Geophysical Research: Solid Earth
Recommended Citation
Matoza, R.,
Chouet, B.,
Dawson, P.,
Shearer, P.,
Haney, M.,
Waite, G. P.,
Moran, S.,
&
Mikesell, T.
(2015).
Source mechanism of small long-period events at Mount St. Helens in July 2005 using template matching, phase-weighted stacking, and full-waveform inversion.
Journal of Geophysical Research: Solid Earth,
120(9), 6351-6364.
http://doi.org/10.1002/2015JB012279
Retrieved from: https://digitalcommons.mtu.edu/michigantech-p/3354