|Title||Source mechanism of small long-period events at Mount St. Helens in July 2005 using template matching, phase-weighted stacking, and full-waveform inversion|
|Publication Type||Journal Article|
|Year of Publication||2015|
|Authors||Matoza RS, Chouet BA, Dawson P.B, Shearer PM, Haney M.M, Waite G.P, Moran S.C, Mikesell T.D|
|Journal||Journal of Geophysical Research-Solid Earth|
|Type of Article||Article|
|Keywords||complex frequencies; eruption; hydrothermal system; inversions; kusatsu-shirane volcano; long-period seismicity; low-frequency earthquakes; moment-tensor; redoubt-volcano; seismic events; signals; source mechanism; stratovolcano; stromboli-volcano; subevents|
Long-period (LP, 0.5-5Hz) 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 8days 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 (approximate to 10m(3)) of a subhorizontal crack located at shallow depth (approximate to 30m) 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.