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 RESEARCH ARTICLE 10.1002/2015JB012279 Key Points: • Source mechanism of small long-period (0.5–5 Hz) subevents at Mount St. Helens • Volumetric source consistent with shallow subhorizontal crack • Similar tiny long-period subevents likely part of source process at other volcanoes Supporting Information: • Figure S1 • Figure S2 • Figure S3 • Figures S1–S3 captions and Table S1 Correspondence to: R. S. Matoza, matoza@geol.ucsb.edu Citation: Matoza, R. S., B. A. Chouet, P. B. Dawson, P. M. Shearer, M. M. Haney, G. P. Waite, S. C. Moran, and T. D. Mikesell (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, J. Geophys. Res. Solid Earth, 120, 6351–6364, doi:10.1002/2015JB012279. Received 11 JUN 2015 Accepted 11 AUG 2015 Accepted article online 14 AUG 2015 Published online 18 SEP 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 Robin S. Matoza 1,2 , Bernard A. Chouet 3 , Phillip B. Dawson 3 , Peter M. Shearer 1 , Matthew M. Haney 4 , Gregory P. Waite 5 , Seth C. Moran 6 , and T. Dylan Mikesell 7,8 1 Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA, 2 Department of Earth Science and Earth Research Institute, University of California, Santa Barbara, California, USA, 3 U.S. Geological Survey, Volcano Science Center, Menlo Park, California, USA, 4 Alaska Volcano Observatory, U.S. Geological Survey Volcano Science Center, Anchorage, Alaska, USA, 5 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, Michigan, USA, 6 Cascades Volcano Observatory, U.S. Geological Survey Volcano Science Center, Vancouver, Washington, USA, 7 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA, 8 Department of Geosciences, Boise State University, Boise, Idaho, USA 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 m 3 ) 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. 1. Introduction Long-period (LP, 0.5–5 Hz) seismicity, observed at volcanoes worldwide, plays a central role in our ability to assess and forecast unrest and eruption [e.g., Chouet, 1996a; McNutt, 1996; Kawakatsu and Yamamoto, 2007; Kumagai, 2009; Neuberg, 2011; Nishimura and Iguchi, 2011; Zobin, 2012; Chouet and Matoza, 2013]. The term LP seismicity includes individual transient LP events and more continuous volcanic tremor signals. Over the past several decades, numerous competing hypotheses and models have emerged to explain LP seismicity [e.g., Chouet and Matoza, 2013, and references therein]. Among these hypotheses, LP events at shallow depth ( < 2 km) in a volcanic ediﬁce are commonly explained by the impulsive excitation and resonance of ﬂuid-ﬁlled cracks resulting from magmatic-hydrothermal interactions [e.g., Chouet et al., 1994; Chouet, 1996a; Kumagai et al., 2002b; Nakano et al., 2003; Nakano and Kumagai, 2005a; Waite et al., 2008; Matoza and Chouet, 2010; Arciniega-Ceballos et al., 2012; Maeda et al., 2013]. ©2015. American Geophysical Union. All Rights Reserved. MATOZA ET AL. The dome-building phase of the 2004–2008 eruption of Mount St. Helens (MSH) produced millions of repetitive seismic events with long-period codas and slowly evolving waveforms [Moran et al., 2008; Thelen et al., 2008]. Many of these events occurred with such precise regularity that they were termed “drumbeats” [Moran et al., 2008], a phenomenon that has been observed at several other volcanoes [e.g., Neuberg, 2000; MOUNT ST. HELENS SMALL LP SOURCE