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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B02305, doi:10.1029/2007JB005259, 2008

Eruption dynamics at Mount St. Helens imaged from broadband seismic waveforms: Interaction of the shallow magmatic and hydrothermal systems

Gregory P. Waite

U.S. Geological Survey, Volcano Hazards, Menlo Park, California, USA


Bernard A. Chouet

U.S. Geological Survey, Volcano Hazards, Menlo Park, California, USA


Phillip B. Dawson

U.S. Geological Survey, Volcano Hazards, Menlo Park, California, USA


Abstract

The current eruption at Mount St. Helens is characterized by dome building and shallow, repetitive, long-period (LP) earthquakes. Waveform cross-correlation reveals remarkable similarity for a majority of the earthquakes over periods of several weeks. Stacked spectra of these events display multiple peaks between 0.5 and 2 Hz that are common to most stations. Lower-amplitude very-long-period (VLP) events commonly accompany the LP events. We model the source mechanisms of LP and VLP events in the 0.5–4 s and 8–40 s bands, respectively, using data recorded in July 2005 with a 19-station temporary broadband network. The source mechanism of the LP events includes: 1) a volumetric component modeled as resonance of a gently NNW-dipping, steam-filled crack located directly beneath the actively extruding part of the new dome and within 100 m of the crater floor and 2) a vertical single force attributed to movement of the overlying dome. The VLP source, which also includes volumetric and single-force components, is 250 m deeper and NNW of the LP source, at the SW edge of the 1980s lava dome. The volumetric component points to the compression and expansion of a shallow, magma-filled sill, which is subparallel to the hydrothermal crack imaged at the LP source, coupled with a smaller component of expansion and compression of a dike. The single-force components are due to mass advection in the magma conduit. The location, geometry and timing of the sources suggest the VLP and LP events are caused by perturbations of a common crack system.

Received 6 July 2007; accepted 22 October 2007; published 19 February 2008.

Keywords: Long-period earthquake; very-long-period earthquake; crack model.

Index Terms: 7280 Seismology: Volcano seismology (8419); 7215 Seismology: Earthquake source observations (1240); 7290 Seismology: Computational seismology; 8424 Volcanology: Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135); 8145 Tectonophysics: Physics of magma and magma bodies.

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Citation: Waite, G. P., B. A. Chouet, and P. B. Dawson (2008), Eruption dynamics at Mount St. Helens imaged from broadband seismic waveforms: Interaction of the shallow magmatic and hydrothermal systems, J. Geophys. Res., 113, B02305, doi:10.1029/2007JB005259.