S44B-02
Subsurface Imaging at Mount St. Helens with a Large-N Geophone Array

Thursday, 17 December 2015: 16:15
307 (Moscone South)
Steve M Hansen1, Brandon Schmandt1, Alan Levander2, Eric Kiser2, John Emilio Vidale3 and Seth C Moran4, (1)University of New Mexico Main Campus, Albuquerque, NM, United States, (2)Rice University, Earth Science Department, Houston, TX, United States, (3)University of Washington, Seattle, WA, United States, (4)USGS Central Region Offices Denver, Denver, CO, United States
Abstract:
The 900-instrument Mount St. Helens nodal array recorded continuous data for approximately two weeks in the summer of 2014 and provides a remarkable opportunity to interrogate the structure beneath an active arc volcano. Two separate imaging techniques are applied to constrain both the distribution of microseismicity and subsurface velocity structure.

Reverse-time source imaging is applied to the 10 km3 region beneath the volcanic edifice where most of cataloged seismicity occurred during the experiment. These efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the Pacific Northwest Seismic Network. Earthquake locations resolve a narrow, ≤1 km wide, vertical lineament of seismicity that extends from the surface to 4 km depth directly beneath the summit crater, consistent with the historical event distribution of Waite and Moran[2009]. This feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface.

Moho imaging is achieved using the near-offset (< 30 km) PmP phase generated by the iMUSH active source shots that occurred during the deployment. The PmP arrivals are enhanced using short-term-average over long-term-average processing and then migrated using a 3D velocity model. The observed Moho depths range from 35-40 km with a slight eastward deepening across the Mt St Helens fracture zone. Significant variations are observed in the Moho reflectivity. Large amplitude PmP energy is observed in shots originating from the north and east whereas shots from the south-west display little-to-no PmP energy. The region above the reflective Moho is approximately coincident with areas displaying reduced lower-crustal velocities in the initial iMUSH tomography models and may therefore contain fluids and/or partial melt.

Additional evidence for lower crustal fluids in this region is provided by deep-long-period (DLP) events which have historically been observed east of the crater and below ~20 km depth [Nichols et al., 2011]. Two DLP events were observed during the nodal deployment [Han and Vidale, this meeting] and are located using the source imaging approach applied to the S-wave coda envelopes. Both events locate at 25-45 km depth, indicating that they occurred at or near the Moho interface.

<< Previous Abstract | Next Abstract >>