Investigating microseismicity and crustal structure beneath Mount St. Helens with a 900-geophone array

Abstract:

During Summer 2014 we deployed ~900 cable-free seismographs within ~12 km of Mount St. Helens. Each seismograph contained a 10-Hz geophone and recorded continuously for two weeks with a sample rate of 250 Hz. The array temporarily provides a major increase in spatial coverage compared to the 10-station long-term monitoring array, but each of the geophone has a high noise floor compared to the force-feedback sensors of the long-term array that is part of the Pacific Northwest Seismic Network (PNSN). We are investigating the utility of the geophone array for source and structural analyses using ambient noise, high-frequency microseismicity, deep long-period seismicity, and 23 controlled sources from the concurrent iMUSH active source project. Surface waves extracted from ambient noise cross-correlation have adequate signal to noise ratios for upper crustal tomography using frequencies ~0.2-0.5 Hz. Efforts to extract higher frequency body waves with interferometry are ongoing and include focusing on time periods with stronger high frequency noise or coda from controlled sources and earthquakes. Continuous back-projection of the array data into the 3-D subsurface was used to automatically detect and locate high-frequency (>5 Hz) microseismicity extending down to ~M-2, with a completeness magnitude of ~0.3. Two deep crustal low frequency earthquakes (<5 Hz) detected by PNSN occurred during our survey. We relocated these events and are using them to optimize back-projection parameters and create matched filters to search for additional deep low frequency seismicity. One of the deep low frequency events locates at approximately Moho depth using back-projection of S-wave energy and S-P times from dense geophone sub-arrays. This event occurs just southeast of Mount St. Helens in an area where controlled source refraction tomography images anomalously slow lower crust and common midpoint stacking images a bright Moho indicative of a locally high impedance contrast between the lower crust and upper mantle.