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Seismic discrimination of controlled explosions and earthquakes near Mount St. Helens using P/S ratios
  • Ruijia Wang,
  • Brandon Schmandt,
  • Eric Kiser
Ruijia Wang
University of New Mexico, University of New Mexico

Corresponding Author:ruijia3@ualberta.ca

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Brandon Schmandt
University of New Mexico, University of New Mexico
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Eric Kiser
University of Arizona, University of Arizona
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Explosions and earthquakes are effectively discriminated by P/S amplitude ratios for moderate magnitude events (M≥4) observed at regional to teleseismic distances (≥200 km). It is less clear if P/S ratios are effective explosion discriminants for lower magnitudes observed at shorter distances. We report new tests of P/S discrimination using a dense seismic array in a continental volcanic arc setting near Mount St. Helens, with 23 single-fired borehole explosions (M 0.9-2.3) and 406 earthquakes (M 1-3.3). The array provides up to 95 three-component broadband seismographs and most source-receiver distances are <120 km. Additional insight is provided by ~3,000 vertical component geophone recordings of each explosion. Potential controls on local distance P/S ratios are investigated, including: frequency range, distance, magnitude, source depth, number of seismographs, and site effects. A frequency band of 10-18 Hz performs better than lower or narrower bands because explosion-induced S-wave amplitudes diminish relative to P for higher frequencies. Source depth and magnitude exhibited negligible influences. Site responses for earthquakes and explosions are correlated with each other and with shallow crustal Vp and Vs from travel-time tomography. Overall, the results indicate high potential for local distance P/S discrimination in a continental volcanic arc setting, with 100% true positives and 4.93% false positives using a frequency band of 10-18 Hz for array-median P/S ratios.
Oct 2020Published in Journal of Geophysical Research: Solid Earth volume 125 issue 10. 10.1029/2020JB020338