Seismic discrimination of controlled explosions and earthquakes near
Mount St. Helens using P/S ratios
Abstract
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.