Local distance seismic source classification during the iMUSH project

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 magnitude events 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_L 0.9–2.3) and 406 earthquakes (M_L 1–3.3). The array provides up to 95 three-component broadband seismographs and most source-receiver distances are <120 km. 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 about 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 weak influences on P/S ratios. 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 explosion discrimination in a continental volcanic arc setting, with ≥98% true positives and ≤6.3% false positives when using the array median from ≥16 stations. Performance is reduced for smaller arrays, especially those with ≤4 stations, thereby emphasizing the importance of array data for discrimination of low magnitude explosions. When only a sparse network is available, preliminary tests show that combining P/S ratios with depth sensitive differential magnitudes, M_L–M_C, can improve source discrimination. Lastly, we analyze the focal mechanisms of explosions and earthquakes through both first motion polarity and full moment tensor inversion. Our integrated source study in such a structurally complex region shows promising results in local distance source discriminations.