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 (ML 0.9-2.3) and 406 earthquakes (ML 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 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.

Advancing Local Distance Discrimination of Explosions and Earthquakes With Joint P/S and ML‐MC Classification

“…Encouraging performance without local correction factors does not imply that such corrections are unimportant. In regions where sufficient data exist to calibrate local corrections, we expect that incremental but important improvements in performance are possible for a given number of stations (e.g., Kintner et al., 2020; Wang et al., 2020). Lastly, both methods used here are fast to calculate, making the joint method practical for near‐real time screening at local scale (e.g., Dempsey et al., 2020; Scafidi et al., 2018).…”

“…All broadband three‐component records are filtered between 10 and 18 Hz, which is the optimal band for discrimination of similar‐magnitude sources at comparable distances in Wang et al. (2020). Modified after O’Rourke et al.…”

Big Data Seismology

“…With growing time‐histories of data and associated event catalogs, recent work using data from regional networks in Utah and the Netherlands has demonstrated that machine learning can play an increasingly important role in this problem (Linville et al., 2019; Trani et al., 2021). At local scales, data from the Mount Saint Helens nodal array and from a dense network in Wyoming have been used to explore the use of classical P / S amplitude ratios for explosion discrimination (O’Rourke et al., 2016; R. Wang et al., 2020). Using the dense data deployments, both studies identified strong site effects that varied over relatively short distances.…”

Natural and Anthropogenic Sources of Seismic, Hydroacoustic, and Infrasonic Waves: Waveforms and Spectral Characteristics (and Their Applicability for Sensor Calibration)

“…In addition, explosion signals show a shorter rise time and duration as well as a higher frequency content compared to earthquakes. Unlike tectonic earthquakes, both nuclear and chemical explosions are dominated by P waves and radiate weaker S waves over all frequencies, especially for frequencies greater than 10 Hz (Wüster 1993 ; Kim et al 1994 ; Prastowo and Madlazim 2018 ; Wang et al 2020 ). The P wave is then followed by high amplitude Rayleigh waves, which are strongly affected by the explosions’ depth and the overlying topography (Stevens et al 2017 ).…”