Evaluating Local Distance Seismic Amplitude Measurements as Predictors of Event Magnitude in a Tectonically Complex Setting

Estimating event size is a critical part of explosion monitoring efforts, and accuracy at local distances is becoming increasingly important. As station-event distances shrink, path effects can play a larger role, especially in tectonically complex areas where geologic heterogeneities may have large effects on signal propagation. Understanding and mitigating these factors is critical to achieving a reliable magnitude estimate. We tested four commonly used measures of seismic magnitude: P-and S-peak amplitude, Rg amplitude (M-Rg), and S-coda amplitude. We chose a dataset of 225 seismometers that recorded 13 local small-magnitude earthquakes, 21 tamped single-fired borehole shots, and 13 open-pit coal mining blasts. S coda achieved the most accurate magnitude estimates for all events and across all geologic regions. The coda shapes also proved to be very similar between event types, meaning that a single type curve can be used to estimate magnitude for an earthquake, mine blast, or borehole shot. M-Rg performed well at stations within the Powder River basin, but suffered nearly an order of magnitude reduction among stations in the mountains due to lowered Rg amplitudes. This discrepancy is readily apparent when using a dense array but presents a significant problem for sparse networks in tectonically complex regions. Body-wave amplitudes were generally very unreliable predictors of magnitude: they performed best for earthquakes, whereas mine blast body-wave amplitudes were more dependent on frequency and geologic region, and body-wave amplitudes from borehole shots had little to no correlation to yield for any frequency or region. We show that in a tectonically complex environment S coda is the most reliable method to estimate event size, and M-Rg may be a useful complement in a more uniform setting or with better calibration to account for path effects due to structure.