Significantly Improving Regional Seismic Amplitude Tomography at Higher Frequencies by Determining S‐Wave Bandwidth

Characterizing regional seismic signals continues to be a difficult problem due to their variability. Calibration of these signals is very important to many aspects of monitoring underground nuclear explosions, including detecting seismic signals, discriminating explosions from earthquakes, and reliably estimating magnitude and yield. Amplitude tomography, which simultaneously inverts for source, propagation, and site effects, is a leading method of calibrating these signals. A major issue in amplitude tomography is the data quality of the input amplitude measurements. Pre‐event and prephase signal‐to‐noise ratio (SNR) tests are typically used but can frequently include bad signals and exclude good signals. The deficiencies of SNR criteria, which are demonstrated here, lead to large calibration errors. To ameliorate these issues, we introduce a semiautomated approach to assess the bandwidth of a spectrum where it behaves physically. We determine the maximum frequency (denoted as F max) where it deviates from this behavior due to inflections at which noise or spurious signals start to bias the spectra away from the expected decay. We compare two amplitude tomography runs using the SNR and new F max criteria and show significant improvements to the stability and accuracy of the tomography output for frequency bands higher than 2 Hz by using our assessments of valid S ‐wave bandwidth. We compare Q estimates, P / S residuals, and some detailed results to explain the improvements. For frequency bands higher than 4 Hz, needed for effective P / S discrimination of explosions from earthquakes, the new bandwidth criteria sufficiently fix the instabilities and errors so that the residuals and calibration terms are useful for application.