Ridgecrest nFI

Earthquakes of similar magnitude can generate different amounts of high‐frequency radiation, contributing to variability in peak ground acceleration and uncertainties in ground‐motion prediction. These source differences are expressed in their P‐ and S‐wave spectra and affect spectral estimates of earthquake stress drop. Relative variations in spectra and related stress‐drop estimates among events are better resolved than their absolute values, and thus provide useful measurements for distinguishing earthquakes and their high‐frequency radiation content.

Here, we quantify these differences in spectra using the frequency index (FI), defined as the logarithmic ratio of the average spectral amplitude in a high‐frequency band to that in a low‐frequency band. We then empirically correct for path and station effects to compute a new quantity, the normalized frequency index (nFI). We identify spatial variations in nFI values for aftershocks of the 2019 Ridgecrest earthquakes that correlate with published stress‐drop estimates. Our data‐driven approach can robustly identify variations in high‐frequency energy among different earthquakes and source regions, providing an observation‐based alternative to commonly applied stress‐drop estimation methods for characterizing source behavior at high frequencies.