Azimuthal Variation in the Velocity Spectra of the 2019 Ridgecrest Earthquake Sequence

Abstract High-frequency energy is often challenging to model because it is influenced by numerous factors in earthquake source processes. Hence, we investigate the factors that impact the high-frequency spectra of Mw 1.5–3 earthquakes from the 2019 Ridgecrest earthquake sequence, by clustering the earthquakes and stacking their velocity spectra to average source effects. We observe an azimuthal variation in the spectral amplitudes, where stations close to fault strike record more high-frequency energy than other stations. The increase in spectral amplitudes for stations that are near fault strikes is most apparent around 15–25 Hz, and a secondary high-frequency peak can be observed in some of the clusters. Our results are consistent across clusters with average depths of 2.0–9.7 km and average magnitudes of Mw 1.6–2.7. We evaluate the factors impacting high-frequency energy by calculating the relative site effects using regional earthquakes and modeling fault zone amplification and rupture directivity using dynamic rupture simulations. The analysis shows fault zone amplification may be the primary factor causing the observed azimuthal variation of high-frequency energy, though a combination of these factors may also contribute to the variability of high-frequency velocity spectra.