Azimuthal Variation in the Spectra of the 2019 Ridgecrest Earthquake Clusters and its Application to Understanding Fault Zone Structure

We first show through dynamic rupture models that FDZs can amplify high-frequency waves along directions close to fault strike and the amplified frequency band may be used to estimate the width and velocity contrast of the FDZ. Then, we identify this high-frequency amplification in the spectra of M1.5–3 earthquakes from the 2019 Ridgecrest earthquake sequence. We cluster the earthquakes by location and waveform similarity, and stack their velocity spectra to average source effects. We find that for 93% of the clusters, stations close to fault strike record more high-frequency energy at 15-25 Hz, close to the characteristic frequency of fault zone reflections. The results are consistent across clusters with average depths of 2.0–9.7km and average magnitudes of M1.6–2.7. Additionally, we analyze the relative site effects of the stations and find that they cannot explain the azimuthal variation in the spectra.