Using a high- to low-frequency spectral ratio to distinguish variations in earthquake source properties

Ian Vandevert, Peter M. Shearer, & Wenyuan Fan

Submitted July 3, 2025, SCEC Contribution #14230

Earthquake stress drop is a widely-used parameter related to source properties that can be estimated from P- and S-wave spectra and is observed to vary among differ- ent events and source regions. However, absolute levels of stress drop depend both on modeling assumptions about rupture dynamics and corrections applied for attenu- ation during wave propagation. For these reasons, stress drop estimates suffer from large uncertainties and show poor agreement among different studies. However, the rel- ative differences in stress drop among different events show much better agreement and reflect variations in high-frequency energy radiated from the source. Here, we introduce a method to resolve these relative differences more directly by using a new parameter, 𝜷, defined as the ratio of the average spectral amplitude in a high-frequency band to that in a low-frequency band. We correct for path effects using small calibration events close to each target event. We identify spatial variations in corrected 𝜷 values for aftershocks of the 2019 Ridgecrest earthquakes that correlate with published stress-drop estimates. Our simple 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, such as spectral decomposition and empirical Green’s function analyses.

Citation
Vandevert, I., Shearer, P. M., & Fan, W. (2025). Using a high- to low-frequency spectral ratio to distinguish variations in earthquake source properties. Bulletin of the Seismological Society of America, (submitted).