Combining two distinct methods to resolve spatial variation in attenuation and earthquake source parameters

Giovanna Calderoni, & Rachel E. Abercrombie

Under Review August 27, 2024, SCEC Contribution #14064

Stress drop is a fundamental parameter in ground motion modeling and seismic hazard assessment, but the application of different methods to the same seismic sequence and dataset can yield different results. Estimates of stress drop are subject to considerable uncertainties, which arise from a variety of factors. These factors include the complexity of the seismic source, the assumptions inherent in the models used, the limited range of frequencies available, and the inherent difficulty in removing the propagation effects along the wave path. One of the primary challenges in computing stress drop is understanding whether the observed variations in the estimates are attributable to the characteristics of the seismic source or to the propagation path.
In this study, we apply and compare two methods, each of which addresses the trade-offs between propagation and source in different ways. The first method, known as the spectral fitting approach, operates on the hypothesis that the path effects remain constant across the spatial and temporal range of the sources under investigation. This approach assumes a level of uniformity in the propagation effects that simplifies the analysis. The second method, referred to as the spectral ratio approach, is based on the hypothesis that a small, co-located event will experience identical propagation effects. This method leverages the similarity in location to isolate the source characteristics more effectively.
Our comparison of these two methods reveals that the choice of method is not only influenced by the specifics of the data and the seismic events but is also significantly constrained by the geological context of the study area. The geological characteristics can impact the propagation of seismic waves and, consequently, the accuracy and reliability of stress drop estimates. Therefore, understanding the local geology is crucial in selecting the appropriate method for stress drop computation in any given seismic study.

Citation
Calderoni, G., & Abercrombie, R. E. (2024). Combining two distinct methods to resolve spatial variation in attenuation and earthquake source parameters. Bulletin of the Seismological Society of America, (under review).

Related Projects & Working Groups
Stress Drop Community Validation Study, Seismology