SCEC Award Number 21045 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Finite-Source Based Stress Drop Estimates for the 2019 Ridgecrest, CA Sequence
Investigator(s)
Name Organization
Douglas Dreger University of California, Berkeley Taka'aki Taira University of California, Berkeley
Other Participants There will be one undergraduate student TBD.
SCEC Priorities 1d, 1e, 1c SCEC Groups Seismology, FARM, SDOT
Report Due Date 03/15/2022 Date Report Submitted 06/20/2022
Project Abstract
 A finite-source inversion method for stress drop that fits seismic moment rate functions for the distribution of fault slip, the kinematic rise time and rupture velocity parameters is applied to Ridgecrest earthquake aftershocks. The seismic moment rate functions are derived from spectral domain deconvolution of nearby smaller earthquakes referred to as empirical Green’s functions (EGF, e.g. Hartzell, 1978). The finite-source slip models are used to estimate the rupture area, the average slip, as well as the coseismic stress change (Ripperger and Mai, 2004) to determine the peak and average stress drop (e.g. Dreger et al., 2007). We have studied 7 aftershocks of the 2019 Ridgecrest earthquake sequence ranging in magnitude from 4.0 to 5.5, and we also compare the results with our published results for the Ridgecrest foreshock (Mw6.4) and mainshock (Mw7.0) (Wang et al., 2020). The results indicate self-similar scaling where the estimated rupture area is consistent with the Leonard (2010) scaling relationship within the 2-sigma uncertainty. The average stress drop of the studied events is between 1-3 MPa, with a total range from 0.36 to 17.0 MPa.
Intellectual Merit This research uses a finite-source based approach to estimate the stress drop of earthquakes. The method does not suffer from uncertainty related to assumed mechanical models, and cubed dependence on corner frequency as does standard spectral methods. Instead seismic moment rate functions are fit to a finite-source model that enables determination of the fault orientation, the rupture velocity and rise time, and slip distribution accounting for possible rupture directivity effects. The slip distribution is then used to determine the spatially variable stress drop. We have found that average stress drops from finite-source models are generally consistent with a Brune stress drop, but that the slip heterogeneity implies significant spatial variability across a fault rupture. This research is helping to provide insight into what standard stress drop approaches may be revealing about complex finite-fault earthquake rupture.
Broader Impacts Jose Magana, a first generation immigrant to the US from El Salvador, and the first in his family to pursue a college education was involved in this research. Jose's contribution involved the application of a finite-source inversion method to seismic moment rate functions derived from empirical Green's function deconvolution, and performing statistical tests on the results. He submitted this work for an Honors Senior Thesis. He has since graduated and pursued a Masters Degree in Civil Engineering from UC Berkeley (geotechnical specialization). He is currently working in the private sector in geotechnical engineering.

In addition the research results are contributing to the working group effort to characterize uncertainty in seismic stress drop determination led by Baltay and Abercrombie.
Exemplary Figure Figure 8. Rupture area from the finite-source inversion is compared to the Leonard (2010) scaling law (red line; dotted red lines are 2-sigma uncertainty). The gray lines show stress drop. Ridgecrest aftershocks scale self-similarly with an average stress drop of 1-3MPa.