SCEC Award Number 16187 View PDF
Proposal Category Individual Proposal (Special Fault Study Area)
Proposal Title Effects of fault geometry on stressing rates and off-fault seismicity on the southern San Andreas and San Jacinto faults
Name Organization
Michele Cooke University of Massachusetts Amherst
Other Participants
SCEC Priorities 4d, 4a, 4b SCEC Groups USR, SoSAFE, SDOT
Report Due Date 03/15/2017 Date Report Submitted 04/24/2017
Project Abstract
We compare focal mechanisms near the San Gorgonio Pass (SGP) to stress patterns from interseismic, co-seismic and steady-state (i.e. across multiple earthquake cycles) crustal deformation models. In re-gions of fault complexity, such as the SGP Special Fault Study Area, slip along faults relieves accumu-lated stresses on the fault and loads the adjacent crust. The spatial mis-correlation of focal mechanism slip rakes with slip sense predicted from interseismic models suggests that the microseismicity of the SGP region may be recording permanent off-fault deformation that is distinctly different from the strike-slip loading of the region between large earthquakes. The discrepancies owe to 1) under-sampling of strike-slip events along the San Andreas Fault, 2) events with M>5.5 produce aftershocks inconsistent with long term loading and 3) deep creep along the northern San Jacinto fault. Because the northern San Jacinto fault has a southward increasing slip gradient, deep aseismic creep can account for the en-igmatic normal slip focal mechanisms in the San Bernardino basin. Consequently, the stress inversions from the catalog provide inaccurate information about the loading of the major faults.

The mean normal compressive stress from the steady-state model shows high spatial correlation (loca-tion and depth) with regions of anomalously high stress drop from the focal mechanisms in the SGP. This correlation suggests that small slip surfaces off of the major faults are strengthened due to localized clamping. These results suggests that the anomalously high stress drop within the SGP may reflect local compression and do not require anomalously high static fault strength.
Intellectual Merit The project uses a new approach for analyzing microseismicity that could impact how we interpret load-ing of faults from focal mechanism inversions. Within the San Gorgonio Pass (SGP), the pattern of stresses from the focal mechanism inversion do not match stresses produced by intersiesmic loading models of the San Andreas fault system. This suggests that the past few decades of microseismicity of the SGP region may record permanent off-fault deformation that is distinctly different from the long term loading of the San Andreas fault. Because our current seismic catalog under-samples strike-slip events along the San Andreas fault, the catalog may not represent the loading on all parts of the San Andreas fault system. The results of this study can guide efforts of the SCEC Community Stress Model to charac-terize the crustal stresses within southern California.

The project also demonstrates that apparent spatial variations in stress drop within the San Gorgonio Pass region do not require explicit fault strength variations. Heterogeneous stresses due to complex fault geometry could also result in local clamping of slip surfaces, which yields locally high stress drop. This result highlights the need for investigation of fault geometry within focused studies such as the Special Fault Study Areas of SCEC4.
Broader Impacts By integrating results of crustal deformation models and the stress inversions from focal mechanisms, this project crosses disciplines within SCEC. This project supported a female graduate student at UMass - Amherst and a hearing-impaired female PI.
Exemplary Figure Figure 3: Slip sense from focal mechanisms (A & B), an interseismic model with uniform 25 km locking depth (B&C) and a model with 10 km locking depth on the northern San Jacinto fault with 20 km locking depth elsewhere. Because the normal slip events occur around 15 km depth, interseismic creep along the northern San Jacinto fault below 10 km, which has southward increasing slip gradient, could account for loading of secondary faults and off-fault deformation with normal slip.