SCEC Award Number 15155 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
Investigator(s)
Name Organization
Michele Cooke University of Massachusetts Amherst
Other Participants
SCEC Priorities 4d, 4a, 4b SCEC Groups SDOT, SoSAFE, FARM
Report Due Date 03/15/2016 Date Report Submitted 03/20/2016
Project Abstract
 Two crustal deformation models examine the short term stressing of the crust between earthquakes and the longer-term off-fault residual stresses over multiple earthquake cycles. In regions of fault complexity, such as the San Gorgonio Pass Special Fault Study Area, not all stresses are relieved by slip along faults. These residual off-fault stresses can produce permanent deformation including microseismicity. The spatial correlation of patterns of stresses from focal mechanism inversions with off-fault stresses over multiple earthquake cycles suggests that the microseismicity of the San Gorgonio Pass region may be recording permanent off-fault deformation that is distinctly different from the long-term strike-slip loading of the San Andreas fault. This suggests that our current seismic catalog under-samples strike-slip events along the San Andreas Fault. Consequently, the stress inversions from the catalog provide inaccurate information about the loading of the major faults and instead reveal permanent distributed off-fault deformation.
The mean normal compressive stress from the multi-cycle model shows high spatial correlation (location and depth) with regions of anomalously high stress drop from the focal mechanism inversions in the San Gorgonio Pass region. This spatial correlation suggests that small slip surfaces off of the major faults are strengthened due to localized clamping. In addition to local variations in explicit fault strength, heterogeneous stress fields can also produce spatial variations in stress drop. The anomalously high stress drop within the San Gorgonio Pass may reflect localized compression rather than high static fault strength.
Intellectual Merit The project provides a new approach for analyzing microseismicity that could greatly impact how we interpret stresses from focal mechanism inversions. Within the San Gorgonio Pass, the pattern of stresses from the focal mechanism inversion better match models of residual off-fault deformation than models of intersiesmic loading of the San Andreas fault system. This suggests that the past few decades of microseismicity of the San Gorgonio Pass region may primarily 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, the catalog may not represent the loading on all parts of the San Andreas fault system. The complex fault geometry in the San Gorgonio Pass special fault study area may promote local off-fault deformation so that stress inversions are less reliable here than other regions of southern California. The results of this study can guide efforts of the Community Stress Model to characterize the crustal stresses within southern California.
The project also demonstrates that apparent spatial variations in stress drop within the San Gorgonio Pass region can arise from heterogeneous stresses due to complex fault geometry rather than explicit fault strength variations. 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 and a female PI with a hearing impairment.
Exemplary Figure Figure 3: Mean normal stressing rate from (A) interseismic and (B) off-fault models compared to (C) the focal mechanism inversions of Yang et al., [2012]. The green lines outline areas of greater relative tension (warmer colors) in the focal mechanism inversion. Within the San Gorgonio Pass region, the stress pattern from the focal mechanism inversions has better match to the off-fault stressing rate (correlation coefficient, r = 0.41) than to the interseismic stressing rate (r = 0.007). This correlation suggests that the seismic catalog in the SGP region under-samples strike-slip events that would reveal interseismic loading of the San Andreas fault and instead these earthquakes record permanent deformation off of the major faults. The stress inversions from focal mechanisms may not accurately inform the stress patterns that drive seismic hazards in southern California.