SCEC Award Number 17173 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Stressing rates and off-fault seismicity associated with the southern San Andreas and San Jacinto faults
Investigator(s)
Name Organization
Michele Cooke University of Massachusetts Amherst
Other Participants PhD candidate Jenn Beyer
SCEC Priorities 1c, 1e, 3e SCEC Groups CXM, FARM, SDOT
Report Due Date 06/15/2018 Date Report Submitted 05/30/2018
Project Abstract
 Within the San Bernardino basin, focal mechanisms show normal slip events that are inconsistent with the inter-seismic strike-slip loading of the nearby San Jacinto and San Andreas faults. The discrepancy may owe to deep (> 10 km depth), creep along the northern San Jacinto fault. The enigmatic normal slip events occur to the northeast of the fault and primarily below 10 km depth, consistent with off-fault deformation due to spatially non-uniform deep creep rates. Consequently, if these normal slip events are included in stress inversions from the seismic catalog, the results may provide inaccurate information about fault loading. Here, we show that models with deep creep on the northern San Jacinto fault that match first-order pattern of observed normal slip focal mechanisms in the basin and that this deep creep cannot be detected with GPS data due to the proximity of the San Andreas fault.
Intellectual Merit The project uses a new approach for analyzing microseismicity that could impact how we interpret loading of faults from focal mechanism inversions. Within the San Bernardino Basin the pattern of stresses from the focal mechanism inversion do not match stresses produced by interseismic load-ing models of the San Andreas fault system. This findings of this study suggests that the past few decades of microseismicity may record permanent off-fault deformation that is distinctly different from the interseismic loading of the San Jacinto and San Andreas faults. For this reason and 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 characterize the crustal stresses within southern California.
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 Figure3: a) Map view of reliable focal mechanisms that pass the completeness test colored by slip sense. Enigmatic normal slip events occur within the San Bernardino basin, between the San Andreas and San Jacinto faults. Dashed fault traces are the graben bounding normal faults imaged by Anderson (2004) in Fig. 1c. b) Focal mechanisms of the San Bernardino basin (grey region of a) projected into a N-S profile. Slip sense color same as in a. The normal slip focal mechanisms within the San Bernardino basin occur pre-dominantly below 7.5 km depth c) Slip sense at locations of microseismicity from the interseismic model with shal-low locking depth (10 km) on the San Jacinto fault to simu-late deep creep. The locking depth on all other faults is 20 km. Inset cartoon shows the set-up of the interseismic model. Normal slip events occur within the San Bernardino basin. GPS stations shown with labeled triangles. b) Mean interseismic slip sense for events within light grey region of A shown with 1 vertical bars. Models with SJf locking depth < 12.5 km better match the mean slip sense of focal mechanisms in the San Bernardino Basin. d) Transect along A-A’ (shown in A) of GPS station velocity parallel to the San Jacinto fault (Herbert et al., 2014), and velocity predic-tions from the interseismic model with a shallow locking depth on the SJf (pink star, same as results shown in A) and interseismic model with a 15 km locking depth on all faults (blue circle). The surface velocities cannot resolve deep slip on the SJf because of its proximity to the SAf.