SCEC Award Number 19158 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Sensitivity of regional interseismic deformation to variations in active fault configuration of the southern San Andreas fault and San Jacinto faults
Investigator(s)
Name Organization
Michele Cooke University of Massachusetts Amherst Scott Marshall Appalachian State University
Other Participants Hanna Elston
SCEC Priorities 3a, 1e, 1a SCEC Groups SDOT, CXM, SAFS
Report Due Date 03/15/2020 Date Report Submitted 03/20/2020
Project Abstract
 We compare interseismic velocities measured by GPS and slip sense of microseismicity to predictions from interseismic deformation models that explicitly incorporate plausible 3D active subsurface fault configurations for the southern San Andreas fault (SAF) through the San Gorgonio Pass. We assess two models, one with and one without the northern pathway of the SAF, whose activity has been debated. Because focal mechanisms occur at a range of depths, they are more sensitive to variations of slipping fault configuration below the locking depth than GPS stations at the Earth’s surface. This study is the first to use interseismic focal mechanisms of off-fault microseismicity to evaluate active fault geometry. A model with slip along both the northern and southern pathways of the SAF fault produces greater contraction in the region between the two pathways than the model with just the active southern pathway. This contraction predicts reverse slip microseismicity that does not match the observed focal mechanisms as well as the model with only the southern pathway of the SAF. We show that focal mechanisms from off-fault microseismicity in the interseismic period can be used to delineate between alternative fault configurations. In regions of complex faulting with abundant off-fault seismicity, focal mechanisms can provide an additional and valuable constraint on slip partitioning. Our analysis shows that the southern SAF model with the inactive northern pathway shows slightly better fit to the GPS-derived surface strain rates and significantly better fit than the dual pathway model to the microseismicity focal mechanisms.
Intellectual Merit Because the focal mechanisms record stress state at a wide range of depths, they are more sensitive to variations of slipping fault configuration below the locking depth within the interseismic period than GPS velocities along the Earth’s surface. This study demonstrates that in regions of complex faulting with abundant off-fault seismicity, focal mechanisms from off-fault interseismic microseismicity can provide additional and valuable constraints on the partitioning of slip among faults and assessment of their activity. Our analysis is the first that we are aware of that compares deformation models results to interseismic focal mechanisms for the purpose of assessing activity level of faults. The southern San Andreas fault model with the inactive northern pathway shows slightly better fit to the surface strain rate and significantly better fit than the dual pathway model to the focal mechanisms.
Broader Impacts This study is the first to demonstrate that in regions of complex faulting with abundant off-fault seismicity, focal mechanisms from interseismic microseismicity can provide an additional and valu-able constraint on partitioning of slip among faults and assessment of their activity. This project supports a female MS student, Hanna Elston at UMass - Amherst and a hearing-impaired female PI.
Exemplary Figure Figure 4: Recorded focal mechanism slip sense and model predictions (20 km locking depth) at focal mechanism locations. Opaque symbols indicate locations where model results differ by more than the focal mechanism nodal plane uncertainty. Histograms show slip sense distribution for locations that differ between models. For the mod-els, we report the % difference between the model predicted and observed slip senses where models differ. In-sets show cross-section along A-A’. The upper inset shows focal mechanisms and the lower two insets show mod-el predicted slip sense with added noise. The opaque portion indicates the region where models differ the most for the whole region. SGPT - San Gorgonio Pass Thrust.