SCEC Award Number 14033 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Joint seismotectonic and source spectra analysis of the Ventura Basin and San Gorgonio Pass SCEC Special Fault Study Areas, southern California
Investigator(s)
Name Organization
Egill Hauksson California Institute of Technology John Shaw Harvard University
Other Participants Thomas Goebel - PostDoc; Andreas Plesch --Research Associate
SCEC Priorities 1a, 2d, 4d SCEC Groups SoSAFE, USR, EFP
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 We have analyzed the spatial variation in source parameters of small and intermediate magnitude earthquakes close to Ventura Basin and San Gorgonio Pass. In San Gorgonio, the regional tectonics are controlled by a restraining bend of the San Andreas fault system, which results in distributed crustal deformation, and heterogeneous slip along numerous strike-slip and thrust faults. Most of these faults have low geologic slip rates. The region exhibits diffuse deformation and seismicity distributions with focal depths of up to 20 km. The Ventura region, on the other hand, is controlled by North-South compression accommodated by series of thrust and reverse faults. The Ventura region shows highly variable seismic activity that extends to depths of 20 km. The rapid convergence rates across the basin suggest that several of these faults slip at high rates and are capable of producing large-magnitude earthquakes. We identified several crustal and faulting parameters that may contribute to local variations in stress drop including the style of faulting, changes in average tectonic slip rates, mineralogical composition of the host rocks, as well as the focal depths of seismic events. We observed anomalously-high stress drops (>20 MPa) beneath the San Gorgonio thrust faults and stress drops show an approximate negative correlation with geologic slip rates within the San Gorgonio region. The Ventura region shows significantly lower stress-drops (∼1 MPa) than the San Gorgonio area. Principle stress orientations are substantially more heterogeneous within the San Gorgonio area as compared to the Ventura region.
Intellectual Merit The proposed research has contributed to improved understanding of the 3D geometry of active faults, and crustal stress distribution within the two Special Fault Study Areas (SFSAs) (research priority 4d) as well as has fostered a better understanding of strain partitioning and style of faulting. We have directly address SCEC research priority 1a through an integration of the geologic and seismicity data. Furthermore, a detailed mapping of spatial stress drop variations has helped answer the question if stress drops are affected by regional variations in crustal structure or state of stress (Research priority 2d).
The proposed research has contributed directly to SCEC efforts in the Ventura and San Gorgonio Special Fault Study Areas (SFSA’s) and the Center’s community structural models (CFM and CVM-H). The proposed collaboration has brought together seismologists (Hauksson, Goebel) and structural geologists (Shaw, Plesch) who are experienced in seismotectonic analysis in California and have made major contributions to SCEC’s earthquake catalogs and community models.
Broader Impacts The project has provided partial support for a PostDoc, Dr. Thomas Goebel. In general, the project contributes to understanding of earthquake hazards in southern California. In particular, the project refines the basic data used in hazards estimates in the San Gorgonio Pass and the Ventura basin regions.
The proposed research has contributed directly to SCEC efforts in the Ventura and San Gorgonio Special Fault Study Areas (SFSA’s) and the Center’s community structural models (CFM and CVM-H). The proposed collaboration has brought together seismologists and structural geologists who are experienced in seismotectonic analysis in California and have made major contributions to SCEC’s earthquake catalogs and community models.
Exemplary Figure Figure 1. Credit: Goebel et al. (2015)