SCEC Award Number 11080 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Investigating Along-Strike Depth Variations of Seismicity along the San Andreas Fault System to Better Resolve Geodetic Locking Depths
Investigator(s)
Name Organization
Bridget Smith-Konter University of Texas at El Paso David Sandwell University of California, San Diego Peter Shearer University of California, San Diego Yuehua Zeng United States Geological Survey
Other Participants 1 graduate student
SCEC Priorities A3, A4, A1 SCEC Groups CDM, Geodesy, Seismology
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 The primary objective of this project is to investigate depth variations of the seismogenic zone beneath the San Andreas Fault System (SAFS) using both seismic and geodetic methods. Both geodetic and seismic estimates of depth are typically an along-strike average of the data sampled for each predefined fault segment. Synthesizing these data, we tune our geodetic deformation model segmentation using variations in seismicity depths from earthquake hypocenter catalogs. Three primary findings have resulted from this work: (1) Using a simplified fault segmentation for the southern SAFS, there is generally good agreement between seismogenic thickness and geodetic locking depth, with 9 out of the 12 fault segments analyzed agreeing to within 2 km. However, three segments (Imperial, Coyote Creek, and Borrego) have significant discrepancies, with seismic estimates that are well outside the error bounds of the geodetic locking depths. In these cases, the geodetically inferred locking depths are much shallower than the seismogenic depths. (2) Along the southern SAFS, 13 of the 20 segments considered in this study require additional sub-segmentation to properly represent along-strike seismicity variations. (3) Comparisons of our strain rate model with other dislocation models illustrates that some regions of the SAFS accommodate strain at rates higher than suggested by alternative methods (i.e., isotropic gridding of sparse PBO GPS data results in strain rates 5 – 8 times lower). Variations in dislocation model strain rates are due to differences in applied slip rates and locking depths. The greatest differences occur within 15 km of major faults.
Intellectual Merit The primary objective of this project was to investigate depth variations of the seismogenic zone beneath the San Andreas Fault System (SAFS) using both seismic and geodetic methods. Seismic hazard models of the SAFS (i.e. the UCERF3 model) rely on an accurate representation of fault depths to properly estimate the earthquake potential of active faults. Such hazard models typically utilize quantities like strain rate or moment accumulation rate to evaluate earthquake potential, both of which depend largely on slip rate and fault depth. Furthermore, relocated earthquake hypocenters in southern California show that the depth above which 99% of the moment release of background seismicity occurs provides a reasonable estimate of the maximum depth of rupture in moderate to large earthquakes [Nazareth and Hauksson, 2004]. Thus accurate estimates of maximum fault depth are fundamental in forecasting the magnitude of future earthquakes.
Broader Impacts A significant component of this project has an emphasis on interdisciplinary (geologic, seismic, and geodetic) collaborations, distribution of model products within the SCEC community, and improving public awareness of earthquake hazards in California. We continued to work in a collaborative exchange of data and models with active members of UCERF3. Our system-wide stress accumulation maps of the SAFS have shown that geodetically-derived fault locking depths have a strong influence on seismogenic stress accumulation rates; from an education and outreach perspective, stress/strain maps developed under this project were used as educational tools for K-12 Earth Science educators and students. We continued our ongoing efforts to share all computer source code and documentation resulting from this research with the SCEC community. These materials are currently available for download at http://topex.ucsd.edu/body_force.
Exemplary Figure Figure 1.
Comparison of seismogenic thickness (depths) estimated by LSH07 (99% seismicity cutoff depth, horizontal axis) and geodetic locking depths (vertical axis). Uncertainty estimates in geodetic depths and seismogenic depths (approximated from 90–99% seismicity cutoff depth estimates) are also plotted. Number labels correspond to segment numbers in Table 1 of Smith-Konter et al. [2011]. The light shaded oval represents data that cluster around the 1:1 match in depths, represented by the diagonal line. The dark shaded circle represents outliers (Imperial, Coyote Creek and Borrego) where seismicity suggests much deeper fault locking depths than geodesy.

Smith-Konter, B., D.T. Sandwell, and P. Shearer (2011), Locking depths estimated from geodesy and seismology along the San Andreas Fault System: Implications for seismic moment release, Journal of Geophysical Research, 116, B06401, doi:10.1029/2010JB008117.
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