SCEC Award Number 13082 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title High-resolution geodetic imaging of damage zones of major seismogenic faults in Southern California
Investigator(s)
Name Organization
Yuri Fialko University of California, San Diego
Other Participants Eric Lindsey
SCEC Priorities 4, 1, 5 SCEC Groups Geodesy, FARM
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 We investigated interseismic deformation across the San Jacinto fault at Anza, California where previous geodetic observations have indicated an anomalously high shear strain rate. We present an updated set of secular velocities from GPS and InSAR observations that reveal a 2-3 kilometer wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two. GPS occupations of an alignment array installed in 1990 across the fault trace at Anza allow us to rule out shallow creep as a possible contributor to the observed 15 strain rate. Using a dislocation model in a heterogeneous elastic half space, we show that a reduction in shear modulus within the fault zone by a factor of 1.2-1.6 as imaged tomographically by Allam & Ben-Zion (2012) can explain about 50% of the observed anomalous strain rate. However, the best-fitting locking depth in this case (10.4 ± 1.3 km) is significantly less than the local depth extent of seismicity (14-18 km). We show that a deep fault zone with a shear modulus reduction of at least a factor of 2.4 would be required to fully explain the geodetic strain rate, ssuming the locking depth is 15 km. Two alternative possibilities include fault creep at a substantial fraction of the long-term slip rate within the region of deep microseismicity, or a reduced yield strength within the upper fault zone leading to distributed plastic failure. Both cases are consistent with a fault segment that is reaching the end of the interseismic period.
Intellectual Merit The San Jacinto fault produced 10 significant (magnitude greater than 6)
earthquakes over the last 120 years, and is currently the most
seismically active fault in Southern California. The conducted work
improved our understanding of the Anza section of the San Jacinto
fault, where available geodetic data reveal a zone of anomalously high
secular strain rate. The Anza section is the only section of the SJF
that has not experienced a large earthquake in historic times, and is
believed to be in the late interseismic phase of the earthquake cycle.
The performed data collection and analysis provided insights into interseismic loading of the Anza gap, and reduce uncertainties in the earthquake hazard estimates.
Broader Impacts This project provided training and support for one graduate student
(Lindsey) and one postdoc (Bartlow). The PI (Fialko) used results of
this study in two graduate classes taught at SIO.
Exemplary Figure Figure 4.