SCEC Award Number 15211 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
SCEC Priorities 1a, 1d, 4b SCEC Groups Geodesy, SDOT
Report Due Date 03/15/2016 Date Report Submitted 03/14/2016
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 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 and 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. We also studied surface deformation at the southern end of the San Jacinto fault. By combining data from the ascending and descending tracks we were able to isolate a horizontal (fault-parallel) velocity field. We conducted several GPS campaign-style surveys of existing benchmarks to complement data from a continuous GPS sites. The data clearly show interseismic accumulation of strain in the San Felipe shear zone. This strain anomaly is asymmetric with respect to the Coyote Creek fault, which is believed to be the main branch of the San Jacinto fault south of the Santa Rosa mountains. The new data confirm our hypothesis that the Clark fault (the central segment of the San Jacinto fault) has a blind southern continuation into the Borrego badlands carrying slip rate as high as 10-12 mm/yr. If so, it may pose a significant seismic hazard.
Intellectual Merit The San Jacinto fault (SJF) is historically the most seismically active fault in Southern California, with 9 strong (M 6-7) earthquakes over the past 120 years. This project focuses on characterizing the near-field interseismic deformation along the San Jacinto and other major faults in Southern California using high-resolution space geodetic observations (InSAR and campaign GPS). Enhanced strain rates are expected in damage zones that are often expressed in reduced effective elastic moduli and seismic velocities. Observations of near-fault strain localization can thus be used to infer in situ mechanical properties of rocks comprising the fault damage zones. In addition, large lateral variations in elastic rigidity due to fault-induced damage may bias estimates of geodetic fault slip rates and locking depths. We developed semi-analytical and numerical approaches to evaluate the effects of low rigidity fault zones on surface deformation and the inferred model parameters.
Broader Impacts Evaluation of seismic hazard is based primarily on historic seismicity and long-term fault slip rates inferred from paleoseismic data. Geodetic observations provide an important additional source of information about contemporaneous accumulation of strain in the seismogenic layer. UCERF3 model now incorporates estimates of fault slip rates based on geodetic data. A major outstanding question is whether geodetic observations can help identify areas of seismic hazard that haven’t been recognized based on available seismic and geologic data. While mature faults such as the San Andreas fault by and large have clear expression in geomorphology, young developing faults and fault zones may be more difficult to recognize. In order to better understand a potential contribution of geodetic observations to estimates of seismic hazard such as UCERF, we investigate several zones of high strain rate along the San Jacinto fault system. The proposed collection and analysis of space geodetic data will improve our understanding of the associated seismic hazard to populated areas in Southern California. This project has provided training and support for three graduate students.
Exemplary Figure Figure 3a.