SCEC Award Number 17032 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Microseismicity, geodetic coupling, and earthquake variability on heterogeneous faults: A case study of the Anza section of the San Jacinto Fault
Investigator(s)
Name Organization
Yuri Fialko University of California, San Diego
Other Participants Junle Jiang - postdoc
SCEC Priorities 1d, 3b, 5a SCEC Groups FARM, SDOT
Report Due Date 06/15/2018 Date Report Submitted 06/19/2018
Project Abstract
 This is a multi-year project in which we developed a 3D model of fault with spatially variable rate-and-state frictional porperties to reproduce the relation between seismicity and geodetic locking depths relevant to the Anza section of the San Jacinto fault (SJF). Through simulating earthquake sequences and aseismic slip, our models successfully reproduce microseismicity much deeper than the geodetic locking interval, suggesting that the discrepancy between seismic and geodetic estimates may be due to a transition zone with highly heterogeneous frictional properties. Our model predicts that large earthquakes on the fault can penetrate beyond the nominal geodetic locking depth, to the maximum depth extent of microseismicity. Besides, abundant aseismic transients occur in the transition zone in our model, sometimes preceding the nucleation of seismic slip. Both of these phenomena are potentially important for understanding seismogenic fault behavior and future major earthquakes in the region.
Our modeling results suggest that the transition zone with heterogeneous friction is the key to reproducing deeper microseismicity and shallower geodetic locking depths on the fault. The simulated microseismicity occurs predominantly at depths of 10--15 km in the transition zone. The model also captures spatially-variable, time-dependent coupling on the fault in the interseismic period. We developed methods to infer the effective geodetic locking depth, together with the plate loading rate, based on the surface displacement that results from the variable fault slip rates.
Intellectual Merit Over the last year, we have further developed models of faults with along-strike variations in frictional properties that are more closely tailored to the case of Anza. We incorporate enhanced dynamic weakening (DW) that is documented at high-slip-rate experiments, in addition to the low-slip-rate rate-and-state friction laws that were only considered in the earlier study. Our focus is on constraining fault frictional properties of the Anza seismicity gap and surrounding areas and the interactions of seismic and aseismic slip in these areas. Our main findings are as follows:
• Along-strike variations in fault frictional properties lead to complex earthquake slip patterns. The Anza seismicity gap can be explained by a larger fault area that is more susceptible to DW during earthquake rupture and has lower interseismic stresses, compared to nearby areas. The lower-stressed region, along with variable seismogenic zone width, contributes to a complex earthquake slip history characterized by partial ruptures of different sizes and system-sized events every hundreds of years, largely consistent with paleoseismic studies at Anza.
• Large-scale geodetic observations constrain the over behavior of faulting, while pronounced smaller-scale variations can still exist. Local deeper penetration of large earthquakes can be reconciled with a shallower regional geodetic locking depth if nearby areas host aseismic transients.
• Interactions of seismic and aseismic slip is highly dependent on the heterogenous state of fault frictional properties. In 2D models with stochastic distribution of frictional properties, a higher moment release through seismic slip is promoted by a larger amplitude in heterogeneity, a larger maximum wavelength, and a faster average loading rate.
Broader Impacts The San Jacinto fault produced nine significant (magnitude greater than 6) earthquakes over the last 120 years, and is currently the most seismically active fault in Southern California. Research performed under this project improved our understanding of the Anza section of the San Jacinto fault which appears to be in the late interseismic phase of the earthquake cycle, and poses a considerable seismic hazard to populated areas in Southern California. The developed models improved our understanding of the on-going deformation in Southern California, and may help reduce uncertainties in the earthquake hazard estimates. The project provided partial support for one postdoc (Junle Jiang).
Exemplary Figure Figure 2.