Earthquake variability, geodetic coupling, and microseismicity on heterogeneous faults: A case study of the Anza seismic gap

Junle Jiang, & Yuri Fialko

Submitted August 15, 2017, SCEC Contribution #7798, 2017 SCEC Annual Meeting Poster #205

The San Jacinto fault zone is the most seismically active fault in Southern California. Robust microseismicity occurs along much of the San Jacinto fault, but is largely absent in the “Anza seismicity gap,” which lies between the Hot Springs and Trifurcation areas. Geodetic observations indicate an anomalously shallow locking depth on the Anza section. The discrepancy between shallower geodetic locking and much deeper microseismicity, along with transient signals recorded by the strainmeter network, suggest that substantial aseismic creep occurs at nominally seismogenic depths. Despite high stressing rates due to vigorous creep at the bottom of the seismogenic zone, the Anza section has not hosted a major earthquake for over two hundred years and hence understanding the behavior of this fault segment is critical to assessing seismic risks. Motivated by geodetic, seismological, and paleoseismic observations at Anza, we use models of faults that incorporate spatial variations in frictional properties to study the relation between fault properties and behavior, and interactions between seismic and aseismic fault slip. We investigate the possibility that the fault section at Anza is conducive to more pronounced weakening during dynamic slip, potentially due to relative geometrical/structural simplicity, compared to adjacent segments. Lateral variations in coseismic fault weakening indeed lead to variability in earthquake patterns, including more frequent smaller ruptures outside of the seismic gap, and occasional through-going ruptures, which resembles the earthquake history inferred from paleoseismic trenching at Hog Lake, northwest of Anza. Enhanced dynamic weakening of fault areas can also facilitate local deeper penetration of system-size earthquakes, thereby reducing the interseismic stressing rates and stress levels of deeper fault areas and potentially explaining the lack of microseismicity. Meanwhile, aseismic transients within the downdip heterogeneous transition zone, at least next to the seismicity gap, are also required to reproduce the observed high surface strain accumulation across the fault. We also explore how large-scale fault loading conditions influence microseismic behavior and how aseismic transients affect nucleation conditions of large events in these transition zones.

Citation
Jiang, J., & Fialko, Y. (2017, 08). Earthquake variability, geodetic coupling, and microseismicity on heterogeneous faults: A case study of the Anza seismic gap. Poster Presentation at 2017 SCEC Annual Meeting.


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)