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Reconciling seismicity and geodetic locking depths on the Anza segment of the San Jacinto Fault

Junle Jiang, & Yuri Fialko

Published July 26, 2016, SCEC Contribution #6376, 2016 SCEC Annual Meeting Poster #066

The depth extent of fault locking can be independently estimated from seismic and geodetic observations. In Southern California, available observations for the San Andreas and San Jacinto Faults suggest a general agreement between the two estimates. Among a few notable exceptions is the Anza segment of the San Jacinto Fault, where geodetically-estimated locking depth is ~10 km while seismicity occurs as deep as 15-18 km. The discrepancy between the seismic and geodetic estimates is at odds with models of faults governed by the rate-state friction with laboratory-based, depth-dependent properties, because such models predict that the effective geodetic locking depth should be close to or deeper than the depth of concentrated seismicity (Jiang and Lapusta, 2015).

We suggest that the discrepancy may be due to a broad transition zone of highly heterogeneous frictional properties, which is not envisioned in models of depth-dependent fault properties based on laboratory data. Using numerical models of faults with stochastic heterogeneity in rate-state frictional properties, we explore the interplay of diverse fault slip phenomena in the long-term fault slip history. Our models successfully reproduce the observed depth relation between seismicity and geodetic locking. Furthermore, we find that the heterogeneous transition zone introduces a great variability in microseismicity, thus also explaining the scarcity of repeating earthquakes in the region of Anza. Such rheological transition may reflect the combined effects of complex structures and properties of the deep fault zone, and probably applies to other similar fault segments. Importantly, our models demonstrate that these faults are capable of hosting large earthquakes rupturing to the greatest extent of seismicity, with increased complexity in rupture fronts downdip. Our simulations predict that spontaneous and triggered aseismic transients should be ubiquitous in the transition zone between large events on the Anza segment, which can be potentially verified using high-precision geodetic observations.

Jiang, J., & Fialko, Y. (2016, 07). Reconciling seismicity and geodetic locking depths on the Anza segment of the San Jacinto Fault. Poster Presentation at 2016 SCEC Annual Meeting.

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