Seismic velocity structure in the Hot Springs and Trifurcation areas of the San Jacinto fault zone, California, from double-difference tomography

A.A Allam,I Kurzon,F Vernon,Y Ben-Zion

doi:10.1093/gji/ggu176

Abstract

We present tomographic images of crustal velocity structures in the complex Hot Springs and Trifurcation areas of the San Jacinto Fault Zone (SJFZ) based on double-difference inversions of earthquake arrival times. We invert for V P , V S and hypocentre location within 50 × 50 × 20 km 3 volumes, using 266969 P and 148249 S arrival times. We obtain high-fidelity images of seismic velocities with resolution on the order of a few kilometres from 2 to 12 km depth and validate the results using checkerboard tests. Due to the relatively large proportion of S-wave arrival times, we also obtain stable maps of V P /V S ratios in both regions. The velocity of the Trifurcation Area as a whole is lower than adjacent unfaulted material. We interpret a 4-km-wide low velocity zone with high V P /V S ratio in the trifurcation itself as related to fault zone damage. We also observe clear velocity contrasts across the Buck Ridge, Clark and Coyote Creek segments of the SJFZ. The Anza segment of the SJFZ, to the NW of the trifurcation area, displays a strong (up to 27 per cent) contrast of V S from 2 to 9 km depth. In the Hot Springs area, a low velocity zone between the Claremont and Casa Loma Strands narrows with depth, with clear velocity contrasts observed across both segments. A roughly 10-km-wide zone of low velocity and low V P /V S ratio at the NW tip of the Hot Springs fault is indicative of either unconsolidated sediments associated with the San Jacinto basin, or fluid-filled cracks within a broad deformation zone. High V P /V S ratios along the Anza segment could indicate a preferred nucleation location for future large earthquakes, while the across-fault velocity contrast suggests a preferred northwest rupture propagation direction for such events.

Highlights

Detailed imaging of fault zone velocity structure can reveal important information about the previous behaviour and likely future properties of earthquakes on the fault
The results show significant low velocity zones in the Trifurcation and Hot Springs areas that narrow with depth
3.1 Inversion benchmarks As in previous double difference tomography works (e.g. Thurber et al 2006; Lin et al 2010; Allam & Ben-Zion 2012), we evaluate the quality of the two final models based on four criteria: (1) improvement of fit to the arrival time data, (2) sufficient resolution calculated by the derivative weight sum, (3) plausibility of hypocentre relocations, in terms of alignment with known fault traces and consistency with the velocity structure and (4) resolution estimated from synthetic tests

Summary

Introduction

Detailed imaging of fault zone velocity structure can reveal important information about the previous behaviour and likely future properties of earthquakes on the fault. Fault zone regions with relatively high and low seismic velocities may be correlated, respectively, with areas of high slip and end points of ruptures Brietzke & Ben-Zion 2006; Zhao et al 2010; Bennington et al 2013). Coherent low velocity fault zone layers can act as seismic trapping structures that may increase considerably the near-fault ground motion Ben-Zion & Shi 2005; Dor et al 2006; Mitchell et al 2011). An accurate knowledge of the velocity structure is important for correct derivations of earthquake locations and focal mechanisms An accurate knowledge of the velocity structure is important for correct derivations of earthquake locations and focal mechanisms (e.g. Oppenheimer et al 1988)

Results

Discussion

Conclusion