Damage Zone Resonance: Observation and Analytical Fitting of Fault Zone Trapped Wave Normal Modes Using Large-N Near-fault Seismic Arrays

Hongrui Qiu, Amir A. Allam, Fan-Chi Lin, & Yehuda Ben-Zion

Published August 12, 2019, SCEC Contribution #9459, 2019 SCEC Annual Meeting Poster #048 (PDF)

We present the first observations of the full spatial eigen-function of fault zone trapped wave normal modes recorded on a dense large-N near-fault geophone array. The cumulative product of brittle deformation surrounding active faults ¬- the damage zone – creates a region of low seismic velocity that traps seismic energy from near-fault earthquakes. Accurate measurement of the spatial distribution of amplitude, phase, and duration of trapped waves can provide important constraints on the fault damage zone structure and correspondingly allow inference of the behavior of large earthquakes. Here we present the results from a near-fault array dense array (~10m spacing in the damage zone) along the San Jacinto Fault Zone near the Hemet Stepover. We show that the amplitude distributions of the trapped waves are consistent for 328 near-fault events, and are thus controlled by fault zone structure rather than source properties. We derive analytical expressions for the spatial distribution of the frequency-dependent eigen-functions of the trapped waves from a simple model consisting of a low-velocity layer sandwiched between two quarter-spaces of different velocities. For each event, we apply a grid search to obtain the damage zone width and location, S wave velocity inside and outside the fault zone. The results indicate a ~400 m wide damage zone with 55% velocity reduction with the NE boundary located at the surface trace of the fault. The asymmetry and large width suggest that the largest damage-creating events tend to rupture SW to NE and terminate in the Hemet Stepover.

Key Words
Fault zone trapped wave; Normal mode; Fault zone imaging

Citation
Qiu, H., Allam, A. A., Lin, F., & Ben-Zion, Y. (2019, 08). Damage Zone Resonance: Observation and Analytical Fitting of Fault Zone Trapped Wave Normal Modes Using Large-N Near-fault Seismic Arrays. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology