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Seismic Imaging of the Mw 7.1 Ridgecrest Earthquake Rupture Zone and Garlock Fault From Data Recorded by Dense Linear Arrays

We analyze seismograms recorded by five arrays with 100 m station spacing and apertures of 2–8 km that cross the surface rupture of the 2019 Mw 7.1 Ridgecrest earthquake (from B1 in the NW to B4 in the SE) and the adjacent Garlock fault (A5). For the Ridgecrest earthquake rupture zone, the results show complex internal structures (velocity contrasts and low-velocity zones) that vary along fault strike. Both teleseismic and local P waves travel faster on the northeast than the southwest side of the fault beneath arrays B1 and B4, but the velocity contrast is less reliably resolved at arrays B2 and B3. We also identify several 1–2 km wide low-velocity zones with much slower inner cores that amplify S waveforms, inferred as damage zones, beneath each array. The damage zones at arrays B2 and B4 also generate fault-zone head and trapped waves. The trapping structure around array B4 has, based on waveform modeling of stacked trapped wave, a width of ∼300 m, depth of 3–5 km, S wave velocity reduction of ∼20% with respect to the surrounding rock, Q-value of ∼30, and S wave velocity contrast of ∼4% across the fault (faster on the northeast side).

Clear P waves reflected from the Garlock fault, oriented northeast-southwest near the southern end of the Ridgecrest rupture zone, are observed at array B4 and identified for 7 events with depths ranging from 4 to 10 km. The polarity of fault zone reflected waves suggests that P waves travel faster in the crustal block north of the Garlock fault. This is in agreement with large-scale tomography models, and results of P-wave delay times of local and teleseismic events, along with fault zone head waves resolved at array A5. We hand-pick good quality reflected signals and image a vertical dipping angle of the Garlock fault interface between 2-6 km via Kirchhoff migration. A ~300-m-wide low velocity zone centered on the Garlock fault is also identified from P wave delay times, but no persistent S wave amplification pattern or fault zone trapped waves are detected, likely due to the complicated surface geology beneath array A5.

Session: Fault Damage Zones: What We Know and Do Not I

Type: Oral

Room: Grand A

Date: 4/20/2022

Presentation Time: 02:00 PM Pacific

Presenting Author: Hongrui Qiu

Student Presenter: No

Additional Authors