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Results from dense nodal-array recordings of the 2019 Ridgecrest Sequence aftershocks

Rufus D. Catchings, Mark R. Goldman, Malcolm C. White, Hongrui Qiu, & Yehuda Ben-Zion

Published August 14, 2020, SCEC Contribution #10597, 2020 SCEC Annual Meeting Talk on TBD

The 2019 Ridgecrest, California earthquake sequence included Mw 6.4 and Mw 7.1 events on orthogonal faults, with tens of thousands of ensuing aftershocks. We recorded aftershocks and ambient noise for a 2-month period following the mainshock to: (1) evaluate connectivity of the faults, (2) determine the 3D fault and velocity (Vp and Vs) structure, and (3) evaluate 3D spatial and temporal variations in seismicity. We deployed 461 three-component nodal seismographs (nodes) at 575 individual recording sites. The nodes recorded continuously, allowing us to collect ~14TB of data, which are available at the IRIS DMC. Here, we summarize some of our initial findings. Combining data from both the nodal arrays and the regional network, White et al. (2020) developed detailed tomographic 3D Vp, Vs, and Vp/Vs models using over 2.47 million P-wave and 1.52 million S-wave arrivals, and they relocated more than 94,000 earthquakes (> 2.5 times the number of events reported in the SCSN catalog). They found strong velocity contrasts across the causative faults, low Vp, Vs, and Vp/Vs ratios along the Garlock fault, and high Vp, low Vs, and a zone of low seismicity in the Coso area. Qiu et al. (2020) used nodal data from four linear arrays across the main NW-SE fault zone and found a 1-2-km-wide, low-velocity, fault-damage zone, with large velocity differences across the fault. The southern part of the fault zone consists of an ∼300-m-wide, ∼4-km-deep, low-S-wave velocity (~20%) zone. Goldman et al. (2020) used nodal data from arrays across both Ridgecrest faults to evaluate the individual faults within the fault zones. They show that the southeastern end of the NW-SE fault zone is more than 5 km wide, with the main fault likely on the southwest side of the rupture zone. The southwestern end of the NE-SW fault is at least 3.5 km wide, with the main fault likely centrally located among the surface ruptures. Catchings et al. (2020) used nodal data from arrays crossing both Ridgecrest faults and the Garlock fault to infer connectivity among the faults. They show that aftershocks on the Ridgecrest faults generated guided waves on the other Ridgecrest fault and on the Garlock fault, suggesting that all three faults are connected in the subsurface. The density and the large foot print of the nodal arrays (50 x 60 km), combined with the highly productive Ridgecrest aftershock sequence, provide ample data to evaluate diverse aspects of the complex faulting in the region.

Key Words
seismicity, Vp, Vs, fault locations, fault connectivity

Catchings, R. D., Goldman, M. R., White, M. C., Qiu, H., & Ben-Zion, Y. (2020, 08). Results from dense nodal-array recordings of the 2019 Ridgecrest Sequence aftershocks. Oral Presentation at 2020 SCEC Annual Meeting.

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