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3d P- and S-Wave Active-Source Seismic Tomography of Rock Valley, the Nevada National Security Site

The Rock Valley Fault Zone (RVFZ) is a ~4 km-wide group of active, left-lateral strike slip faults that trend northeast within Rock Valley (RV), in the southern Nevada National Security Site. The RVFZ has hosted a number of magnitude > 3 events, and in May 1993, a shallow (< 3 km) magnitude 3.7 event occurred at the RVFZ and triggered over 600 aftershocks in the five months following. Precise hypocentral locations require a local seismic study in order to refine seismic velocities and constrain the faults at depth. We conducted an active-source accelerated weight drop (AWD) experiment over the central RVFZ area in April and May of 2021. We deployed 188 3-component seismometers along several intersecting transects at ~100 m spacing and recorded 553 AWD vertical and side hits. The waveform data were processed into shot gathers from which we picked P- and S-wave travel-times. We then inverted these picked travel-times for 3D P-wave velocity (Vp) and S-wave velocity (Vs) structure using a linearized, iterative, least squares inversion scheme. We resolved velocities as deep as 600 m, with Vp and Vs structure consistent with the local geology of the RVFZ. We imaged lower velocities at depth to the northwest in a volcanic domain and higher velocities to the southeast in a carbonate domain, with Vp reaching 3 km/s at ~200 m below the surface to the northwest and ~150 m below the surface to the southeast. Just to the north of, and bounded by, the central fault strands is a lower velocity region that is ~ 1 km wide and persists to the deepest part of the velocity model. Vp in this region reaches 3 km/s at ~300 m below the surface. The lower velocity region could be caused by enhanced faulting and fracturing. These local 3D Vp and Vs models will be incorporated into regional models to be used for more precise earthquake hypocentral locations.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525

Session: Development, Enhancement and Validation of Seismic Velocity Models [Poster]

Type: Poster

Room: Evergreen Ballroom

Date: 4/21/2022

Presentation Time: 08:00 AM Pacific

Presenting Author: Jennifer L. Harding

Student Presenter: No

Additional Authors