Double beamforming ambient noise tomography of the Wasatch Fault System using a temporary linear array

Abstract:

We provide new constraints on the shallow structure of the East Bench segment of the Wasatch Fault System by exploiting continuous data recorded from a month-long temporary linear array of 32 nodal stations in Salt Lake City. By cross-correlating the ambient noise records, we extract Rayleigh wave signals at periods 0.4 s to 1.1 s and apply double-beamforming to enhance signal and measure phase velocities across the array. We invert each beam center dispersion curve using an uncertainty-weighted least-squares inversion scheme and combine all inverted 1-D Vs models to obtain a pseudo-2D Vs model for the area, sensitive to the upper 400 m beneath the array. The derived model has higher velocities to the east, with a low-velocity layer gradually thickening towards the west corresponding to the Salt Lake basin sediments. In addition, there is a 400m-wide low-velocity zone which narrows with depth adjacent to the surface trace of the East Bench fault which we interpret as a fault-related damage zone. The proposed geometry of the damage zone is asymmetric: wider in the hanging wall and with greater velocity reduction. Our results provide important information regarding the shallow structure and mechanics of the Wasatch Fault System and for the seismic hazard assessment in Salt Lake City. Furthermore, the good correlation between our model features and the local geologic and tectonic structures encourage future applications of the double beamforming method in more urban environments.