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Seismic Identification and Location of Blind, Near-Vertical Faults in Granitic Rocks With Application to Wide-Ranging Geologic Settings

Description: 

Identifying and locating near-vertical faults using seismic methods can be difficult in granitic rocks, especially for blind faults in sediment-covered areas. We used multiple seismic methods, including two-dimensional Vp tomography, MASW-based Vs, Vp/Vs ratios, Poisson’s ratios, and reflection imaging to identify and locate subsurface faults in the western Mojave Desert of southern California. Below the groundwater table within fault zones and relative to non-faulted rocks, Vp is high due to saturation, and Vs is low due to shearing. As a result, Vp/Vs and Poisson’s ratios are high (up to 4.0 and 0.45, respectively) in the fault zones. These observations allow us to identify faults based on prominent co-located near-vertical zones of high Vp, low Vs, high Vp/Vs ratios, and high Poisson’s ratios, and in unmigrated reflection images, these same faults appear as zones of near-vertically aligned diffractions. Our western Mojave Desert seismic profile traverses a sediment-covered valley and bedrock exposures in local hills. Geologic mapping by others identified surface fault traces in the hills that coincide geographically with the seismically imaged subsurface faults. Furthermore, the seismic images show that there are prominent blind faults beneath the sediment-covered valleys that were not identified from surface mapping. Laterally varying depths of the 1500 m/s velocity contour in the Vp tomographic images coincide with the depths of the groundwater table measured in multiple wells along the seismic profile. Vertical offsets in the groundwater table (1500 m/s) are seen across imaged near-vertical faults, demonstrating that the faults act as barriers to lateral groundwater flow and further confirming their subsurface locations. These combined seismic methods allow effective identification and location of faulting in both subsurface sediments and granitic or crystalline rocks, and the methods can be applied to image faults in most other terrestrial settings.

Session: Above the Seismogenic Zone: Fault Damage and Healing in the Shallow Crust [Poster]

Type: Poster

Date: 4/19/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Rufus D. Catchings

Student Presenter: No

Invited Presentation: 

Authors