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Locating an Urban Fault Along the San Francisco Peninsula Using High-Resolution Active-Source Seismology

Description: 

In July 2021, the U.S. Geological Survey conducted a series of seismic surveys in San Carlos, California across suspected traces of the Quaternary-active Serra Fault, a poorly mapped fault trending subparallel to the San Andreas fault along much of the San Francisco Peninsula. We deployed a series of seismic profiles along suburban streets, consisting of 3-component nodal seismometers spaced at 5-m intervals, and co-located seismic sources (shots). Here, we present results from a 61-node, 300-m-long profile along Graceland Avenue. P-wave shots were generated using a 500-kg weight drop mounted on a Bobcat tractor, and S-wave shots were generated by horizontally striking an anchored metal block with a 3.6-kg hammer. We developed 2D tomographic velocity models from P- and S-wave data separately and combined the models to create 2D Vp/Vs and Poisson's ratio models. Our P-wave model shows a zone of high velocities (> 3500 m/s) in the upper 30-m depth, centered near the western end of the profile. Our S-wave model shows a large increase in velocities (600 to > 1200 m/s) east of the same area. Our Vp/Vs and Poisson's ratio models indicate a zone of high ratios (Vp/Vs > 3.5; PR > 0.48) in the upper 30-m depth, centered near meter 180 of the profile. We interpret the velocity differences and high ratios near meter 180 as resulting from the presence of the Serra fault, which is likely a water-saturated fault barrier at that location.

Session: Crustal Imaging of High Seismic Hazard Regions [Poster]

Type: Poster

Date: 4/20/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Mark Goldman

Student Presenter: No

Invited Presentation: 

Authors