Velocities and Upper Crustal Structure of the Hayward Fault Zone: Results From the 2016 East Bay Seismic Experiment

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

The 2016 East Bay Seismic Investigation (EBSI-16) was designed to examine the greater Hayward Fault Zone (HFZ) and the San Leandro Block (SLB) that defines the block’score near San Leandro, California. The EBSI-16 data allowus to examine the subsurface of the entire East Bay plain and a wide swath of the East Bay hills using a variety of seismic methods. We deployed vertical- and horizontal-component seismographs, spaced at 100 m intervals along a 15-km-long, ~55°-trending profile from the East Bay Shore to the eastern East Bay hills, centered on the active trace of the Hayward Fault (HF). Where the profile crossed known or suspected faults, we deployed closely-spaced (20-m) vertical-component seismographs to better locate these faults with multiple seismic methods. Seismic energy was generated by buried explosions (up to 45 kg) spaced at ~1-km intervals. From the data, we developed refraction tomography images of the HFZ to depths of 5–6 km for Vp and ~2 km for Vs. Our Vpimages show that the San Leandro Block (SLB) is a low-velocity structure in the upper ~1 km, with nearly the same Vpas the adjacent Great Valley sediments to the east. Deeper (>1 km) Vpand Vsimages show high velocities and significant basement topography within the SLB, west of the active trace of the HF, and beneath much of the East Bay plain. Shallow depth (<1 km) Vp/Vsand Poisson’s ratio images imply several fault zones east of the active trace of the HF, which is consistent with known splay faults of the HFZ. At >1 km depth near the western end of our profile, we observe a Vp/Vsanomaly (~3.2) that far exceeds anomalies within the HFZ, including the active trace of the HF (~2.2). The westernmost Vp/Vsanomaly is near-vertical and is on-strike with the northward projection of the Silver Creek Fault, which trends through downtown San Jose and parts of the East Bay toward our seismic profile. The EBSI-16 data provide the best constraints on upper crustal Vp(1900 – 5750 m/s), Vs(700 – 3100 m/s), and structure of the HFZ to date.

Presenting Author: Luther M. Strayer

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