East Bay Seismic Experiment

Panorama photo of Warren Hall

Warren Hall towers above the California State University East Bay Campus, with the city of Hayward and San Francisco Bay in the valley below. The Hayward Fault runs along the base of the hills.

Map showing Hayward Fault and instrument sites

Map showing the deployment of hundreds of seismographs in the city of Hayward that the USGS will use to record and observe the implosion of Warren Hall on the campus of California State University East Bay, Hayward.

California State University East Bay (CSU-EB) will implode Warren Hall, which is located near the active trace of the Hayward Fault. This implosion provides an excellent opportunity to use a “free” seismic source that is practically located on the Hayward Fault. There are some important data that the implosion (if recorded) could provide to us. The plan is to deploy 500 to 600 temporary seismographs in the Hayward area to capture the seismic signal generated by the implosion.

The effort is primarily a collaboration between CSU-EB and the USGS, but other organizations and agencies are also planning to conduct additional seismic investigations using the Warren Hall source. From the source, the USGS and CSU-EB would like to record the seismic energy as it moves from the source along concentric circles. Because attenuation is generally greater with distance from the source, the concentric circles allow us to measure the same theoretical amplitude (a) in the hard rock of the hills, (b) in the soft rocks/sediments of the valley, (c) within the fault zone, and (d) on the peaks, regardless of the source function. From these measurements, we can compare the relative amplification effect of the various geologic terrains. The seismographs will be densely spaced so that we can also look at the effect of relative amplification within each geologic terrain, which may help to explain why seismic energy can be stronger on one side of a street relative to the other side of the same street. Because the seismographs will be deployed for several days, we hope to also record natural seismic events on the fault. From any of the events on the fault, we can measure guided waves that travel along the fault. This will allow us to determine if sub parallel faults are connected to the Hayward Fault at depth because both the Hayward Fault and the connected faults will transmit high-amplitude guided waves only if they are connected.

Along several of the radii extending from Warren Hall, we plan to measure seismic velocities (Vp, Vs) in the hills, within the fault zone, and in the valley. These data are very useful in modeling expected ground shaking, and the data may also allow us to determine if there are additional faults east and west of the main surface trace. Using the combined P- and S-wave data, we may be able to determine Vp/Vs and Poisson’s ratios, which can especially highlight the fault zones. The regional earthquake-monitoring network of seismographs used to locate earthquakes in the Bay Area will be turned on before the planned implosion to record the signal. From these data, we can determine how accurately the network locates the implosion and provide correction factors, essentially providing a calibration for the network.

Photos

Example of amplification of seismic waves in fault zones

s=“six column align-right”> Figure depicting seismic waves

(a) Plot of peak ground velocity (amplitude) as a function of distance across the San Andreas Fault. Note the high amplitudes centered over the San Andreas Fault.
(b) Cross section of seismic velocities across the San Andreas Fault. The main trace of the San Andreas Fault that ruptured in 1906 is located at about meter 31 of the seismic profile (see red arrow). Two other minor faults are shown at meters 4 and 41. Note the very low seismic velocities of material within the main fault zone relative to the areas to either side. Many other fault zones have shown similar amplification.

Research scientist: Rufus Catchings. Photos by Scott Haefner

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