San Francisco Bay Area Arrays and the East Bay Seismic Experiment

Science Center Objects

Portable Seismograph Deployments to Research the Effects of Basins, Topography, and Fault Zones on Seismic Waves.

The implosion of the Warren Hall building on California State University East Bay (CSU-EB) campus in August of 2013 provided an excellent opportunity to use a “free” seismic source that was practically located on the Hayward Fault.

Overview  |  Tracking Stress Buildup and Crustal Deformation  |  Fault Slip Rates and Post-Earthquake Motions

Ground Movement and Ground Shaking  |  Cone Penetration Testing (CPT)  |  Rock Physics Lab

San Francisco Bay Area Arrays and East Bay Seismic Experiment  |  Neogene Deformation

 

San Francisco Bay Area Arrays

Seismologists have observed that both topographic highs and basins have complex and varying effects on seismic waves. By deploying arrays of seismic recorders our understanding is improved of what specific features have what specific effects on the seismic waves. These studies use basins, ridges, and fault zones in the San Francisco Bay area as a laboratory to explore in greater detail what is happening as seismic waves propagate through them. The results can be used in similar urban areas throughout the world to estimate the expected shaking from both local and more distant earthquakes.

location of portable arrays of seismometers

Portable arrays of seismometers across the San Ramon, Pleasanton, and Livermore Valleys. (Public domain.)

Seismic waves traveling through basins and topography are affected by:

  • The edges of the basin
  • The age of the basin, which is related to the density of the material in the basin
  • Fault zones in these areas
  • The direction of approach of the seismic waves and the shape of the feature
  • The distance between the location of the earthquake and the feature
  • The frequency content of the seismic waves

The basic approach employed in these studies is to record seismic waves produced from both earthquakes and man-made sources such as explosions, and then to use several different mathematical methods and models to try to explain the observed levels of shaking. The portable seismographs are set up in different arrays in different locations in order to record various effects. Our studies are facilitated by using a combination of different analysis methods to reveal the wave type, direction, and speed of the important components of the ground motion. Both basins and topographic highs can trap seismic waves and cause them to be amplified. An understanding of these amplification effects is important for proper evaluation of seismic hazards in urban areas.

Download Report on The Effects of Basins, Topography, and Fault Zones on Seismic Waves: San Francisco Bay Area Portable Seismograph Deployments

East Bay Seismic Experiment

Warren Hall towering above California State University East Bay Campus

Before August 2013 Warren Hall towered 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. (Public domain.)

deployment of hundreds of seismographs

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

California State University East Bay (CSU-EB) imploded Warren Hall in August of 2013, which was located near the active trace of the Hayward Fault. This implosion provided an excellent opportunity to use a “free” seismic source that was practically located on the Hayward Fault. 500 to 600 temporary seismographs were deployed in the Hayward area to capture the seismic signal generated by the implosion.

The effort was primarily a collaboration between CSU-EB and the USGS, but other organizations and agencies also conducted additional seismic investigations using the Warren Hall source. From the source, the USGS and CSU-EB recorded the seismic energy as it moved from the source along concentric circles. Because attenuation is generally greater with distance from the source, the concentric circles allowed scientists 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, they compared the relative amplification effect of the various geologic terrains. The seismographs were densely spaced so that scientists could 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.

Along several of the radii extending from Warren Hall, scientists measured 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 can help determine if there are additional faults east and west of the main surface trace. The regional earthquake-monitoring network of seismographs used to locate earthquakes in the Bay Area was turned on before the implosion to record the signal. From these data, scientists can determine how accurately the network located the implosion and provide correction factors, essentially providing a calibration for the network.

Photos by Scott Haefner

Example of Amplification of Seismic Waves in Fault Zones

Example of amplification of seismic waves in fault zones.

Example of amplification of seismic waves in fault zones.
(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. (Public domain.)

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