USGS to conduct seismic imaging tests

By Nathan R. Huff

Campbell will be rocked by explosions in the coming weeks, but residents need not worry about gathering supplies or digging out bomb shelters.

The explosions, from blank shotgun shells to 200-pound charges of ammonium nitrate, are part of a U.S. Geological Survey (USGS) seismic imaging investigation in the Los Gatos Creek area. The government agency hopes to use the survey to, among other things, locate buried faults that may pose significant earthquake hazards to the Santa Clara Valley.

Other goals of the survey include determining the thickness and structure of the sedimentary basin beneath the West Valley, mapping the stratigraphy--layers of underground rock--associated with groundwater in the area, and evaluating the effects of faulting on groundwater resources in Los Gatos' end of the valley. The Santa Clara Valley Water District (SCVWD) will work in collaboration with the USGS.

The area to be studied begins north of Lexington Reservoir and travels along Los Gatos Creek approximately six miles to the intersection of Los Gatos Creek and Hamilton Avenue in Campbell.

Two separate tests are to be conducted, the first a high resolution seismic imaging survey of the shallow subsurface, and the second a large scale imaging survey designed to map out the entire subterranean basin of the West Valley.

The first test will involve stringing a mile-long cable along the ground--most likely along the creek trail. About every 15 feet, a "geophone," which measures seismic vibrations, would be attached to the cable. The three inch wide geophone has a spiked tip, which would be pushed about three inches into the ground.

Alongside each geophone, a blank shotgun shell would be buried eight to 10 inches into the ground. When the shells are fired, the geophones record the subterranean reaction.

The second test will involve small seismographs, the devices used for measuring earthquakes. Ten units will be placed evenly along the entire six-mile survey area, and buried eight inches deep.

This time, the seismic source will be provided by 50 to 200 pound charges of ammonium nitrate, placed at depths of 75 to 150 feet. While the explosions will occur between the hours of midnight and 5 a.m. to minimize outside noise interference, the USGS says it is doubtful anyone other than those present at the test sites will hear or feel anything.

The agency will also secure the areas around the second test sites and provide literature describing the survey to any early morning joggers passing through the area.

The USGS has conducted regular seismic surveys throughout California since the 1960s. The surveys are intended to identify and reduce earthquake hazards, as well as provide information to other agencies, like SCVWD, to ensure water supplies are safe and available.

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