Evergreen Basin Seismic Imaging Study
June 16-30, 2002
Ever wonder whether a strong earthquake could occur near your home or business, or how the ground will shake due to any large earthquake in the region, or whether there will be enough water to meet future needs in the Santa Clara Valley?
These are important issues facing the rapidly growing Santa Clara Valley.
In a collaborative effort between the U.S. Geological Survey and the Santa Clara Valley Water District, this study will provide subsurface images or "pictures" that will help answer these questions. The subsurface pictures resulting from this study will help determine which faults are moving and how they move, an essential step in assessing earthquake hazards. Knowing the configuration of buried faults is crucial to understanding how the earthquake-producing mechanism works in the Santa Clara Valley.
Other important structures that will be imaged are sedimentary basins-- large valleys filled with sand, clay and other erosional deposits-- such as the Evergreen basin. Information on the thickness and shape of sedimentary basins is essential for predicting how the ground will shake in future earthquakes and why different parts of the valley shake more strongly than others, work that could lead to improved forecasts of future ground shaking during earthquakes.
Faults also significantly impact the occurrence and movement of ground water, so the position and geometry of faults in the Evergreen basin will also be used to improve the resolution of the basin hydrologic model. An improved hydrologic model will allow better management of the current water supply and provide a more accurate assessment of future resources.
During June 2002, scientists from the U.S. Geological Survey, using an echo-sounding method are collecting data over the Evergreen sedimentary basin within the San Jose Array (see map below left). The echo-sounding method involves sound waves traveling beneath the Earth's surface to create a picture of the subsurface that shows the depth and thickness of deposits similar to the images that we obtained from a similar project in San Bernardino, California, in 1999. The method is based on the same principle as is used to make a sonogram image. This San Bernardino profile (see "Example profile" below right) shows the depth of rock and soil layers refracting and reflecting the sound waves back to the surface. By measuring the speed of sound through the ground and analyzing the echoes, we get an idea of the strength of the ground, its potential to amplify earthquake ground motion, and the possible flow pattern for subsurface water.
The USGS and the Santa Clara Valley Water District will collect data along a line that starts near 1st St in downtown San Jose, then eastward under Highways 101 and 680, following Penitencia Creek to the edge of the Diablo Range. The line was chosen in 2001 in response to the earthquake recordings made by the San Jose Dense Seismic Array, and will provide information about many underground structures.
(MiniVibe photo above courtesy of Industrial Vehicles International)