Seismic Study Of Santa Clara Valley Gets Underway
The project will involve detonation of 11 explosive shots buried to about 60 feet, and about 2,000 small shotgun blasts at depths of about one foot. As energy waves from the explosions travel through the ground they will be recorded by portable seismometers throughout the area. Because the ground motion being recorded is less than that caused by normal daylight activities such as vehicular traffic, people and livestock moving about, vibrations from pumps, etc., most detonations and recording will be at night, usually between 10 p.m. and 3 a.m.
Analysis of the seismic recordings will reveal various types of sub-soil rock layers and places those layers have been disrupted by faults. In addition to known faults in the western Santa Clara Valley, other geologic mapping studies indicate there may be numerous thrust faults beneath Santa Clara Valley. The strong shaking that can be generated by thrust faulting beneath valley sediments, as happened in the 1994 Northridge Earthquake, added to the dense population of the Santa Clara Valley and the economic implications of earthquakes in the Silicon Valley, has prompted the USGS Earthquake Hazards Team to attempt to locate possible thrust faults and better understand their hazard potential.
USGS scientist Rufus Catchings said the planned investigation will consist of two efforts, a high-resolution seismic imaging effort to precisely locate possible faults, from depths of about 2.5 meters (8 feet) to about one kilometer (1.6 miles), and a lower-resolution seismic imaging effort to measure seismic velocities and image faults from depths of about one kilometer to more than five kilometers. Catchings said this information is necessary to help determine the hazard and likely intensity of shaking from earthquakes in the Santa Clara Valley.
USGS hydrologist Randy Hanson said in sediments like those of the Santa Clara Valley, faults are known to affect groundwater migration paths. "The location of faults may affect whether recharged groundwater, which is often pumped in the subsurface during wet periods, can be successfully recovered during dry period," Hanson said. "Furthermore, the stratigraphic sequence within a given basin can determine if recharge of the groundwater supply is possible. " Hanson said questions related to groundwater supply can be partially determined using drilling techniques, but those techniques are expensive and cannot give a complete picture. The seismic data will help answer questions related to groundwater in the Santa Clara Valley.
Seismic refraction experiments like this have been conducted by USGS and other scientific agencies in parts of the US and Canada for 20 years.
As the nation's largest water, earth, and biological science and civilian mapping agency, USGS works in cooperation with more than 2,000 organizations nationally to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize loss of life and property from natural disasters, contribute to sound conservation, economic and physical development of the nation's natural resources, and enhance quality of life by monitoring water, biological, energy, and mineral resources.