Shallow Subsurface Resistivity Profiles Across the San Jose Fault As It Transects the Cal Poly Pomona Campus

Abstract:

The San Jose fault is a left-lateral strike-slip fault located in the San Gabriel Valley in Southern California. The 1988 (M4.6) and 1990 (M5.2) Upland earthquakes have been attributed to this fault and it has been suggested that it is capable of producing a magnitude M6.0-6.5 earthquake. Sections of the fault are considered to run through the campus of California State Polytechnic University, Pomona as inferred from a 2001 geotechnical engineering report (Geocon, 2001). As it cuts across the campus, the geotechnical engineering report concluded that it has a reverse component of motion. Ascertaining the precise location of the San Jose fault traces on campus is crucial as the university plans its future buildings. Resistivity surveys were conducted across several suggested traces of the fault. The surveys consisted of 24 electrodes in a Wenner electrode configuration with an electrode spacing that varies between 1-5m. An IRIS Instruments Syscal KID switcher unit provided the power source and data recording hardware. The data was processed using IRIS Prosys II software suite before using Geotomo’s Res2Dinv software to obtain 2D images of subsurface resistivity for these profiles. A total of 23 surveys were conducted throughout the campus. Surveys were performed before and after rainfall to compensate for the variation of water content and its effect on resistivity. Preliminary results indicate shallow, north-dipping contrasts in resistivity across many of the areas where the fault was previously identified in the Geocon 2001 report. More data will be analyzed to present an enhanced understanding of the San Jose fault in the vicinity of the Cal Poly Pomona campus at AGU.