GRAVITY PROFILES ACROSS THE SAN JOSE FAULT ON CAL POLY POMONA CAMPUS

Traces of the San Jose Fault are known to run through the California State Polytechnic University, Pomona campus. Several geotechnical investigations have been conducted in an attempt to locate and classify these traces, but the results have been conflicting. The disagreement in the literature about the fault type (left-lateral strike slip versus reverse) coupled with the uncertainty of the fault’s location adds to the mystery of the San Jose Fault and what kind of a role it plays on the Cal Poly campus. The Seismic Review Board has classified several buildings on campus as some of the most seismically hazardous buildings of the entire CSU system. The CSU Board of Trustees recently voted to raze the iconic Cal Poly Pomona Classroom, Laboratory and Administration Building because of poor construction and seismic code concerns, giving rise to more questions about the fault’s true location.

Six profiles across the campus were chosen to run gravity surveys, to determine whether lateral variations in rock density could be detected, corresponding to the proposed locations of the fault. The choice of sites of the gravity profiles were based on the traces of the San Jose Fault as mapped by the GeoCon geotechnical investigation and practical considerations of accessibility and terrain. The surveys were conducted using a LaCoste and Romberg Gravimeter and a total station surveying instrument. The use of the total station ensures accurate elevation measurements, which are required for high quality gravity corrections. Profiles of elevation, free air, and Bouguer gravity anomalies are presented and the results are compared with those from previous geotechnical trenching and geological mapping studies.