A seismic refraction and reflection study across the central San Jacinto Basin, Southern California

The San Jacinto Basin is a northwest-trending, pull-apart basin in the San Jacinto fault zone of the San Andreas fault system in southern California. About 24 km long and 2 to 4 km wide, the basin sits on a graben bounded by two strands of the San Jacinto fault zone: the Claremont Fault on the northeast and the Casa Loma Fault on the southwest. We present a case study of shallow structure (less than 1 km) in the central basin. A 2.75-km refraction line running from the northeast to southwest across the regional structural trend reveals a groundwater barrier (Offset I). Another line, bent southward and continued for 1.65-km, shows a crystalline basement offset (Offset III) near an inferred trace of the Casa Loma Fault. Although a basement refractor was not observed along the 2.75-km line, a mismatch between the estimate of its minimum depth and the basement depth determined for the 1.65-km line suggests that an offset in the basement (greater than 260 m) exists around the junction of the two refraction lines (Offset II). By revealing more faults and subtle sedimentary structures, the reflection stack sections confirm the two refraction offsets as faults. Offsets I and III each separate sediments of contrasting structures and, in addition, Offset III disrupts an unconformity. However, the sense and amount of the offset across Offset III contradict what may be expected across the Casa Loma Fault, which has its basinward basement down-thrown to about 2.5 km in the better defined southeastern part of the graben. The Casa Loma Fault trace has been mislinked in the existing geological maps and the trace should be remapped to Offset II where the reflector disruptions spread over a 400-m wide zone. Our Offset III is an unnamed, concealed fault.