DELINEATION OF FAULTING AND BASIN GEOMETRY BENEATH URBANIZED SAN BERNARDINO VALLEY, CALIFORNIA, FROM SEISMIC REFLECTION AND GRAVITY DATA

Fourteen kilometers of continuous, shallow seismic reflection data acquired through the urbanized San Bernardino Valley, California, have revealed numerous faults between the San Jacinto and San Andreas faults as well as a complex pattern of downdropped and uplifted blocks. More than 1.4 km of apparent vertical displacement on the basement is observed across the San Jacinto fault at this location. These data also indicate that the Loma Linda fault continues northeastward at least 4.5 km beyond its last mapped location on the southern edge of the valley and to within at least 2 km of downtown San Bernardino. Previously undetected faults within the valley northeast of the San Jacinto fault are also imaged, including the inferred western extension of the Banning fault and several unnamed faults. The Rialto-Colton fault is imaged southwest of the San Jacinto fault. The seismic data image the top of the crystalline basement complex across 70 percent of the profile length, and show that the basement has an overall dip of roughly 10° SW between Perris Hill and the San Jacinto fault. Gravity data corroborate the interpreted location of the San Jacinto fault and better constrain the basin depth along the seismic profile to be as deep as 1.7 km. Gravity data also corroborate the fault locations and the general dip of the basement surface. The basin geometry delineated by these data was used to generate modeled ground motions that show peak horizontal amplifications of 2-to-3 above bedrock response, which is consistent with recorded earthquake data in the valley.