NORTHERN COACHELLA VALLEY AND SAN GORGONIO PASS FAULT STRUCTURE, SOUTHERN CALIFORNIA

We use seismic tomography and reflection images from multiple crustal-scale and shallow-depth profiles, combined with geologic mapping and seismicity cross sections, to characterize the crustal and fault structure of the northern Coachella Valley and San Gorgonio Pass. We interpret composition and fault structure in the upper few hundred meters of the crust from shallow-depth, high-resolution seismic images and geologic mapping. In the upper ~ 10 to 15 km, we interpret composition and fault structure from crustal-scale seismic images. Our seismic profiles cross many of the major faults within the region, including the Mission Creek, Banning, and Garnet Hill Strands of the San Andreas Fault Zone (SAFZ) and the Pinto Mountain Fault. In the shallow crust, seismic tomography reveals differing subsurface materials across faults, from which fault dip can be measured. Because the shallow faults are groundwater barriers, the top of ground water (Vp =1500 m/s on tomography images) is vertically offset, allowing us to further evaluate shallow-depth fault dip. At upper crustal depths, strands of the SAFZ juxtapose rocks of differing velocities, allowing us to estimate upper crustal composition and fault dip. From upper crustal to mid-crustal depths, shearing of rocks within fault zones creates low-velocity (Vp and Vs) zones relative to the velocities of the adjacent intact crustal rocks. Thus, upper and mid-crustal fault dip also can be determined from the dips of the low-velocity zones. Along our profiles, combinations of coincident data sets (seismic tomography, reflection, seismicity, and/or resistivity) are internally consistent with respect to fault dip, structure, and composition. From our combined data, we observe a thin cover of sediments (< 1 km) and dominantly northeasterly dipping faults in the shallow crust in San Gorgonio Pass. In the northern Coachella Valley, the major strands of the SAFZ juxtapose relatively high- and low-velocity rocks to mid-crustal (~10 to 15 km) depths. Our images indicate that the Garnet Hill and Banning faults dip northeasterly, but the Mission Creek Strand dips vertically to slightly southwesterly. We interpret these faults to merge by mid-crustal depths, thereby forming a near-vertical fault zone.