3D Basin Depth Map for the San Gabriel, Chino, and San Bernardino Basins

The San Gabriel, Chino, and San Bernardino sedimentary basins in Southern California amplify earthquake ground motions and prolong the duration of shaking due to the basins shape and the low seismic velocities of the soft sediments. In the event of a major rupture along the southern segment of the San Andreas fault, the physical proximity and connection of the basins can produce a waveguide effect and channel the amplified energy into the Los Angeles area, threatening the megacity of Los Angeles to intensified ground motions. Improving estimates of the shape of the sediment-basement interface is required for more accurate ground shaking models in this high seismic-hazard zone. From gravity, seismic data, and borehole observations, we obtain a three-dimensional sediment-basement map of the San Gabriel, Chino, and San Bernardino basins. Using the least-squares method on travel time of the sediment-basement P-to-s conversion, obtained from receiver functions, and Bouguer gravity along the 10 nodal lines, we extend the 2D constraints to a 3D map of basement time, constrained by gravity. We calculate the depth of the basement by taking an average S-wave velocity within the basin and convert the time of basement constrained by gravity to depth. Due to the ambiguity of gravity data interpretation, data from ~14 boreholes provide further constraints to the model. The basement map shows the shape of the San Gabriel, Chino, and San Bernardino basins. The south-central part of San Gabriel basin is in the deepest part and a significant gravity signature is associated with our interpretation of the Raymond fault. The Chino basin deepens towards the south and shallows to the northeast, towards the San Gabriel Mountains. The San Bernardino basin, bounded by the San Jacinto fault (SJF) and San Andreas fault zone, deepens from the northern to the southern end of the SJF within the basin. In addition, we demonstrate the improvement of using gravity data to aid in the interpretation of the sediment-basement interface in receiver functions profiles.