Three‐Dimensional Basin Depth Map of the Northern Los Angeles Basins From Gravity and Seismic Measurements.

Villa, Valeria; Li, Yida; Clayton, Robert W.; Persaud, Patricia

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 low seismic velocities. In the event of a major earthquake rupture along the southern segment of the San Andreas fault, their connection and physical proximity to Los Angeles (LA) can produce a waveguide effect and amplify strong ground motions. Improved estimates of the shape and depth of the sediment‐basement interface are needed for more accurate ground‐shaking models. We obtain a three‐dimensional basement map of the basins by integrating gravity and seismic measurements. The travel time of the sediment‐basement P‐to‐S conversion, and the Bouguer gravity along 10 seismic lines, are combined to produce a linear relationship that is used to extend the 2D profiles to a 3D basin map. Basement depth is calculated using the predicted travel time constrained by gravity with an S‐wave velocity model of the area. The model is further constrained by the basement depths from 17 boreholes. The basement map shows the south‐central part of the San Gabriel basin is the deepest part and a significant gravity signature is associated with our interpretation of the Raymond fault. The Chino basin deepens toward the south and shallows northeastward. The San Bernardino basin deepens eastward along the edge of the San Jacinto Fault Zone. In addition, we demonstrate the benefit of using gravity data to aid in the interpretation of the sediment‐basement interface in receiver functions. Plain Language Summary: The shaking levels in the Los Angeles (LA) metropolitan area due to an earthquake on the San Andreas fault are underestimated. Northeast of LA, the San Gabriel, Chino, and San Bernardino basins influence the amount of shaking the LA area will experience. Sedimentary basins like these can amplify and trap seismic waves. Understanding these basins' shapes will improve our Earth model of the area and therefore seismic hazard estimates. The Basin Amplification Seismic Investigation project installed several small seismic instruments across these basins to characterize the structure of the basins. Along with gravity measurements, which capture information about the rock's density variations, we determine the basins' depth and shape. The depth model is then combined with a new velocity model of the area to produce an improved Earth model. Future studies of ground shaking should take these improved models into account. Key Points: Passive seismic and gravity measurements are integrated to estimate the 3D depth of the northern Los Angeles basinsThe maximum depth in the San Gabriel basin is ∼4.5 km, and Chino and San Bernardino basins are less than 2 km deepThe use of gravity helps to delineate faults across the basin, which helps our sediment‐basement interpretation in the receiver functions

LOS Angeles (Calif.); CHINO (Calif.); GRAVIMETRY; EARTHQUAKE hazard analysis; SURFACE fault ruptures; TRAVEL time (Traffic engineering); GROUND motion; SEISMIC waves; FAULT zones

Journal of Geophysical Research. Solid Earth, 2023, Vol 128, Issue 7, p1

2169-9313

Academic Journal

10.1029/2022JB025425