Three-Dimensional Basin Depth Map of the Northern Los Angeles Basins
from Gravity and Seismic Measurements
Abstract
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 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 models 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 towards the south and
shallows northeastward. The San Bernardino basin, bounded by the San
Jacinto fault (SJF) and San Andreas fault zone, deepens along the edge
of the SJF. In addition, we demonstrate the benefit of using gravity
data to aid in the interpretation of the sediment-basement interface in
receiver functions.