Abstract
A mathematical model for historical horizontal crustal deformation is presented for the Los Angeles region of California. Model parameters are estimated from triangulation, trilateration, and astronomic azimuth data observed during the past 120 years. Also geologic and seismic data are used indirectly in specifying the model which relates crustal deformation to strain rates over large geographic areas, secular slip rates along geologic faults, and to episodic movements associated with large ( M >- 6) earthquakes. The episodic movements are modeled in accordance with the theory of dislocation in an elastic halfspace. For the secular motion, the region is partitioned into a mosaic of proposed microplates or “districts” that are allowed to individually translate, rotate, and deform homogeneously as a linear function of time. The model thus accounts for the secular and episodic time variability of the geodetic markers coordinates. The results are compared with geodetic, geologic, and seismic studies by other investigators to evaluate the reliability of the procedure and the estimates. In some cases the model provides the first geodetic determinations of strain rates, slip rates, directions of maximum right-lateral shears, and values of coseismic displacements experienced within the region. The shear strain rates reveal that maximum right-lateral shears are oriented parallel to the San Andreas fault system and decrease in magnitude with distance from the central Transverse Ranges. The directions of shear strain form a funnel-shaped pattern that converges on the point outside the region where the San Andreas, Garlock, Big Pine, and San Gabriel faults meet. The velocity for a point near San Pedro relative to a point at the region's northeast corner is 1.8 ± 0.6 cm/yr in the direction N15 δW ± 11 δ. These results are discussed in light of data limitations and modeling inadequacies, subsidence due to oil and gas withdrawal, and postseismic relaxation following the 1971 San Fernando earthquake.
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