Global Positioning System measurements of crustal deformation across the Pacific-North American plate boundary in Southern California and Northern Baja, Mexico

The Pacific-North American plate boundary in southern California and northern Baja, Mexico undergoes a complex transition from crustal spreading in the Gulf of California to right-lateral transform motion along the San Andreas and associated fault systems. Historically, this has been one of the most seismically active segments of the plate boundary. We use GPS observations collected during the period from 1986 to 1995 to investigate the nature of ongoing crustal deformation in this complicated region and to estimate the contemporary rate of Pacific-North American relative plate motion. By allowing for episodic deformation associated with earthquakes, the time evolution of GPS coordinate estimates reveals a steady-state crustal deformation signal. By enlisting a simple block model to explain both the distribution and sum of deformation across the plate boundary, we use the horizontal components of the estimated secular site velocities to infer deep slip rates of 26 ± 2 mm/yr, 9 ± 2 mm/yr, 7 ± 2 mm/yr, and 7 ± 2 mm/yr for the San Andreas, San Jacinto, Elsinore, and San Clemente faults, respectively. We also infer rates of 35 ± 2 mm/yr and 42 ± 1 for the Imperial and Cerro Prieto faults, and a total Pacific-North America relative plate motion rate of 49 ± 3 mm/yr. Our results are highly consistent with both geologic estimates for long term slip rates and previous space geodetic results and are statistically consistent with, though slightly larger than, the NUVEL-1A plate motion estimate. We detect no systematic trends in the residual velocity field. We cannot reject the hypothesis that Pacific-North American relative plate motion is accommodated across a finite set of discrete, relatively narrow shear zones which lie below fault systems known to have undergone significant Quaternary offset. Neither can we reject the elastic Poisson Earth hypothesis.

Coseismic surface deformation associated with the M_w 6.1, April 23, 1992, Joshua Tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from Global Positioning System and trilateration measurements. We apply a Tikhonov regularization operator to these estimates to infer a slip distribution yielding a geodetic moment estimate of 1.7X10¹⁸ N m with corresponding maximum slip around 0.8 m which compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations. From empirical Green's function analyses, a rupture duration of 5 s is obtained which implies a rupture radius of 6-8 km. Most of the inferred slip lies to the north of the hypocenter, consistent with northward rupture propagation. Stress drop estimates are in the range of 2-4 MPa. In addition, predicted Coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters; most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0.1 MPa. In contrast, predicted stress changes are near zero at the hypocenter of the M_w 7.3, June 28, 1992, Landers earthquake which nucleated about 20 km beyond the northernmost edge of the Joshua Tree rupture. Based on aftershock migrations and predicted static stress field, we speculate that redistribution of Joshua Tree-induced stress perturbations played a role in the spatio-temporal development of the earthquake sequence culminating in the Landers event.