Crustal deformation across the Imperial Fault: Results from kinematic GPS surveys and trilateration of a densely spaced, small-aperture network

Kinematic Global Positioning System (GPS) surveys in 1991 and 1993 of a 66-station, small-aperture network across a 12-km-long segment of the Imperial Fault near El Centro, California, reveal a fairly uniform secular displacement field consistent with right-lateral slip along a single, vertical dislocation surface. Total relative motion across the array equals 20+/-3 mm/yr at N49+/-7 degrees W for sites 5-6 km away from the fault trace on opposite sides. These rates agree with simple dislocation models that assume 10 mm/yr of creep along the fault trace from the surface to a locking depth of 9 km and a plate boundary rate of 45 mm/yr oriented N40 degrees W at depth. Model fit does not significantly improve if creep rates are permitted to vary along strike by several millimeters per year. A comparison of 1991/1993 GPS with 1987 high-precision electronic distance measurements validates the dislocation models but suggests a smoother deformation field than that defined by the 1991/1993 velocities. The difference can be attributed to the poorer 1991 kinematic positioning accuracy rather than to temporal variations in strain. There is evidence for 20 mm of sympathetic coseismic slip from the 1987 Superstition Hills earthquake, although aseismic slip along both faults appears to be completely decoupled.