Global positioning system measurements of deformations associated with the 1987 Superstition Hills Earthquake: Evidence for conjugate faulting

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Abstract
Large station displacements observed from Imperial Valley global positioning system (GPS) campaigns are attributed to the November 24, 1987, Superstition Hills earthquake sequence. Thirty sites from a 42‐station GPS network established in 1986 have been reoccupied during 1988 and/or 1990. Displacements at three sites within 3 km of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (MS6.6) within a preexisting GPS network. Best‐fitting uniform slip models of rectangular dislocations in an elastic half‐space indicate 130 ± 8 cm right‐lateral displacement along the northwest‐trending Superstition Hills fault and 30 ± 10 cm left‐lateral displacement along the conjugate northeast‐trending Elmore Ranch fault. The geodetic moments are 9.4 × 1025dyn cm and 2.3 × 1025dyn cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest that postseismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using singular value decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of nonseismic secular displacement is observed across the Imperial Valley, which is attributed to interseismic plate‐boundary deformation.