RELATION OF THE SOUTHERN SAN JACINTO FAULT ZONE TO THE IMPERIAL AND CERRO PRIETO FAULTS

The fate of the southern San Jacinto fault zone in the Imperial Valley, southern California, has been uncertain because the fault features are covered by young sedimentary deposits. I relocate 43,000 earthquakes in an inversion for hypocenters and 3D crustal seismic velocities. The hypocenter patterns define the configuration of the buried southeastward extensions of the Superstition Mountain and Superstition Hills faults (the southernmost elements of the San Jacinto fault zone), and the northwestward extension of the Cerro Prieto fault. The Superstition Hills fault continues 25 km southeast past its mapped surface trace (the Weinert fault) and steps 5 km left to the Imperial fault 10 km north of the international border. A seismicity lineament diverges from the Superstition Mountain fault and continues 30 km southeast to the northeast side of the Heber geothermal area. The Cerro Prieto fault continues 30 km northwest beyond its mapped trace. The Heber geothermal area occupies a 9 km wide right (extensional) step between the Cerro Prieto and Superstition Mountain faults. A fault defined within the Heber production zone corresponds to the Superstition Mountain fault. The enigmatic Dixieland creep zone lies on strike with the Cerro Prieto fault extension and may be a manifestation of the same basement feature. The 12 mm/yr slip of the San Jacinto fault zone apparently is distributed onto the southern Imperial fault (via the Superstition Hills fault) and the Cerro Prieto fault (via the Superstition Mountain fault). If the San Jacinto fault zone is considered as a strand of the San Andreas system formed to simplify the geometric complexity, further development of the Superstition Mountain - Cerro Prieto fault connection can be speculatively anticipated, leading to the abandonment of the southern San Andreas fault, and the Brawley and Cerro Prieto spreading centers. This evolution is analogous to that recently proposed for the northern Gulf of California.