Faults in Imperial Valley

by Harold Magistrale
Department of Geological Sciences
San Diego State University

3-4pm
Friday Mar 21, 2003
Refreshments served at 2:45pm
Munk Conference Room
Cecil and Ida M. Green Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California, San Diego
http://mahi.ucsd.edu/seminar/

Abstract

The configuration of fault features in the Imperial Valley, southern
California, have been uncertain because sedimentary cover obscures them.
I use high precision hypocenters to define elements of the southern San
Jacinto fault zone and the Brawley seismic zone. The Superstition
Mountain and Superstition Hills faults comprise the southernmost
elements of the San Jacinto fault zone. The Superstition Hills fault
continues ~30 km southeast past its mapped surface trace 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 ~20 km to the southeast. The Cerro Prieto fault continues
~35 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. 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).

The BSZ is divided into four segments: (1) The north segment, between
the east shore of the Salton Sea and the northeast-striking Elmore Ranch
fault, representing a continuation of the San Andreas fault striking
~N25°W. (2) The Salton Sea geothermal area, near the south shore
of the Salton Sea. The Salton Sea geothermal area lies over a small
northeast-striking spreading center. Seismicity in the geothermal area
is unusually shallow, and is controlled by geothermal power production
activities. (3) The central segment, between the Salton Sea geothermal
area and the northeast-striking fault defined by aftershocks of the
1979 Imperial Valley earthquake. This segment comprises an extensional
transform fault, with seismicity driven by magmatic dikes. (4) The
southern segment, south of the 1979 aftershock-defined fault to the north
end of the Imperial fault. The extensional transform fault continues in
the segment, and it includes the stepover of the transform fault to the
Imperial fault.