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U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
Abstract
Parts of the Imperial and the Superstition Hills faults moved right laterally at the ground surface at the time of or shortly following the ML 5.6 Westmorland earthquake of 26 April 1981. The displacements probably occurred before any significant aftershocks on either fault and thus are classed as triggered slips. Although the main shock was located in an exceptionally seismogenic part of Imperial Valley, about 20 km distant from either fault, no clear evidence of past surface faulting is known in the epicentral area. Horizontal displacement on the Imperial and Superstition Hills faults, southeast and southwest of the epicenter, respectively, reached maxima of 8 and 14 mm, and the discontinuous surface ruptures formed along approximately equal lengths of northern segments of the two structures (16.8 and 15.7 km, respectively). The maximum vertical component of slip measured on surface cracks on the Imperial fault (6 mm) was located on the west side of Mesquite basin near Harris Road. Leveling data at Harris Road suggest slightly larger vertical movement at the surface there. Fault dislocation modeling of this leveling data further suggests that the maximum shallow subsurface dip-slip component of the triggered movement was as much a factor of 4 larger than that measured at the fault trace. The map pattern of the rupture may reveal the principal strand among the several near the north end of the Imperial fault.
No surface displacement was found along traces of the Brawley fault zone, the San Andreas fault, or the part of the Coyote Creek fault that slipped during the 1968 Borrego Mountain earthquake. Ground search in the epicentral area of the main shock and in the widely dispersed aftershock region in northern Imperial Valley failed to locate any definite evidence of new surface faulting.
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