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U. S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
Abstract
Two right-lateral slip events, about 3 weeks apart in November 1987, broke the surface discontinuously along probably similar, nearly 20 km lengths of the northern Imperial fault. The first displacement, at about the beginning of November, was accompanied by a surface tilt representing deep vertical motion or distributed strain. This movement may have been part of a more regional event that also involved the southern San Andreas fault, although the evidence there is questionable. The later surface offset was triggered, probably by the second main shock of the 24 November earthquakes located in the Superstition Hills, about 37 km northwest of the Imperial fault. The maximum observed displacement was less than 4 cm on both occasions; for the triggered movement the maximum slip occurred on a branch strand near the northern extremity of the fault.
Pre-earthquake resurveys of short-length leveling lines indicated combined surface displacement and an eastward tilt at two locations between surveys 
months apart and 13 months apart; the tilt at Harris Road during this pre-earthquake interval is modeled as a 1.4 cm vertical component of slip deeper than 100 m on a 70° northeastward-dipping fault. Later resurveys showed a marked reduction of deep movement in the time period including the triggered slip, and between December 1987 and January 1988, it had ceased.
No definite evidence of surface fracturing was found in the Brawley fault zone during either period of time when the Imperial fault moved. Remeasurement of leveling lines there indicated small, very near-surface vertical components of movement in the northern half of the zone. Elevation changes in the southern part indicated distributed strain and/or movement only deeper than about 50 m. The Brawley zone changes are not fixed in time well enough to clearly relate them to the surface movements on the Imperial fault.
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