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SEISMOLOGICAL LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA 91125
Abstract
The Brawley earthquake swarm provided a unique opportunity for studying a highly interesting tectonic region. The swarm was most intense for a period of 4 days including 75 events with ML between 3.0 and 4.7 with a spatial extent of 12 km. Precise relative hypocenters were obtained for 264 earthquakes (ML 
1.5) using a master event method to calibrate the USGS Imperial Valley array. These locations together with well-constrained focal mechanisms for 16 of the largest events suggest faulting on at least three distinct structures. Hypocentral depths ranged from 4 to 8 km, compared to a basement depth of about 6 km for this part of the Imperial Valley. The swarm began on a nearly vertical right-lateral fault striking N8°W (Brawley fault) about 8 km southeast of Brawley at a point which had experienced enhanced shallow seismicity during the preceding 4 days. The seismicity migrated bilaterally north and south from this point at a constant velocity of 0.5 km/hr terminating to the north on a steeply south dipping, N50°E-striking fault. This structure is on trend with splays associated with the northern end of ground breakage of the 1940 Imperial Valley earthquake. To the south the seismicity ended near the northern end of the 10 km of surface rupture mapped by R. V. Sharp, which continues on strike to a point near the Imperial fault. Tectonic interpretations include the transfer of right-lateral offset from the Imperial fault to the Brawley fault associated with the formation of a closed depression bounded on the west and east by these two faults.
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