abstract

Surface ruptures formed along a 10.4-km segment of the Brawley fault during the earthquake swarm of January and February, 1975, in central Imperial Valley, California. This newly recognized fault appears to branch northward from the Imperial fault about 8 km east-northeast of El Centro. Displacement at the surface was apparently vertical and reached a maximum of probably more than 0.2 m at Keystone Road. En echelon cracking suggested an incipient right-lateral component, but roads and irrigation canals were not laterally offset by the faulting. Surface cracks and most of the displacement probably developed suddenly at the time of the largest shock (M 4.7) in the swarm on January 23. At Harris Road, vertical movement continued at least until March 24, but significant afterslip across the fault zone may have ceased by February 13. Vertical movement after the 13th apparently took the form of a relative subsidence centered on the fault with no important net slip across the fault. Pre-1975 historic displacements on the Brawley fault occurred in 1940 at the time of the M 7.1 Imperial Valley earthquake and probably as intermittent creep between 1940 and 1975.

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