Slip Along the San Andreas Fault Associated with the Earthquake

Some of the fault slip associated with the 1979 Imperial Valley \nearthquake occurred along other than the Imperial fault and the \nBrawley fault zone. More than 90 km to the north of the seismogenic \nfault, a 39-km-long section of the San Andreas fault developed a \ndiscontinuous set of surficial fractures soon after the earthquake. This \nset of fractures consisted of small left-stepping echelon cracks displaying \nextensional and dextral components of movement. Average dextral \nslip was about 4 mm, and slip reached 10 mm at one point along \nthe fault. In one locality the cracks formed between Va and 4Vfe days \nafter the main shock, although slippage at depth may have been \nnearly simultaneous with the earthquake. \nIn general, this set of breaks duplicates the location, style, and slip \nmagnitude of the set that was mapped in 1968 after the Borrego \nMountain, Calif, earthquake. Such near-duplication indicates that \nthis section of the San Andreas fault, in particular, is susceptible to \nsmall amounts of triggered slip. Although the reasons for such behavior \nare far from clear, similar behavior of the Imperial fault before \n1979 suggests that this section of the San Andreas fault may generate \na moderate earthquake within the next few decades.