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