argued that foreshock activity is probably a
manifestation of the nucleation process that ultimately results in the
main earthquake. He noted that foreshocks tend to occur in the
immediate vicinity of the hypocenter of the later main shock, they
increase in frequency of occurrence as the time of the main shock is
approached, and they are typically much smaller in magnitude than
the main shock. Dilatancy may explain short-term quiescences just
prior to the main shock in some foreshock sequences. The models for
precursory crustal deformation, described earlier, also can be applied
to explain foreshock sequences since rapid crustal deformations may
be associated with some seismic energy release. The individuality of
foreshock sequences from one earthquake to another may mean that
foreshocks are not an intrinsic part of the nucleation process on a fault
but rather are part of that nuclear process