The most important summaries of foreshocks on a global basis
were published by Jones and Molnar (1976) and Reasenberg (1999).
The former study reported on MN7.0 earthquakes from 1950 to 1973
and showed that 44% of these strong earthquakes had a least one
foreshock (MN4.5) within 40 days of the main shock. The latter study
analyzed MN6.0 earthquakes from 1977 to 1996 and showed that
13.2% had a least one foreshock (MN5.0) with 10 days and 75 km of
the main shock. It is likely that many earthquakes have smaller
foreshocks than those reported in these studies, and so these results
probably represent a lower bound on global foreshock rates before
strong earthquakes. However, no statistical work to document the
rates of smaller magnitude foreshocks has been done due to uneven
earthquake detection worldwide.