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.