Sometimes these small quakes are foreshocks to the big one and
after the big one, smaller aftershocks follow.
Some large earthquakes are preceded by foreshocks.
Knowledge of past earthquake patterns allows scientists to estimate
the odds that an earthquake striking today is a foreshock and will
soon be followed by a larger mainshock in the same area. These odds
depend on the earthquake's magnitude and the seismic history of the
fault on which it occurred. When a moderate earthquake hits
California, scientists immediately estimate the probability that a
damaging mainshock will follow. If the threat is significant, a
warning is issued.
This warning process was put into action in June 1988 when a
magnitude 5.1 shock--one of the largest in the San Francisco Bay
region since the great 1906 earthquake--struck 60 miles south of
San Francisco. Alerted by the USGS that there was a 1 in 20 chance
of a larger earthquake in the next 5 days, the California OES
issued an advisory to warn the public. (The usual daily odds of a
large quake in the Bay region are 1 in 15,000.) The warning period
passed without further activity. In August 1989, another earthquake
hit the same area and a similar advisory was issued. Again nothing
happened in the specified warning period. However, 69 days later,
the area was rocked by the magnitude 7.1 Loma Prieta earthquake,
which killed 63 people and caused $6 billion damage in the San
Francisco Bay region.
BE FOREWARNED about foreshocks...
