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EARTHQUAKE INFORMATION

AFTERSHOCK FORECAST
Mon 13 May 2002 10:25 PM PDT

U. S. Geological Survey, Menlo Park, California
U. C. Berkeley Seismological Laboratory, Berkeley, California

This forecast is based on the statistics of aftershocks typical
for California. This is not an exact prediction, but only a
rough guide to expected aftershock activity. This forecast may
be revised as more information becomes available.

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MAINSHOCK: Mon 13 May 2002 10:00:29 PM PDT MAGNITUDE 5.2
5 km ( 3 miles) SW (215 degrees) of Gilroy, CA

STRONG AFTERSHOCKS (Magnitude 5 and larger)

At this time (immediately after the mainshock) the probability of
a strong and possibly damaging aftershock IN THE NEXT 7 DAYS is
approximately 10 PERCENT.

EARTHQUAKES LARGER THAN THE MAINSHOCK

Most likely, the recent mainshock will be the largest in the
sequence. However, there is a small chance (APPROXIMATELY 5 TO
10 PERCENT) of an earthquake equal to or larger than this
mainshock in the next 7 days.

WEAK AFTERSHOCKS (Magnitude 3 to 5)

In addition, approximately 3 to 20 SMALL AFTERSHOCKS are expected
in the same 7-DAY PERIOD and may be felt locally.

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Background Information About Aftershocks

Like most earthquakes, the recent earthquake is expected to be fol-
lowed by numerous aftershocks. Aftershocks are additional earthquakes
that occur after the mainshock and in the same geographic area. Usu-
ally, aftershocks are smaller than the mainshock, but occasionally an
aftershock may be strong enough to be felt widely throughout the area
and may cause additional damage, particularly to structures already
weakened in the mainshock. As a rule of thumb, aftershocks of magni-
tude 5 and larger are considered potentially damaging.

Aftershocks are most common immediately after the mainshock; their
average number per day decreases rapidly as time passes. Aftershocks
are most likely to be felt in the first few days after the mainshock,
but may be felt weeks, months, or even years afterwards. In general,
the larger the mainshock, the longer its aftershocks will be felt.

Aftershocks tend to occur near the mainshock, but the exact geographic
pattern of the aftershocks varies from earthquake to earthquake and is
not predictable. The larger the mainshock, the larger the area of
aftershocks. While there is no "hard" cutoff distance beyond which an
earthquake is totally incapable of triggering an aftershock, the vast
majority of aftershocks are located close to the mainshock. As a rule
of thumb, a magnitude 6 mainshock may have aftershocks up to 10 to 20
miles away, while a magnitude 7 mainshock may have aftershocks as far
as 30 to 50 miles away.

EDIS-05-13-02 2227 PDT

• Aftershock warning