ates may not be appropriate for other purposes, such as the
development of seismic design criteria for a specific site. The
development of such criteria commonly requires detailed analyses of
the site and its immediate geologic environment beyond the scope of
this report. Consequently, detailed site analyses may require
modification of the conclusions reached in this report, particularly
fault systems other than the San Andreas and Hayward faults.

B. GEOLOGIC EVIDENCE

Some of the possible earthquakes listed are repeat occurrences of
historical events, others are not, but geologic evidence indicates
that earthquakes occurred on these faults before settlement of the
region. Based on available data, the postulated earthquake magnitudes
would be the largest events that could be expected at a reasonable
level of probability. They represent a selection of events useful for
planning purposes, but are by no means the only such events likely to
occur either on these or other fault systems.

The historic record of seismicity in California is too short to
determine confidently how often large earthquakes reoccur. Information
on past earthquakes must be gleaned from the geologic record and
therefore, presents a picture of past seismicity that is incomplete
and not yet fully deciphered. Current knowledge about the recurrence
of large earthquakes on specific faults is rudimentary. The
probabilities of occurrence shown above are order-of-magnitude
estimates and subject to considerable uncertainty, especially for the
less probable events.

C. DESCRIPTION OF EVENTS

Following are brief descriptions of postulated events. Figure 1 gives
their geographic location.

1. Los Angeles-San Bernardino/Southern San Andreas Fault
(Magnitude 8.3)

For the past several thousand years, great earthquakes have been
occurring over a 300 km length of the San Andreas fault approximately
every 100 to 200 years, 140 years on the average. The last such event
took place in 1857. The probability of occurrence of this earthquake
is estimated to be currently as large as 2 to 5 percent per year and
greater than 50 percent in the next 30 years. The fault skirts the
edge of the Los Angeles-San Bernardino metropolitan region, thus most
of the urbanized area lies further than 20 miles from the source of
strong shaking. Because of the distance, shaking would be more
hazardous for large structures than for one- to two-story houses. The
l