GEOLOGIC EARTHQUAKE SCENARIOS
A. MAJOR EVENTS
For purposes of assessing the consequences of a major California
earthquake, scenarios for seven large earthquakes were developed. The
scenarios depict expectable earthquakes that could severely impact on
the major population centers of California. In each case they are
representative of only one possible magnitude of earthquake that could
occur on the indicated fault system. On each fault system there is a
greater probability of one or more damaging earthquakes of somewhat
smaller magnitude than the postulated event. The postulated
earthquakes are listed in the following table.
TABLE 1
MAJOR CALIFORNIA EARTHQUAKES
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Current
Annual Likelihood
Probability of
of Occurrence
Richter Occurrence in Next
Region Fault System Magnitude[1] (Percent) 20-30 Years
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Los Angeles- Southern
San Bernardino San Andreas 8.3 2-5 High
San Francisco Northern
Bay Area San Andreas 8.3 1 Moderate
San Francisco
Bay Area Hayward 7.4 1 Moderate
Los Angeles Newport- Moderate
Inglewood 7.5 0.1 -Low
San Diego Rose Canyon 7.0 0.01 Low
Riverside Moderate-
San Bernardino Cucamonga 6.8 0.1 Low
Los Angeles Santa Monica 6.7 0.01 Low
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[1] This is the estimated largest magnitude earthquake expected
at a reasonable level of probability. The main shock can be
expected to be followed by large aftershocks over a period of
weeks or longer. Each large aftershock would be capable of
producing additional significant damage and hampering disaster
assistance operations.
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These earthquake scenarios represent the largest magnitude events
estimated on the basis of a variety of geologic assumptions. The
appropriateness of these assumptions depends on the intent of the
analysis and the state of geologic knowledge. Therefore, the resulting
estimates 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
long duration of shaking could trigger numerous slides on steep
slopes and cause liquefaction in isolated areas.
2. San Francisco Bay Area/Northern San Andreas Fault
(Magnitude 8.3)
A repeat occurrence of the 1906 earthquake, in which the San
Andreas fault broke over 400 km of its length, would cause severe
damage to structures throughout the Bay Area and adjacent regions. The
extensive urban development on lowlands and landfill around San
Francisco Bay would be especially hard hit and liquefaction in many of
these areas would intensify the damage to structures erected on them.
3. San Francisco Bay Area/Hayward Fault (Magnitude 7.4)
The last large events to occur on this fault were in 1836 and
1868. Should a major earthquake occur, severe ground shaking and
liquefaction is expected to cause damage throughout the entire
circum-bay area nearly as severe as that resulting from a 1906-type
earthquake on the San Andreas fault. This earthquake would be of
particular concern because of the many dams located along or near the
fault.
4. Los Angeles/Newport-Inglewood Fault (Magnitude 7.5)
This earthquake would be a serious threat to the nearby,
densely-populated areas of Los Angeles. Shaking would cause extensive
structural damage throughout the Los Angeles Basin and liquefaction
near the coast would add still more destruction.
5. San Diego Area/Rose Canyon Fault (Magnitude 7.0)
This fault--a segment of an active zone of faults extending from
the Newport-Inglewood fault to Northern Mexico--would present the
greatest earthquake risk to the San Diego area. Severe damage due to
shaking and liquefaction could be expected in the coastal areas.
Because of unstable sea-bed sediments in the offshore area, local
tsunamis (tidal waves) are possible.
6. Los Angeles/Santa Monica Fault (Magnitude 6.7 and 7.0) and
Riverside/San Bernardino/Cucamonga Fault (Magnitude 6.8)
These faults are part of a system of east-west tending faults
bordering the northern edge of the Los Angeles basin. This fault
system caused the 1971 San Fernando earthquake and is geologically
similar to the system that generated the large 1952 Kern County
earthquake. Although smaller in magnitude than the earthquakes
previously described, these postulated events are potentially quite
dangerous because of their vicinity to high population densities in
Southern California.
D. EARTHQUAKE EFFECTS
Detailed maps were prepared for each event showing qualitative
estimates of ground shaking intensity resulting from each earthquake.
These estimates are indicative of the general severity of damage to
ordinary structures. Empirical formulae providing quantitative
estimates of peak ground motion at various distances from the
postulated earthquakes were developed for use in the effects of
severe ground shaking on individual structures or critical
facilities. No estimates were made of localized effects, such as
ground failures related to liquefaction (the complete failure or loss
of strength, of a saturated soil due to shaking), landslides, and
fault rupture. These effects can be far more destructive than ground
shaking alone.
[Illustration: Figure 1. Geographic Locations of Selected Regional
Events]