An Assessment of the Consequences and Preparations for a Catastrophic California Earthquake: Findin | Page 8

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