Part 2 (1/2)

Riverside Moderate- San Bernardino Cucamonga 6.8 0.1 Low

Los Angeles Santa Monica 6.7 0.01 Low ------------------------------------------------------------------------- [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 a.s.sistance operations.

These earthquake scenarios represent the largest magnitude events estimated on the basis of a variety of geologic a.s.sumptions. The appropriateness of these a.s.sumptions depends on the intent of the a.n.a.lysis 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 a.n.a.lyses of the site and its immediate geologic environment beyond the scope of this report. Consequently, detailed site a.n.a.lyses 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 Francis...o...b..y 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 Francis...o...b..y would be especially hard hit and liquefaction in many of these areas would intensify the damage to structures erected on them.

3. San Francis...o...b..y 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 circ.u.m-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 offsh.o.r.e 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