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
| Region | Fault System | Richter Magnitude[1] | Current Annual Probability of Occurrence (Percent) | Likelihood of Occurrence in Next 20-30 Years |
| Los Angeles-San Bernardino | Southern San Andreas | 8.3 | 2-5 | High |
| San Francisco Bay Area | Northern San Andreas | 8.3 | 1 | Moderate |
| San Francisco Bay Area | Hayward | 7.4 | 1 | Moderate |
| Los Angeles | Newport-Inglewood | 7.5 | 0.1 | Moderate-Low |
| San Diego | Rose Canyon | 7.0 | 0.01 | Low |
| Riverside San Bernardino | Cucamonga | 6.8 | 0.1 | Moderate-Low |
| Los Angeles | Santa Monica | 6.7 | 0.01 | Low |
[1] This is the estimated largest magnitude earthquakeexpected at a reasonable level of probability. The main shock can beexpected to be followed by large aftershocks over a period of weeks orlonger. Each large aftershock would be capable of producing additionalsignificant damage and hampering disaster assistance operations. | ||||
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