An Assessment of the Consequences and Preparations for a Catastrophic California Earthquake Part 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

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 quant.i.tative 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.

[Ill.u.s.tration: Figure 1. Geographic Locations of Selected Regional Events]

CHAPTER III

a.s.sESSMENT OF LOSSES FOR SELECTED POTENTIAL CALIFORNIA EARTHQUAKES

A. INTRODUCTION

As part of a program that FEMA and its predecessor agencies have had underway for a number of years, property loss and casualty estimates were prepared in 1972 and 1973 for a number of potential maximum credible earthquakes that could impact on the San Francisco and the Los Angeles areas--North San Andreas (Richter magnitude 8.3), Hayward (Richter magnitude 7.4), South San Andreas (Richter magnitude 8.3), and Newport-Inglewood (Richter magnitude 7.5). These estimates have now been updated as part of the current a.s.sessment.

Estimates of property loss and casualties are based on the expected type and distribution of damage for each postulated earthquake as determined by the size and location of the earthquake and the distribution and character of the buildings and structures within the affected area. Methodologies for estimates of this type are approximate at best. Consequently, the figures shown below may vary upward or downward by as much as a factor of two or three. This degree of uncertainty does not affect the validity of the conclusions of this report, however, since there are greater uncertainties in all other aspects of emergency response planning.

B. PROPERTY LOSS ESTIMATES

The property loss estimates were obtained by first estimating the total replacement dollar value of buildings and their contents, multiplying them by percentage loss factors (inferred from the antic.i.p.ated strength of shaking in each county), and then summing to obtain the aggregate loss. Included in the estimates are private as well as Federal, State, and local government buildings, insured and uninsured. Excluded from consideration is the replacement value of transportation and communication facilities, dams, utility installations, and special purpose structures (e.g., convention centers and sports arenas). Also excluded is the potential damage resulting from a major dam failure or the indirect dollar losses due to such factors as higher unemployment, lower tax revenue, reduced productivity, and stoppage of industrial production. Experience indicates that indirect losses could be approximately equal to the dollar amounts lost in buildings and their contents. The property loss estimates for four postulated earthquakes on the faults listed below are as follows.

TABLE 2

ESTIMATES OF PROPERTY LOSSES FOR REPRESENTATIVE EARTHQUAKES[1]

-------------------------------------------------------------------------- Loss to Loss of Building Contents Total Loss Fault ($ in Billions) ($ in Billions) ($ in Billions) -------------------------------------------------------------------------- Northern San Andreas 25 13 38 Hayward 29 15 44 Newport-Inglewood 45 24 69 Southern San Andreas 11 6 17 ---------- [1] Uncertain by a possible factor of two to three.

C. CASUALTY ESTIMATES

Deaths and injuries in these earthquakes princ.i.p.ally would occur from failures of man-made structures, particularly older, multistory, and unreinforced brick masonry buildings built before the inst.i.tution of earthquake-resistant building codes. Experience has shown that some modern multistory buildings--constructed as recently as the late 1960's, but not adequately designed or constructed to meet the current understanding of requirements for seismic resistance--are also subject to failure. Consequently, the number of fatalities will be strongly influenced by the number of persons within high-occupancy buildings, capable of collapsing, or by failure of other critical facilities such as dams. Additional imponderables are the degree of saturation of the ground at the time of the event and the possibility of weather conditions conducive to the spread of fire. A conflagration such as occurred in the 1906 San Francisco earthquake, is not considered likely to occur in any of the a.n.a.lyzed events, however, because of improvements in fire resistance of construction and firefighting techniques. Nonetheless, numerous smaller fires must be antic.i.p.ated in any of the a.n.a.lyzed events and a "Santa Ana type" wind could cause serious problems.

An additional element of uncertainty in estimating casualties from earthquake stems from not knowing where most of the population will be at the time of the earthquake. In the early morning (i.e., 2:30 a.m.) most people are at home, by far the safest environment during a seismic emergency. At 2:00 in the afternoon, on the other hand, the majority of people are at their places of employment and therefore vulnerable to collapse of office buildings. Around 4:30 p.m. many more people are in the streets and thus subject to injury due to falling debris or failures of transportation systems. Consequently, depending on the time of day, wide variations in the number of casualties can be expected.

Following are estimates of dead and injured (requiring hospitalization) for each of the four representative faults and for the three time periods just discussed.

TABLE 3

ESTIMATES OF CASUALTIES[1]

------------------------------------------------------------------- Fault Time Dead Hospitalized[2]