More recently, a cooperative Federal, State, local, and private-sector effort was initiated to prepare for responding to a credible large-magnitude earthquake, or its prediction, in Southern California. B. SUMMARY The review provided the overall assessment that the Nation is essentially unprepared for the catastrophic earthquake (with a probability greater than 50 percent) that must be expected in California in the next three decades. While current response plans and preparedness measures may be adequate for moderate earthquakes, Federal, State, and local officials agree that preparations are woefully inadequate to cope with the damage and casualties from a catastrophic earthquake, and with the disruptions in communications, social fabric, and governmental structure that may follow. Because of the large concentration of population and industry, the impacts of such an earthquake would surpass those of any natural disaster thus far experienced by the Nation. Indeed, the United States has not suffered any disaster of this magnitude on its own territory since the Civil War. The basis for this overall assessment is summarized below and discussed in more detail in the subsequent chapters of this report. C. LIKELIHOOD OF FUTURE EARTHQUAKES Earth scientists unanimously agree on the inevitability of major earthquakes in California. The gradual movement of the Pacific Plate relative to the North American Plate leads to the inexorable concentration of strain along the San Andreas and related fault systems. While some of this strain is released by moderate and smaller earthquakes and by slippage without earthquakes, geologic studies indicate that the vast bulk of the strain is released through the occurrence of major earthquakes--that is, earthquakes with Richter magnitudes of 7.0 and larger and capable of widespread damage in a developed region. Along the Southern San Andreas fault, some 30 miles from Los Angeles, for example, geologists can demonstrate that at least eight major earthquakes have occurred in the past 1,200 years with an average spacing in time of 140 years, plus or minus 30 years. The last such event occurred in 1857. Based on these statistics and other geophysical observations, geologists estimate that the probability for the recurrence of a similar earthquake is currently as large as 2 to 5 percent per year and greater than 50 percent in the next 30 years. Geologic evidence also indicates other faults capable of generating major earthquakes in other locations near urban centers in California, including San Francisco-Oakland, the immediate Los Angeles region, and San Diego. Seven potential events have been postulated for purposes of this review and are discussed in chapter II. The current estimated probability for a major earthquake in these other locations is smaller, but significant. The aggregate probability for a catastrophic earthquake in the whole of California in the next three decades is well in excess of 50 percent.