A SEVERE NATIONAL THREAT

It is likely that one or more severely damaging earthquakes, which equal or exceed the 1994 Northridge earthquake in magnitude, will strike the United States within the next decade. Repeats of the 1906 San Francisco and the 1964 Alaska earthquakes loom somewhere in the future for California and Alaska. Although most people associate them with the nation’s West Coast, earthquakes pose a significant risk in at least 39 states. The New Madrid, Missouri, earthquake of 1811 was as powerful as the 1906 San Francisco earthquake and was felt across the entire eastern United States. The National Research Council has estimated that a repeat of the 1811 New Madrid earthquake could result in hundreds to thousands of lives lost and over $100 billion dollars of damage in a 26-state area. In areas such as the Midwest that experience earthquakes infrequently, the earthquake hazard awareness, vulnerability, and risk sensitivity of the residents is low. Even in areas that have frequent earthquakes, preparedness is often highly variable.

Earthquakes release the strain built up in the earth’ otentially damaging earthquakes are caused by sudden movements along faults. Earthquakes may result in offsets of up to thirty feet which extend up to hundreds of miles along the length of the faults. The 1906 San Francisco earthquake and the 1964 Alaska earthquake were of this scale. Lesser earthquakes, like the 1971 San Fernando earthquake, the 1989 Loma Prieta earthquake are intermediate in magnitude but were still felt over thousands of square miles. Even in relatively well-studied areas surprises can occur. The 1994 Northridge earthquake, which occurred along an unrecognized, buried fault, is a prime example. In the Central and Eastern United States, where earthquakes are less frequent than in the West, there are potentially more surprises; because the risk is less well understood, mitigation practices are less commonly implemented and the potential for damage, should an earthquake occur, is much greater.

Earthquake effects include violent ground shaking and earthquake-induced ground failure such as liquefaction (the sudden conversion of soil to a liquid mass due to shaking as occurred in the 1995 Kobe earthquake), landslide, or ground surface rupture. Submarine earthquakes can induce damaging tsunami (seismic sea waves or “tidal” waves), which can travel undiminished thousands of miles before bringing destruction to coastal areas. Earthquakes may also cause permanent changes in sea-level elevation through local ground subsidence or uplift.

The principal threat from earthquakes is shaking damage and the collapse of buildings and other structures that have been inadequately designed or constructed to resist seismic forces. Major earthquakes can severely interrupt regional or national economic activity by damaging lifelines such as roads, railways, water, power, and communication lines. Seismic damage interrupts the flow to users of vital resources and services, thereby increasing the risk to life safety and impeding economic growth. Ground failure hazards such as subsidence, landslides, liquefaction, and settlement also cause damage to structures and lifelines, and are a major threat to dams, waterfront structures, highway facilities, and buried lifelines.

Although much remains to be learned about the most effective and economical techniques for enhancing the seismic safety of structures, many proven cost-effective measures are already being applied in the United States. Considering that little to no strong earthquake ground motion data was collected prior to the 1933 Long Beach earthquake, there have been great accomplishments in the design and construction of earthquake-resistant structures. Because of improved building codes, land use planning, and preparedness, the losses in the San Francisco Bay area from the 1989 Loma Prieta earthquake and in the Los Angeles area from the 1994 Northridge earthquake were much lower than would have occurred in a less well-prepared region .