Although earthquakes are inevitable natural hazards, they neednot be inevitable disasters. Through prudent actions our nation can reducelosses of life, casualties, property losses, and social and economic disruptionsfrom future earthquakes.
A SEVERE NATIONAL THREAT
It is likely that one or more severely damaging earthquakes, which equalor exceed the 1994 Northridge earthquake in magnitude, will strike the UnitedStates within the next decade. Repeats of the 1906 San Francisco and the1964 Alaska earthquakes loom somewhere in the future for California andAlaska. 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 earthquakeand was felt across the entire eastern United States. The National ResearchCouncil has estimated that a repeat of the 1811 New Madrid earthquake couldresult in hundreds to thousands of lives lost and over $100 billion dollarsof damage in a 26-state area. In areas such as the Midwest that experienceearthquakes infrequently, the earthquake hazard awareness, vulnerability,and risk sensitivity of the residents is low. Even in areas that have frequentearthquakes, preparedness is often highly variable.
Earthquakes release the strain built up in the earth's crust by the ongoingaction of geologic deformation. Potentially damaging earthquakes are causedby sudden movements along faults. Earthquakes may result in offsets of upto thirty feet which extend up to hundreds of miles along the length ofthe faults. The 1906 San Francisco earthquake and the 1964 Alaska earthquakewere of this scale. Lesser earthquakes, like the 1971 San Fernando earthquake,the 1989 Loma Prieta earthquake are intermediate in magnitude but were stillfelt over thousands of square miles. Even in relatively well-studied areassurprises can occur. The 1994 Northridge earthquake, which occurred alongan unrecognized, buried fault, is a prime example. In the Central and EasternUnited States, where earthquakes are less frequent than in the West, thereare potentially more surprises; because the risk is less well understood,mitigation practices are less commonly implemented and the potential fordamage, should an earthquake occur, is much greater.
Earthquake effects include violent ground shaking and earthquake-inducedground failure such as liquefaction (the sudden conversion of soil to aliquid 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 undiminishedthousands of miles before bringing destruction to coastal areas. Earthquakesmay also cause permanent changes in sea-level elevation through local groundsubsidence or uplift.
The principal threat from earthquakes is shaking damage and the collapseof buildings and other structures that have been inadequately designed orconstructed to resist seismic forces. Major earthquakes can severely interruptregional or national economic activity by damaging lifelines such as roads,railways, water, power, and communication lines. Seismic damage interruptsthe flow to users of vital resources and services, thereby increasing therisk to life safety and impeding economic growth. Ground failure hazardssuch as subsidence, landslides, liquefaction, and settlement also causedamage to structures and lifelines, and are a major threat to dams, waterfrontstructures, highway facilities, and buried lifelines.
Although much remains to be learned about the most effective and economicaltechniques for enhancing the seismic safety of structures, many proven cost-effectivemeasures are already being applied in the United States. Considering thatlittle to no strong earthquake ground motion data was collected prior tothe 1933 Long Beach earthquake, there have been great accomplishments inthe design and construction of earthquake-resistant structures. Becauseof improved building codes, land use planning, and preparedness, the lossesin the San Francisco Bay area from the 1989 Loma Prieta earthquake and inthe Los Angeles area from the 1994 Northridge earthquake were much lowerthan would have occurred in a less well-prepared region .
The current legal requirements for constructing buildings, highways, bridges,and other lifelines in earthquake-prone regions vary greatly from one regionto another, or even from one local jurisdiction to another, despite thefact that seismic safety can often be incorporated in new buildings andlifelines at little or no extra cost for design, construction, or operation.Local action to provide earthquake mitigation measures depends largely uponthe awareness and education of public officials, engineers, planners, thebusiness community, and the general populace.
While the United States has lost comparatively few lives in earthquakesin recent years, the number can be reduced further. The cost of earthquakedamage is still unacceptably high. All regions that are prone to earthquakesmust begin to undertake mitigation measures to reduce future human and propertylosses. While earthquakes are inevitable natural hazards, they need notbe inevitable disasters. Our nation can reduce losses of life, casualties,property losses, and social and economic disruptions from future earthquakesthrough prudent actions.