Although earthquakes are inevitable natural hazards, they need
not be inevitable disasters. Through prudent actions our nation can reduce
losses of life, casualties, property losses, and social and economic disruptions
from future earthquakes.
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's crust by the ongoing
action of geologic deformation. Potentially 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 .
The current legal requirements for constructing buildings, highways, bridges,
and other lifelines in earthquake-prone regions vary greatly from one region
to another, or even from one local jurisdiction to another, despite the
fact that seismic safety can often be incorporated in new buildings and
lifelines at little or no extra cost for design, construction, or operation.
Local action to provide earthquake mitigation measures depends largely upon
the awareness and education of public officials, engineers, planners, the
business community, and the general populace.
While the United States has lost comparatively few lives in earthquakes
in recent years, the number can be reduced further. The cost of earthquake
damage is still unacceptably high. All regions that are prone to earthquakes
must begin to undertake mitigation measures to reduce future human and property
losses. While earthquakes are inevitable natural hazards, they need not
be inevitable disasters. Our nation can reduce losses of life, casualties,
property losses, and social and economic disruptions from future earthquakes
through prudent actions.