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What causes an earthquake?
According to now generally accepted theory of plate tectonics, the earth’s crust is divided into several major plates, some 50 miles thick, which move slowly and continuously over the interior of the earth. Earthquakes are initiated when, due to slowly accumulating pressure, the ground slips abruptly along a geological fault plane on or near a plate boundary. The resulting waves of vibration within the earth create ground motion at the surface that vibrates in a very complex manner.
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What are an earthquake’s characteristics?
Once the sudden rupture occurs, the earth begins to shake. This shaking is caused by a series of waves known as seismic waves moving from the center of the earthquake out to other parts of the earth. The type of waves involved in an earthquake is a key characteristic of the phenomenon. The four types of seismic waves are grouped into two main categories according to the way they travel from the source, or focus, of an earthquake. P waves and S waves are “body” waves. Love waves and Rayleigh waves are “surface” waves.
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How do we measure earthquakes?
Once an earthquake occurs, it is important to know where the seismic event took place, how intense it was, and its impact on the built environment. The more we know about earthquakes and about how and when they occur, the more we can do to lessen their effects on our communities. Two scales are frequently used to measure earthquakes: the Modified Mercalli Intensity Scale, which measures the intensity or impact of an earthquake on people and the built environment, and the Richter Scale, which measures the amount of energy released by an earthquake, or its magnitude.
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What is the history of earthquakes in the United States?
The 1906 San Francisco earthquake was estimated as a Richter Magnitude 8.3 event, lasting 45 seconds. The 1964 Alaska earthquake (a Richter 8.4) lasted over 3 minutes. Larger events can be expected in the United States, and they will not be confined solely to the West Coast region. In fact, two of the most severe earthquakes in U.S. history occurred east of the Rockies: one in Charleston, South Carolina, in 1886 and the other a series of three shocks centered near New Madrid, Missouri, in 1811-12. Measuring an estimated 8.5 on the Richter scale, the New Madrid earthquake sent shock waves as far west as the Rocky Mountains and as far east as Washington, D.C. and Boston.
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What are some of the effects of earthquakes on the natural and built environments?
The seismic waves associated with earthquakes produce several different effects on the natural environment, including liquefaction, landslides, tsunamis and seiche, and fault rupture. All of these effects can cause tremendous damage to the built environment (buildings, transportation structures, utilities, and communication lines).
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What building characteristics are significant to seismic design?
Several important characteristics of buildings affect performance during an earthquake. Buildings of different construction materials or configurations will respond in different ways to the same ground motion; some may collapse while others survive. Knowing how buildings respond in an earthquake helps architects, engineers, and builders design and construct buildings to withstand ground motion without collapsing. Some of the structural characteristics of buildings that influence how they behave during an earthquake include natural period, damping, ductility, stiffness, drift, and building configuration.
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What is mitigation?
Mitigation is a set of actions taken to prevent or reduce the risk to life, property, social, and economic activities from such natural hazards as earthquakes, floods, and hurricanes. An effective mitigation plan anticipates actions a community must take before a disaster strikes. Planning is one of the most important parts of any mitigation effort. Taking the time up front to make people aware of the earthquake risk to their community, making a plan of how to reduce that risk over time, and what to do in the event of an earthquake can make a tremendous difference in post-disaster recovery efforts.
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How can my community be protected?
There are many different mitigation measures a community can take. There is education and planning. A community also can adopt zoning regulations and guidelines on land-use practices. Making sure that new buildings are not constructed on or near land that is subject to liquefaction, landslides, faults, and tsunamis can be an important mitigation step to prevent or reduce the loss of life and property. Building codes are one of the most powerful mitigation tools that can be adopted by a community in anticipation of a seismic event.
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Why use building codes?
Building codes, which first came into use to protect communities from potential earthquake damage in the 1930’s, are established to ensure uniform minimum standards of health and safety across the United States. A building code requires that a building or facility be located, designed, and constructed so that any threat to life, health, and welfare of its occupants and the public is minimized or prevented. Over and over again, the enforcement of effective building codes have saved lives and property.
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