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Earthquakes

Don W. Steeples and Liz Brosius

Introduction

Though we seldom feel them, earthquakes are not rare. Every 24 hours, more than 1000 earthquakes occur around the world.

Earthquakes are produced when rocks beneath the earth's surface suddenly move along faults, fractures that occur at weak points in the earth. This movement releases stress-energy that has been built up by forces inside the earth. The strength of the earthquake depends on the amount of stress released.

Earthquakes and Plate Tectonics

To understand earthquakes, we need to know something about the theory of plate tectonics. According to this theory, the earth's 120-mile (200-km)-thick shell, called the lithosphere, is broken into several rigid slabs called plates that slide over the uppermost layer of the mantle (Fig. 1). Seven major oceanic and continental plates have been identified, along with a number of smaller plates. Moving at rates of 10 to 130 millimeters (0.4 to 5 inches) per year, these plates interact with one another in various ways, producing mountain belts, volcanoes, and earthquakes.

Figure 1--Simplified map of the earth's crustal plates (U.S. Geological Survey, 1990, Professional Paper 1515).

Map of the Earth showing plates

Geologists have identified several types of plate movements associated with earthquakes. A well-known example is the movement of plates along the San Andreas fault in California. Here, the North American and Pacific Plates scrape past one another, the Pacific Plate moving to the northwest and the North American Plate moving in the opposite direction. Because friction prevents the plates from gliding easily past one another, enormous stresses build up and are periodically relieved by large earthquakes. The 1906 San Francisco and the 1989 "World Series" (Loma Prieta) earthquakes resulted from sudden movement along this fault system.

Along the western coast of South America is an example of another type of plate movement associated with the collision of oceanic and continental plates. Here, in what is called a subduction zone, the Pacific plate is thrust or subducted under the South American plate, forming a deep oceanic trench. Earthquakes along this boundary are either shallow or deep, depending on where they occur on the downsloping plate. The oceanic plate eventually descends to a depth at which the mantle temperature is high enough to melt part of it into molten rock, or magma. This magma then returns to the surface during volcanic activity, such as that which occurs around the rim of the Pacific Ocean, including Mt. St. Helen's.


Next Page--Where Earthquakes Occur

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