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What Causes Earthquakes?

-- Excerpt from: Noson, Qamar, and Thorsen, 1988,
Washington State Earthquake Hazards: Washington State Department of Natural Resources, Washington Division of Geology and Earth Resources Information Circular 85

An earthquake is the shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth, called a fault. Within seconds, an earthquake releases stress that has slowly accumulated within the rock, sometimes over hundreds of years. ...

The size of an earthquake is indicated by a number called its magnitude. Magnitude is calculated from a measurement of either the amplitude or the duration of specific types of recorded seismic waves. Magnitude is determined from measurements made from seismograms and not on reports of shaking or interpretations of building damage. ... The intensity of and earthquake is a measure of the amount of ground shaking at a particular site, and it is determined from reports of human reaction to shaking, damage done to structures, and other effects. ...

Earth scientists believe that most earthquakes are caused by slow movements inside the Earth that push against the Earth's brittle, relatively thin outer layer, causing the rocks to break suddenly. This outer layer is fragmented into a number of pieces, called plates. Most earthquakes occur at the boundaries of these plates. In Washington State, the small Juan de Fuca plate off the coast of Washington, Oregon, and northern California is slowly moving eastward beneath a much larger plate that includes both the North American continent the land beneath part of the Atlantic Ocean. Plate motions in the Pacific Northwest result in shallow earthquakes widely distributed over Washington and deep earthquakes in the western parts of Washington and Oregon. The movement of the Juan de Fuca plate beneath the North America plate is in many respects similar to the movements of plates in South America, Mexico, Japan, and Alaska, where the world's largest earthquakes occur. ...

Plate Tectonics Theory:

The plate tectonics theory is a starting point for understanding the forces within the Earth that cause earthquakes. Plates are thick slabs of rock that make up the outermost 100 kilometers or so of the Earth. Geologists use the term "tectonics" to describe deformation of the Earth's crust, the forces producing such deformation, and the geologic and structural features that result.

Earthquakes occur only in the outer, brittle portions of these plates, where temperatures in the rock are relatively low. Deep in the Earth's interior, convection of the rocks, caused by temperature variations in the Earth, induces stresses that result in movement of the overlying plates. The rates of plate movements range from about 2 to 12 centimeters per year and can now be measured by precise surveying techniques. The stresses from convection can also deform the brittle portions of overlying plates, thereby storing tremendous energy within the plates. If the accumulating stress exceeds the strength of the rocks comprising these brittle zones, the rocks can break suddenly, releasing the stored elastic energy as an earthquake.

Three major types of plate boundaries are recognized. These are called spreading, convergent, or transform, depending on whether the plates move away from, toward, or laterally past one another, respectively. Subduction occurs where one plate converges toward another plate, moves beneath it, and plunges as much as several hundred kilometers into the Earth's interior. The Juan de Fuca plate off the coasts of Washington and Oregon is subducting beneath North America.

Ninety percent of the world's earthquakes occur along plate boundaries where the rocks are usually weaker and yield more readily to stress than do the rocks within a plate. The remaining 10 percent occur in areas away from present plate boundaries -- like the great New Madrid, Missouri, earthquakes of 1811 and 1812, felt over at least 3.2 million square kilometers, which occurred in a region of southeast Missouri that continues to show seismic activity today.

Plate Tectonics and Earthquakes in the Northwestern United States:

The Cascadia subduction zone off the coast of Washington, Oregon, and northern California is a convergent boundary between the large North America plate and the small Juan de Fuca plate to the west. The Juan de Fuca plate moves northeastward and then plunges (subducts) obliquely beneath the North America plate at a rate of 3 to 4 centimeters per year.

Washington has features typical of convergent boundaries in other parts of the world:

  1. A zone of deep earthquakes near the probably boundary between the Juan de Fuca plate and the North America plate. The 1949 magnitude 7.1 Olympic earthquake and the 1965 magnitude 6.5 Seattle-Tacoma earthquake occurred within this deep zone.

  2. The active or recently active volcanoes of the Cascade range created by the upward migration of magma (molten rock) above the Juan de Fuca plate. Rock in the subducting plate may melt at depths of 100 kilometers or more in the Earth. Because melted rock is lighter, it can sometimes rise to the surface through weakened areas in the overlying materials.

  3. Young, highly deformed mountains composed of formerly oceanic rocks scraped off the Juan de Fuca plate during subduction and piled up on the Olympic peninsula.

  4. Deformed young sediments offshore in the Pacific Ocean where the converging plates meet.

In sum, the subduction of the Juan de Fuca plate beneath the North America plate is believed to directly or indirectly cause most of the earthquakes and young geologic features in Washington and Oregon.

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03/21/01, Lyn Topinka