USGS/Cascades Volcano Observatory, Vancouver, Washington
Earthquakes and Plate Tectonics
From: Noson, Qamar, and Thorsen, 1988, Washington State Earthquake
Hazards: Washing State Department of Natural Resources, Washington Division
of Geology and Earth Resources Information Circular 85
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. ...
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.
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.
... 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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03/02/01, Lyn Topinka
