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.( Figure 8) 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 ( Figure 9. ). 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 (Figure 10). 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 ( Figure 11 ).
Ninety percent of the world's earthquakes occur along plate boundaries ( Figure 8 ) 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 (Schnell and Herd, 1984).
Plate Tectonics and Earthquakes in the Northwestern United States
The Cascadia subduction zone off the coasts 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 (Figures 11, 12). 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 (Chase and others, 1975; Adams, 1984; Riddihough, 1984).
Washington has features typical of convergent boundaries in other parts of the world. These are illustrated in Figure 11 :
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.
The major plate boundaries in the Pacific Northwest are graphically delineated by the locations of recent earthquakes ( Figure 12 ). Narrow zones of shallow offshore earthquakes result from the movement of the Juan de Fuca plate relative to the Pacific plate, particularly along transform boundaries such as the Blanco Fracture Zone off the coast of Oregon. As expected, a few shallow offshore earthquakes occur along the Juan de Fuca Ridge, a spreading boundary between the Juan de Fuca and Pacific plates. Scattered earthquakes occur to the east in Washington, Oregon, and northern California, both in the subducting Juan de Fuca plate and in the overlying North America plate.
The world's greatest earthquakes occur on subduction-zone boundaries. These magnitude 8+ thrust-type earthquakes, sometimes called subduction earthquakes, occur from time to time as the two converging plates jerk past one another. There are no reports of such earthquakes in Washington since the first written records of permanent occupation by Europeans in 1833 when the Hudson Bay Trading Company post was established at Fort Nisqually (Hawkins and Crosson, 1975). And, since the installation in 1969 of a multistation seismograph network in Washington, there has been no evidence of even small thrust-type earthquakes between the plates in Washington and Oregon and offshore.
In fact, few earthquakes of any kind or size have been recorded along the coastal region of the Pacific Northwest. However, parts of subduction zones in Japan and Chile also appear to have had very low levels of seismicity prior to great subduction earthquakes (Heaton and Kanamori, 1984; Heaton and Hartzell, 1986). Therefore the seismic quiescence observed historically along coastal region of Washington and Oregon does not refute the possibility that an earthquake having a magnitude of greater than 8 could occur there. Heaton and Hartzell (1986) note the problem of incomplete seismic data when comparing one subduction zone with another, but they still conclude that available data support the finding that low levels of seismicity may exist in subduction zones prior to a magnitude 8 earthquake.
The convergence of the Juan de Fuca and North America plates is quite slow, so great subduction earthquakes may be rare. Savage and others (I 98 1) interpret geodetic strain measurements near Seattle as indicating that compressional strain is accumulating parallel to the direction of convergence between the
Juan de Fuca and North America plates, as would be expected prior to a great, thrust earthquake off the coast of Washington and British Columbia.
Atwater (1987) has found geologic evidence that he believes shows that the last great subduction earthquake in Washington occur ed as recently as 300 years ago.
Historically, many earthquakes have occurred in the
subducting Juan de Fuca plate deep beneath Puget
Sound and at shallow depths in many places in
Washington, Oregon, and British Columbia in the overlying North America plate. It is reasonable to expect
future earthquakes in these areas to have magnitudes
comparable to the magnitudes of past earthquakes. The
biggest historical earthquakes include the shallow magnitude 7.4 earthquake in the North Cascades in 1872 and
the deep magnitude 7.1 earthquake in the southern Puget
Sound area in 1949 (Rasmussen, 1967; U.S. Geological
Survey, 1975; Malone and Bor, 1979). Therefore, even
without the occurrence of great subduction-style
earthquakes in the Pacific Northwest, Washington is still
earthquake country.