WHAT CAUSES EARTHQUAKES?
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.
Figure 8
Quote:
|
Figure 8. Relation between major tectonic plates and earthquakes. The Earth's surface is made up of 10 major plates and several smaller plates. Most earthquakes occur along plate margins. Small dots represent earthquake epicenters; large dots indicate locations of volcanoes. An enlargement (bottom) shows tectonic plates along the Pacific coast of North America. Arrows show motions of the Pacific and Juan de Fuca plates relative to North America. (World plate map from "Earthquakes" by Bruce A. Bolt. Copyright @1978, 1988 W. H. Freeman and Company. Reprinted with permission; the explanation has been modified)
|
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.
Figure 9
Quote:
|
Figure 9. Cutaway view of the Earth showing the rocky mantle and iron core. The outermost layer consists of tectonic plates that are commonly about 100 km thick. Earthquakes occur within or at the boundaries of these plates. Although the mantle is solid, the rocks that comprise it act like a very viscous liquid and may move a few centimeters a year in great convection cells driven by temperature differences in the Earth. The plates move slowly with these currents. Spreading plate boundaries are thought to lie above areas of upwelling currents, and converging plate boundaries above areas where the currents move towards the center of the Earth. (See also Figure 10.)
|
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).
Figure 10
Quote:
|
Figure 10. Three types of plate boundaries. A spreading boundary (a) marks the divergence of two plates. Material welling up from the mantle creates a rise or ri dge bordering the rift between separating plates. A convergent boundary (b) occurs where one plate moves t owards another. If one of these plates slides beneath the other, the motion is called subduction. A tra nsform boundary (c) occurs where relative plate motion is neither divergent or convergent, but is parallel to the plate edges. The geometry of plates off the coast of Washington is schematically shown in this figur e; plate locations are shown in Fig. 8.
|
Figure 11
Quote:
|
Figure 11. Cross sections of Washington showing plate convergence (top fig ure) and earthquake hypocenter locations. Some major topographic features and underl ying geologic structures of Washington are shown diagrammatically in the upper figure. In th e lower figure, selected hypocenters of earthquakes that occurred in 1982 through 1986 between latitudes 47' and 48'N are projected onto a vertical plane that generally corresponds to the diagram in the upper figure. Because of the great number of shallow earthquakes that occurred between 1982 and 1986, only hy pocenters of those having magnitudes equal to or greater than 1.8 are shown in the lower figure. Belo w 30 km, hypocenters of all earthquakes having magnitudes of 1.0 or greater that occurred during this pe riod are shown. The distribution of deep earthquakes indicates the slope of the zone of subduction. In th e lower figure there is a vertical exaggeration of 2 to I below sea level; this creates the illusion that the subducting Juan de Fuca plate dips more steeply than it actually does. Topography indicated on the lower figure has a vertical exaggeration of 12 to 1.
|
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).
Figure 12
Quote:
|
Figure 12. Epicenters of earthquakes in the Pacific Northwest since 1960. Only the largest earthquakes near Mount St. Helens are indicated. Note the position of the Ca scadia subduction zone relative to Washington's coast and that epicentral locations mark plate boun daries shown in Figure 8. (Data from the National Oceanic and Atmospheric Administration and the Univ ersity of Washington.)
|
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:
- (1) A zone of deep earthquakes near the probable boundary between the Juan de Fuca plate and North America plate (Crosson, 1983; Taber and Smith, 1985; Weaver and Baker, 1988). The 1949 magnitude 7.1 Olympia 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 (Tabor and Cady, 1978).
- (4) Deformed young sediments offshore in the Pacific Ocean where the converging plates meet (Barnard, 1978).
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.