Figure 1.57 Global distribution of earthquakes with magnitudes greater than 5.1 for the period 1964–95 (Engdahl et al.,1998). oceanic crust also is believed to play a significant role
in removing the heat at ocean ridges and in concen- trating minerals on the seafloor and in the oceanic crust. Exploration with the deep submersible Alvin has provided actual observations of hot water from the crust venting directly to the ocean. Submarine thermal springs have been discovered on the Galápa- gos rift and the East Pacific Rise crest. Spectacular submarine hot springs with exit water temperatures near 700 K have also been discovered at the latter location.
1.13 Seismicity and the State of Stress in the
Lithosphere
Just as in the case of volcanoes, the occurrences of
earthquakes strongly correlate with plate boundaries.
The worldwide distribution of seismicity is illustrated inFigure 1.57. Earthquakes occur on all types of plate boundaries; however, subduction zones and areas of continental collision are noted for their very large
earthquakes. Large earthquakes also occur in plate interiors but with a much lower frequency.
Earthquakes are associated with displacements on preexisting faults. A typical displacement in a very large earthquake is 10 m. If the relative velocity across a plate boundary is 50 mm yr−1, it would take 200 years to accumulate this displacement. Large earth- quakes at subduction zones and major transform faults such as the San Andreas reoccur in about this period. Since regular displacements do not have to be accommodated in plate interiors, the period between major intraplate earthquakes is much longer.
The near-surface expressions of major faults are broad zones of fractured rock with a width of a kilo- meter or more (Figure 1.58). Smaller faults may have zones of fault gouge with widths of a few centime- ters or less. Small faults grade down to rock fractures across which there is no offset displacement. The total offset across major faults may be hundreds of kilo- meters. A fault zone is a zone of weakness. When the regional stress level becomes sufficiently large, the fault
Figure 1.58 View along the San Andreas fault in Choia Valley. Note the streams have been offset by the right-lateral displacement on the fault (R. E. Wallace 304, U.S. Geological Survey).
ruptures and an earthquake occurs. There is exten- sive geological evidence that faults become reactivated. Large stresses can reactivate faults that have been inactive for tens or hundreds of millions of years.
The direction of the displacement on a fault can be used to determine the state of stress responsible for the displacement. Since voids cannot be created in the Earth’s deep interior, displacements on faults are parallel to the fault surface. If a region is in a state of tensional stress, normal faulting will occur, as illustrated in Figure 1.59a. If a region is in a state
of compressional stress, thrust faulting will occur, as illustrated inFigure 1.59b.
If a region is in a state of shear, strike–slip faulting will occur, as illustrated inFigures 1.59c and1.59d. If,
to an observer standing on one side of the fault, the motion on the other side of the fault is to the left, the fault is a left-lateral strike–slip fault or sinistral strike– slip fault. If the motion on the other side of the fault is to the right, it is a right-lateral or dextral strike– slip fault. The displacement during many earthquakes
Figure 1.59 Cross sections of (a) a normal fault and (b) a thrust fault and top views of (c) right-lateral and (d) left-lateral strike–slip faults.
combines the horizontal displacement associated with strike–slip faulting and the vertical displacement asso- ciated with either normal or thrust faulting. A com- bination of strike–slip and thrust faulting is known as transpression. A combination of strike–slip and normal faulting is known as transtension.
As discussed previously, the lithosphere can trans- mit stress over large distances. There are several sources for the stress in the lithosphere. One source is the body forces that drive the motion of the sur- face plates. These include the negative buoyancy on the descending plate at a subduction zone and the gravitational sliding of a plate off an ocean ridge. Changes of temperature lead to thermal stresses. When the temperature increases or decreases, rock expands or contracts. The expansion or contraction can cause very large stresses. Erosion and sedimentation also cause a buildup of stress through the addition or removal of surface loads. Glaciation and deglaciation act similarly. Because the Earth is not a perfect sphere but rather a spheroid with polar flattening and an equatorial bulge, plates must deform as they change latitude. This deformation leads to membrane stresses in the lithosphere. Plate interactions such as continen- tal collisions are an important source of stress. Large displacements of the cool, near-surface rocks often occur in these zones. If these deformations occur on faults, high stress levels and major earthquakes can be
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1.13 Seismicity and the State of Stress in the Lithosphere 61
Figure 1.60 Distribution of seismicity in the geological provinces of the western United States (stars). Solid arrows give relative plate velocities; open arrows give stress directions inferred from seismic focal mechanism studies.
expected. The state of stress in the lithosphere is the result of all these factors and other contributions.
As a specific example of seismicity and stress we again turn to the western United States. The distribu- tion of seismicity in this region is given inFigure 1.60. Also included in the figure are the relative veloci- ties between plates and the directions of lithospheric stress inferred from displacements on faults. The Juan de Fuca plate is being formed on the Juan de Fuca ridge with a half-spreading rate of 29 mm yr−1. The seismicity on a transform fault offsetting two seg- ments of the ridge is clearly illustrated. Because the
lithosphere is thin at the ridge and the rocks are hot and weak, relatively little seismicity is associated with the spreading center at the ridge crest. The Juan de Fuca plate is being subducted at a rate of about 15 mm yr−1at a trench along the Oregon and Wash- ington coasts. The seismicity in Oregon and Washing- ton associated with this subduction is also relatively weak. However, there is convincing evidence that this subduction resulted in a magnitude 8.7–9.2 earth- quake on January 26, 1770. Major subsidence along the coasts of Washington and Oregon resulted in the deaths of many trees; a date was established using tree
rings. The specific date was obtained from Japanese records of a major tsunami (Atwater et al.,2005).
Although the extensive seismicity usually associ- ated with active subduction is absent in the Pacific Northwest, a well-defined line of active volcanoes lies parallel to the trench. The volcanoes extend from Mount Baker in Washington to Mount Shasta in northern California. These volcanoes have had vio- lent eruptions throughout the recent geological past. An eruption about 6000 years ago removed the upper 2 km of Mount Mazama, creating Crater Lake in Ore- gon. The spectacular eruption of Mount St. Helens, Washington (Figure 1.10), on May 18, 1980, blew out its entire north flank, a volume of about 6 km3.
The velocity between the Pacific and North Ameri- can plates is 47 mm yr−1in California. A large fraction of this is accommodated by displacements on the San Andreas fault. In the north the fault terminates in a fault–fault–trench (FFT) triple junction with the Juan de Fuca plate. In the south the fault terminates in a series of small spreading centers (ocean ridges) extend- ing down the Gulf of California. Along much of the fault, displacements are almost entirely right-lateral strike–slip. However, north of Los Angeles the fault bends, introducing a thrusting component. Motion on thrust faults in this region is responsible for the for- mation of a series of mountain ranges known as the Transverse Ranges.
A great earthquake occurred on the northern section of the San Andreas fault in 1906; the average displacement was 4 m. A great earthquake occurred on the southern section of the San Andreas fault in 1857; the average displacement was 7 m. A detailed discus- sion of the San Andreas fault is given inSection 8.8.
It is clear that the displacements on accreting plate margins, subduction zones, and transform faults can- not explain the entire distribution of seismicity in the western United States. Major earthquakes occur throughout the region. Rapid mountain building is associated with the Rocky Mountains and the Sierra Nevada. The Basin and Range province is a region of extensive normal faulting. The presence of many graben structures is evidence of crustal extension due to tensional stresses. The asthenosphere rises to the base of the continental crust in this region, and the
lithosphere is thin and weak. Considerable volcan- ism occurs throughout the province. The Rio Grande rift, which marks the eastern boundary of this area of volcanism, seismicity, and mountain building, is also an extensional feature. The stress directions shown in Figure 1.60indicate the entire western United States appears to be extending because of tensional stresses. Although there is no comprehensive understanding of this area, it is likely that the seismicity, volcan- ism, and mountain building are the result of complex interactions of the Pacific, North American, and Juan de Fuca plates that are deforming the entire region. It is likely that there is a geometrical incompatibility between the strike–slip motion on the San Andreas fault and the time-dependent relative displacement between the Pacific and North American plates. As a result the western part of the North American plate is being deformed.
China is another region of extensive tectonics. It is the site of extensive seismicity and mountain building. Deformation associated with the continental collision between India and Asia extends several thousands of kilometers north of the suture zone.
Seismicity can also occur in plate interiors. An example is New Madrid, Missouri, where three very large earthquakes struck in 1811 and 1812. A sig- nificant number of small earthquakes occur in this region at the present time. It should not be surpris- ing that earthquakes occur in plate interiors, since the elastic lithosphere can transmit large stresses. These intraplate earthquakes are likely to occur where the elastic properties of the plate change and stress con- centrates.