Strike-Slip Environments

Strike-slip faults release only a small percentage of total energy, yet they are found in great variety and have produced some of the most destructive and infamous earthquakes, including the 1906 San Francisco (Box 2.2) and the 1995 Hyogo-ken Nanbu (Box 2.5). The most common, but least seismic, of these lateral faults are the transform faults that connect oceanic spreading centers. Although this class includes some of the longest faults in the world—the Romanche transform in the equatorial Atlantic has been mapped as a nearly continuous fault for 950 kilometers—and the ones with the fastest slip rates (as much as 8.5 meters per century for the 480-kilometer-long Tharp transform in the southeastern Pacific), they generate earthquakes larger than M 7 rarely and M 8 almost never (68). Essentially all ridge-ridge transforms are under the oceans and therefore difficult to study, but some have been the subject of detailed oceanographic surveys (69). The low seismicity of ridge-ridge transforms is correlated with the high mantle temperature in these regions, although the peculiar mechanics of these faults may have something to do with the thinness of oceanic crust along ridge-ridge transforms and the abundance of weak, serpentinized upper mantle.

Transforms that run between trenches are far less common than ridge-ridge transforms, because they do not occur as closely spaced arrays separating trench segments. Most often, trench-trench transforms act as the lateral boundaries of large plates. They are typically many hundreds of kilometers long, involve continental lithosphere, and display complex and idiosyncratic geometry (70). Examples include the Alpine fault system of New Zealand (Figure 3.20) and the Macquarie Ridge, farther south. The latter structure generated an unusually large, enigmatic earthquake in 1989. Ridge-trench transforms connecting extensional and contractional zones also occur as lateral plate margins. The left-lateral Dead Sea fault, for example, connects the Red Sea spreading center between the African and Arabian plates to the oblique collisional Bitlis zone between the Arabian and Anatolian plates (71). The right-lateral Sagaing fault system runs through Myanmar (Burma) along the eastern edge of the Indian plate, between spreading in the Andaman Sea and convergence in the Himalayan system.

Many strike-slip faults are not transform faults; that is, they do not transform at their termini into other types of plate boundaries. A major class involves the trench-parallel strike-slip faults associated with the strain partitioning in oblique subduction zones, discussed above. Other strike-slip faults accommodate the considerable horizontal motions associated with the lateral advection of crust escaping continent-continent collision zones. The great faults in central Asia, north of India, are the most prominent cases of such indent-linked strike-slip faults, and they



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