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Active Tectonics: Studies in Geophysics
FIGURE 8.4 Assemblage of landforms associated with active strike-slip faulting. Modified from Wesson et al. (1975).
zone suggests that earthquakes are likely to be generated there in the future.
Active strike-slip faulting in the offshore southern California continental borderland also has distinctive geomorphic expression characterized by linear troughs, sags, tectonic benches, fault scarps, and offset or deflected channels associated with submarine fans. A recent study of the San Clemente Fault zone (Legg and Luyendyk, 1982), utilizing topographic data from Seabeam surveys, demonstrated that the data base and resolution is now sufficient to begin studying submarine-tectonic geomorphology to improve mapping of active faults and to evaluate long-term earthquake hazard. Figure 8.6 shows topography and tectonic landforms associated with the San Clemente Fault zone (M.R.Legg, University of California, San Diego, personal communication, 1983).
Many of the topographic features associated with active strike-slip faulting such as sags, pressure ridges, and fault scarps can be explained by simple shear that produces contraction and extension as illustrated on Figure 8.7 (Wilcox et al., 1973; Sylvester and Smith, 1976). Others are better explained by extension or contraction associated with releasing or constraining bends or steps of fault traces as illustrated on Figure 8.8 (Crowell, 1974; Dibblee, 1977).
PROCESS-RESPONSE MODELS: RATES OF ACTIVE TECTONICS
Process-response models in active tectonics are broadly defined to include the investigation of earth ma-
FIGURE 8.6 Sketch map of part of the San Clemente Fault zone. Data are from Seabeam survey. Courtesy of Mark R.Legg, University of California, San Diego.