FIGURE 8.7 Simple shear associated with strike-slip faulting produces preferred orientation of fractures, faults and folds (A) as well as extensional and contractional landforms (B). Figure modified from Wilcox et al. (1973), Sylvester and Smith (1976), and Keller et al. (1982a).

terials, landforms, and late Pleistocene-Holocene chronology, the purpose of which is to derive rates of active tectonics (slip rates on faults, rates of uplift or subsidence, and recurrence intervals of damaging earthquakes). Such investigations have been very successful in recent years and are providing basic data necessary for long-term (tens to hundreds of years) earthquake prediction. Examples include (1) paleoseismic construetion (occurrence and recurrence intervals of prehistoric earthquakes) obtained from studying faulted Holocene-alluvial sequences of stream, marsh, lake, or landslide deposits (Clark et al., 1972; Sieh, 1978; Davis, 1981; Rust, 1982); (2) paleoseismic construction based on fault-scarp morphology change with time (Wallace, 1977; Bucknam and Anderson, 1979; Nash, Chapter 11, this volume); and (3) rates of uplift, slip rates on active faults, and/or recurrence intervals of assumed earthquakes based on chronology and offset of landforms such as alluvial fans (Keller et al., 1982a), marine terraces (Matsuda et al, 1978; Lajoie et al., 1979, 1982; Keller et al., 1982b), offset streams (Sharp, 1981; Sieh, 1981), and glacial deposits (Schubert, 1982).

Fault-Scarp Morphology

Slope morphology of scarps produced by faulting is a successful geomorphic indicator of active tectonics. Figure 8.9 shows generalized slope elements associated with a fault scarp. All the elements shown need not be present for a particular scarp, and because slopes are dynamic changing landforms, the dominance of one element relative to others changes with time. Table 8.1 and Figure 8.10 summarize form-process relationships for fault-scarp morphology change with time as discussed by Wallace (1977) for the Great Basin area in Nevada. Wallace was able to develop the chronology shown on Figure 8.10 and Table 8.1 by studying fault scarps that truncate 14C-dated shorelines of Pleistocene Lake Lahontan, are associated with volcanic ash of known age, were produced by known earthquakes, or can be dated by tree rings (dendrochronology).

Recurrent displacement along the same fault line produces a composite fault scarp. Wallace (1977) stated that multiple displacements on a compound fault scarp

FIGURE 8.8 Pressure ridges and sags associated with restraining and releasing bends and/or steps along strike-slip faults. Modified after Crowell (1974) and Dibblee (1977).

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