Geophysical Methods

Observations of seismicity can sometimes help to delineate active faults. Persistent alignments of seismicity, especially at the ends of identified faults, can occasionally be considered seismic sources or seismogenic extensions of a fault. Many active faults have associated seismicity, including the Calaveras, Hayward, and central San Andreas Fault zones of central California, which, however, may only indicate the creeping segments of these faults. Other sections of the San Andreas Fault system that are not currently creeping are not clearly delineated by small earthquakes. Areas of relatively high seismicity may warrant examination for active faults.

In the eastern United States, alignments of high seismicity such as near New Madrid are associated active subsurface faults. Other major basement faults are associated with seismicity and may be active (Gordon, 1985); these also could be examined.

Seismic reflection techniques can help to delineate subsurface faults in sedimentary basins, both on land and beneath lakes and oceans. These techniques are used for recent fault detection and delineation studies, particularly in offshore California (Greene et al., 1973) and along the central California coastal margin near the Hosgri Fault (Crouch et al., 1984) near Point Conception (Pipkin and Ploessel, 1985), and in the offshore zone of deformation between the Inglewood Fault and Rose Canyon Fault (San Diego). Seismic reflection profiling by the Consortium for Continental Reflection Profiling (COCORP) has revealed the down-dip nature of many faults and a major detachment surface under the Sevier Desert of Utah (Allmendinger et al., 1983).

Gravity methods are most effective for studying fault zones where a strong density contrast exists between materials on either side of the fault. This situation occurs along faults where basement rocks are displaced against sediments or fault offsets in basins where the thickness of sediment differs across the fault. These methods are especially effective for regions of extensional faulting. Zoback (1983) used gravity techniques to delineate the geometry of range bounding normal faults in the Basin and Range province along the Wasatch Fault zone in Utah.

Application of surface magnetic and aeromagnetic survey methods for evaluation of active faults is discussed by Cluff et al. (1972), Krinitzsky (1974), and Sherard et al. (1974). These methods can be used to detect and delineate faults concealed by recent sediments and provide a relatively inexpensive method of contouring the thickness of basin fill. Smith (1967) located intra-basin, largely concealed, major fault grabens within the Dixie Valley graben using aeromagnetic methods. Some of these graben boundary faults were also accurately delineated by the faulting of the 1954 Dixie Valley earthquake. Smith provided a detailed outline for applying magnetic methods to the Basin and Range province with normal- and oblique-slip faults.

Bailey (1974) used a magnetometer to determine the surface fault location of the Chabot Fault, California, where anomalous drops in magnetism suggested locations of fracturing and subsequent leaching related to faulting. The eastern limit of the Chabot Fault zone was identified so that buildings could be sited to avoid potential surface rupture. Similar applications may be useful in defining active faults that are concealed by young alluvium or bodies of water.

Exploratory Methods

Exploratory methods for fault assessment advocated by Louderback (1950) were little used until the late 1960s when they assumed an important role in fault evaluations to assess such features as for activity, age dating, paleorupture and liquefaction events, slip direction, recurrence intervals, and slip rates. An adequate exploratory trenching and borehole program is critical in evaluation of active faults and is a major part of both domestic and foreign assessments. Specific applications to fault assessment are included in Taylor and Cluff (1973). The use of trenching as an exploratory method for nuclear power plant siting is discussed in Hatheway and Leighton (1979).

DETERMINATION OF FAULT ACTIVITY

Definition of Activity

Before 1950, most geologists did not distinguish between inactive faults and those with a potential for renewed displacements and associated earthquake activity, yet this is a critical part of man’s planning and design. Slemmons (1982a) listed over 30 definitions for “active” or “capable” faults of which only three were made before 1950. No definition for active faults is universally accepted, although two elements are present in most definitions: (1) the potential or probability of future displacements in the present tectonic setting and (2) the time of most recent displacement (e.g., historical, Holocene, Quaternary, or “in the present seismotectonic regime”). The first element, potential, is critical to all assessments for larger earthquakes; the second, recency, relates indirectly to rate of activity, which provides a more quantitative measure of degree of fault activity. Fault activity can also be classified by fault slip rate. Figure 3.2 shows the general relationships between



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