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Active Tectonics: Studies in Geophysics
from the main fault and generally have lesser displacements. Bonilla (1970) showed that secondary rupture displacements decrease with increasing distance from the fault.
Sympathetic offsets occur when strain release along the main fault or vibratory ground motion disturbs the state of stress of another fault, causing it to undergo displacement. During the 1968 Borrego Mountain earthquake, sympathetic displacement occurred on three faults, the Imperial, Superstition Hills, and San Andreas. Investigations by Allen et al. (1972) reported sympathetic offsets of 1- to 2.5-cm right-lateral displacement and that the longest of these ruptures was 30 km long and 50 km from the epicenter.
Surface displacement can occur gradually in areas of tectonic creep and nontectonic fluid withdrawal. In California, several faults are undergoing tectonic creep, with a maximum reported creep of 3.2 cm/yr along the San Andreas Fault in San Benito County (Burford and Harsh, 1980).
Tectonic deformation refers to areal or regional deformation that may or may not be associated with moderate or large earthquakes. Dramatic effects of tectonic deformation have been noted by observing the vertical displacement and tilting of shorelines during the 1964 Alaskan earthquake, which shows evidence of land movement relative to sea level. An area of south central Alaska, of probably over 110,000 mi2 of land and sea bottom, was affected by warping, horizontal distortion, and faulting (Plafker, 1972). The upper growth limit of barnacles showed a maximum of 37.8 feet of vertical displacement when the pre-earthquake and post-earthquake shorelines at Montague Island were compared (Plafker, 1972). In the Kodiak Islands, a maximum subsidence of 6.3 feet of the shoreline was recorded (Plafker, 1972). Plafker commented, “Regional uplift and subsidence occurred mainly in two nearly parallel elongate zones, together about 600 miles long and as much as 250 miles wide, that lie along the continental margin.” Earth movements, detected by geodetic measurements, were recorded after the 1971 San Fernando, California, earthquake (ML=6.5). These measurements showed the mountains to the northeast of the causative fault shifted upward as much as 2 m and horizontally as much as 2 m (Savage et al., 1975).
Folds and large crustal uplifts and tilts have generally been assumed to be aseismic and to represent gradual plastic failure. Recent activity at Coalinga, California, has shown that this, however, is not always the case. The damaging 1983 Coalinga earthquake (MS=6.6) occurred in an area where active faults and large-magnitude earthquakes were previously unrecognized. Post-earthquake investigations concluded that no surficial faulting accompanied the earthquake (Clark et al., 1984). Studies by King and Stein (1983) showed that uplifted Holocene terraces on the main fold associated with the earthquake could be identified and are consistent with the regional deformation accompanying the earthquake (Stein, 1983). Other earthquakes that were associated with folding and compressional regimes are the 1978 Tabas-e-Golshan, Iran, earthquake (MS= 7.5) and the 1980 El Asnam, Algeria, earthquake (MS= 7.25).
The relationships recognized at Coalinga have led to a re-examination of major folds of the California Coast and Transverse Ranges and adjustment of seismic design of nearby major engineering structures. Yeats (1982) suggested that surface faults along anticlines may be weakly seismic or low-shake faults. These faults, also called flexural-slip faults, are thought to be related to the folding structure, with displacement occurring along bedding planes of the units being folded (see Yeats, Chapter 4, this volume). Hill (1984) suggested that some of the Coalinga earthquakes may be related to flexural-slip events. Late Quaternary fault scarps are associated with the Toppenish anticline of the Yakima fold belt in Washington (Campbell and Bentley, 1981), but whether these are seismogenic has not been resolved.
Criteria for discriminating between seismogenic and aseismic geologic structures have yet to be developed for evaluating folds or areas actively undergoing tectonic deformation. Resolution of these issues is important for seismic evaluation of many engineering structures in areas with Late Cenozoic folding. Tectonic tilting or warping must also be considered for level or gradient-sensitive structures, such as aqueducts in Owens Valley and also on the western edge of the San Joaquin Valley.
Secondary effects associated with earthquakes include landslides and rockfalls, liquefaction, seiches, and tsunamis. These effects can cause severe and widespread damage; although the effects may be severest in the epicentral region, they may extend out to distances of as much as 1000 km. Landslides are often induced by earthquake shaking. The scale of these features can vary from slides a few meters long to slides kilometers in length. During the 1976 Guatemalan earthquake (MS= 7.5) over 10,000 landslides were generated in an area of 16,000 km2 (Harp et al., 1981). One of the most notable slides occurred in Peru in 1970, where a large, seismi-