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Active Tectonics: Studies in Geophysics
FIGURE 6.15 Tilted strandlines on Sado Island, Japan.
A, Isobases defined by 120-ka and 6-ka strandlines reveal continuous southeastward tilt of two structural blocks (Osado and Kosado) that constitute the island. Note—data points occur only along coastlines; isobases interpolated across inland areas.
B, Longitudinal profiles of Pleistocene and Holocene strandlines also reveal continual tilt. The strandline data indicate that the tilt rates of the two blocks have been constant but significantly different. If the fault separating the two parts of the island is vertical, the 120-ka strandline is offset about 75 m and the 6-ka strandline is offset about 4 m, which yield similar average slip rates of 0.6 m/ka and 0.7 m/ka, respectively. See Figure 6.24 for location. Modified from Tamura (1979).
FIGURE 6.16 Tilted strandlines on the Muroto Peninsula, Japan.
A, Isobases on 6-ka strandline reveal short-term northward (landward) tilt of peninsula at the rate of 5.5×10−5/ka, which is similar to the long-term rate derived from the 120-ka strandline but much lower than the 5.3×10−4/ka rate preduced by historical earthquakes. Note—data points occur only along coastlines; isobases interpolated across inland areas.
B, Geodetic data show that significant landward tilt accompanied the 1947 Nankai earthquake. This coseismic movement indicates that tilt in this area is episodic not continuous. Gradual southward (seaward) tilt documented by geodetic data prior to the 1947 earthquake probably reflected pre-earthquake strain accumulation. See Figure 6.24 for location. Modified from Yoshikawa et al. (1964) and Kanaya (1978).
cur above and closely mimic tighter bedrock structures, which indicates that strandline deformation represents the most recent increment of long-term crustal movement (Figures 6.17A and 6.18B) (Wellman, 1971a,b; Gahni, 1978; Lajoie et al., 1982b). In many areas, however, folded strandlines are not associated with obvious bedrock structures and, therefore, are the only clear evidence for recent crustal folding (Figure 6.17B; also see Figure 6.21 below) (Plafker, 1972; Kaizuka et al., 1973; Lajoie et al., 1979a). In either case, progressively tighter folds in successively older strandlines reflect continual differential crustal movement (Figure 6.17). Along