in most areas isostatic adjustments are sufficiently rapid to compensate for (and thus prevent) higher rates of vertical crustal movement on a regional scale.
Although most short-term, vertical tectonic displacements recorded by Holocene marine strandlines are episodic or otherwise variable (see Figures 6.25–6.28 below) (Matsuda et al., 1978; Plafker and Rubin, 1978; Plafker et al., 1981), most average, long-term displacements recorded by Pleistocene strandlines appear to be relatively constant, at least over the past 100–500 ka (Bloom et al., 1974; Konishi et al., 1974; Moore and Samayajulu, 1974; Chappell and Veeh, 1978; Bender et al., 1979; Harmon et al., 1981; Dodge et al., 1983; Chappell, 1983; Hanks et al., 1984). Along some slowly uplifting coastlines, strandline data indicate that vertical displacement was fairly constant over the past 2–2.5 m.y. (Ward, 1985). Constant uplift can be demonstrated by comparing actual strandline elevations with those predicted by assuming constant uplift and using the New Guinea sea-level curve as a tectonic datum. Close agreement supports both the assumption of constant uplift and, of course, the assumption that the sea-level curve was a common tectonic datum.
Along a few coastlines, strandline data indicate that long-term tectonic uplift was clearly variable. For example, in the Christchurch area on the island of Barbados radiometrically dated Pleistocene strandlines yield an average uplift rate of 0.5 m/ka between 300 ka and 200 ka BP, and a lower rate of 0.3 m/ka after 200 ka BP (Bender et al., 1979). Interestingly, in the nearby Saint George’s Valley area uplift was relatively constant and averaged 0.3 m/ka over the past 640 ka.
Where data from marine strandlines and other tectonic markers are sufficiently abundant, regional compilations of vertical crustal movements provide valuable insights into both local and regional tectonic processes. Regional data are most conveniently and clearly expressed planimetrically by isobases (Figure 6.12) (Research Group for Quaternary Tectonics Map of Japan, 1969; Dambara, 1971; Wellman, 1979). Not surprisingly, most long-term regional isobases closely mimic general topographic contours, which simply means that the highest uplift rates occur in mountainous areas and the lowest rates or subsidence occur in low-lying regions. Usually, short-term isobases derived from historical information (tide-gauge and geodetic data) agree with longer-term isobases derived from geologic information, but locally short-term and long-term displacement rates differ drastically or the sense of displacement is reversed. For example, along the coastlines of northern California, Oregon, and Washington, net (long-term) displacement rates derived from Pleistocene strandlines are very low (0–0.5 m/ka; derived from dates
given in Kennedy et al., 1982), but short-term rates derived from tide-gauge data locally reach 3 mm/yr (3 m/ ka) (Hicks and Crosby, 1974). Some differences and reversals between short-term and long-term displacement rates probably represent pre-earthquake strain accumulation (Matsuda, 1976) or postearthquake crustal relaxation and, therefore, are important in determining earthquake potential (Thatcher, 1984). On the Muroto Peninsula of Shikoku, Japan, the 120-ka and 6-ka strandlines yield similar long-term uplift rates of 1.7 and 2.0 ka, respectively (Yoshikawa et al., 1974) (see Figure 6.16A below), whereas episodic uplift associated with earthquakes over the past 300 yr averaged 12.5 mm/yr (see Figure 6.26A below). Geodetic and tide-gauge data show that the peninsula actually subsided between these coseismic uplift events (see Figure 6.16B below).
Crustal tilt (T) is the differential vertical displacement (DI−DII) of a horizontal tectonic marker (strandline or platform) divided by the horizontal distance (d) between any two observation points (I and II)