some highly active coastlines coseismic strandlines are warped into progressively tighter folds (Figure 6.17A) (Wellman, 1971a,b; Kaizuka et al., 1973; Gahni, 1978), which suggests that tectonic folding, like uplift and tilt, is incremental, not continuous.
On local to regional scales, vertical displacement data from deformed marine strandlines are conveniently summarized as structural contours and isobases that express folds planimetrically (Figure 6.18; also see Figure 6.21 below). Structural contours and standard isobases depict deformation of a single tectonic marker (Figure 6.18A; also see Figure 6.21A below), whereas integrated isobases depict deformation normalized from two or more tectonic markers of different ages (Figure 6.18B) (Gahni, 1978).
Isobases on the surface defined by the 1964 strandline in the Gulf of Alaska (Figure 6.18A) reveal broad, gentle warps produced by coseismic crustal deformation over an area of 200,000 km2 (Plafker, 1965). These isobases indicate that during the great 1964 earthquake an area of 60,000 km2 was uplifted an average of 1–2 m and was tilted northward; maximum uplift was 3–10 m above local, north-dipping secondary faults (Figure 6.17C) (Plafker and Rubin, 1978). The isobases also show that an area of 110,000 km2 north of the uplifted zone subsided a maximum of 2 m during the earthquake. The pattern of coseismic crustal warping associated with the earthquake is similar to the general pattern of long-term deformation documented by older Holocene strandlines. However, drowned vegetation in the uplifted area south of the epicenteral region documents subsidence during the 1.4 ka prior to the earthquake (Plafker and Rubin, 1978). This subsidence was probably pre-earthquake strain accumulation above the interplate megathrust on which the slip occurred.
The general pattern of coseismic uplift and subsidence that accompanied the 1964 earthquake in the Gulf of Alaska is similar to the pattern of crustal defor-