bouldery beach ridges up to 27 m above sea level on the rapidly uplifting coastline at Turakirae Head, the southernmost tip of North Island (Figure 6.28) (Wellman, 1969; Stevens, 1973). The two lowest strandlines at 7 and 3 m above sea level, which were formed during historical earthquakes in 1460 and 1855, respectively, appear to record regional northwestward tilt across the entire southern tip of the island, whereas the four highest strandlines appear to record intense crustal warping between two local faults parallel to the offshore subduction zone (Wellman, 1969). However, all six strandlines may reflect regional tilt (Stevens, 1973). In either case, the five coseismic uplift events represented by these strandlines range from 2.5 to 9 m, and average 5.4 m (Figure 6.28). An assumption of constant uplift yields tentative ages for the four highest strandlines and also yields an average uplift rate of about 4 m/ka (see Figure 6.7) (Wellman, 1969). These graphically derived dates appear to indicate that uplift events follow a time-predictable pattern, which would indicate that the next event should occur in about 500 yr (Wellman, 1969). However, the pattern of uplift is inherent in the assumption of constant uplift—it is not independently demonstrated. Consequently, no firm estimate for the date of the next coseismic uplift event can be made.
The often disparate and inconclusive estimates of past and future seismic events derived from sequences of emergent Holocene strandlines in the three areas discussed here (Japan, Alaska, and New Zealand) illustrate many of the difficulties encountered in interpreting even excellent historical and geological strandline data. However, in spite of these difficulties sequences of coseismic strandlines are among the most complete records of past earthquakes and in many coastal areas provide extremely valuable insights into the distribution of earthquakes in both space and time.
This brief review of coastal tectonics illustrates the breadth and scope of neotectonic deformation and seismic history that can be derived from the study of marine strandlines, which so conspicuously record the dynamic interaction between the fluctuating sea level and mobile tectonic plates along many of the world’s active coastlines. Without sea-level changes, discrete strandlines would not be produced and the long-term record of Pleistocene crustal movements in coastal areas would be extremely difficult if not impossible to extract from a relative sea-level record. Conversely, without vertical crustal movements, a detailed history of sea-level fluctuations would be virtually impossible to reconstruct. As our understanding of this complex interaction increases we discover new problems that require the reinterpretation of existing data and the acquisition of new, more precise information. It should be stressed, however, that even at our present level of understanding the uncertainties inherent in extracting past crustal movements and seismic histories from relative sea-level records are far outweighed by the wealth of useful information obtained.
Following is a list of some of the most pressing problems and needs that must be addressed if we are to progress in the new and rapidly evolving field of coastal tectonics.
Theoretical models relating sea-level changes and vertical crustal movements should be developed. General relationships are expressed by Eqs. (6.1) and (6.8) and Figures 6.5, 6.6, and 6.7. Theoretical models will provide a framework in which tectonic and sea-level data can be interpreted and evaluated. They will also provide graphical and mathematical means of correlating and dating strandlines. Models will also provide a coherent framework for compiling and comparing strandline data from different areas.
The resolution of existing radiometric, chemical, and paleontologic dating techniques should be im-