FIGURE 6.27 Coseismic uplift, Gulf of Alaska.

A, Radiocarbon-dated and historical coseismic strandlines on Middleton Island, Alaska. Average uplift rate is decreasing and interval between events is increasing while the size of each event has remained fairly constant. Average uplift is 0.95 m/ka. The 1964 coseismic uplift did not bring the island up to the long-term rate, which suggests that another event is due in the near future. Present sea level used as tectonic datum. Modified from Plafker and Rubin (1978).

B, Radiocarbon-dated coseismic strandlines at Icy Cape (about 300 km east of Middleton Island on the Alaska mainland) yield a fairly constant uplift of about 1 m/ka. Uplift events appear to follow a time-predictable pattern. If this pattern continues, the next event is due in the near future. Modified from Plafker et al. (1981). See Figures 6.17C and 6.18A for 1964 coseismic strandline deformation.

about 1.4 ka BP has yet to be released. Consequently, if the Holocene trend continues, coseismic uplift at least as large as the 1964 event should occur on Middleton Island sometime in the near future (Plafker and Rubin, 1978).

The focal region of the 1964 Alaskan earthquake lies north and west of Middleton Island (Figure 6.18A), and a seismic gap lies to the east. Consequently, the next uplift event to affect the island may accompany a megathrust earthquake in the area of the seismic gap, possibly on the submarine Pamplona Fault (Plafker and Rubin, 1978). This interpretation is supported by data from Icy Cape, a broad sedimentary headland about 300 km east of Middleton Island on the Alaska mainland and within the area of the seismic gap. There, depositional strandlines dated at 4.9 ka, 2.4 ka, and 1.3 ka BP by radiocarbon techniques record at least three uplift events that followed a time-predictable pattern over the past 5 ka (Figure 6.27B) (Plafker et al., 1981). The lowest (1.3 ka) strandline may have been formed during the same seismic event that produced the second lowest strandline dated at 1.35 ka on Middleton Island (Figure 6.27A). If the 1.2–1.3-ka time span between strandlines at Icy Cape is the recurrence interval for the area of the present seismic gap, the next uplift event is overdue. Presumably, this event would make up some or all of the apparent deficiency in uplift on Middleton Island. An interesting aspect of the strandline sequence at Icy Cape is that the vertical distance between the 4.9-ka and 2.4-ka strandlines is 27 m (Figure 6.27B), which seems too large for a single uplift event. If uplift averaged 10–15 m per event, a 14-m uplift at about 3.8 ka BP may not have been recorded or its strandline may have been destroyed by subsequent wave erosion (Plafker et al., 1981).

New Zealand

The large southwest Pacific islands of New Zealand straddle the convergent boundary between the Australia-India tectonic plate to the west and the Pacific plate to the east. Northeast of the islands, the Australia-India plate is being thrust eastward over the Pacific plate, and southwest of the islands the relationship is reversed. The islands themselves are a manifestation of the uplift along the transition between the two opposing subduction zones on the same interplate boundary (Walcott, 1984). Within this complex structural area both interplate and intraplate earthquakes are common. At several localities, especially along the southeast coast of North Island, large earthquakes are recorded by emergent Holocene strandlines.

At least five coseismic uplift events are recorded by six

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