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Abstract
Excavations for drainage of Ahuriri Lagoon near Poraiti has exposed a sequence, up to 8 m thick, of peat containing remains of two separate human skeletons, overlying primary airfall Waimihia Lapilli, and overlain by estuarine sediments. Stratigraphy together with radiocarbon dating of peat, wood, human bone, and shell samples is interpreted to indicate tectonic subsidence of 8 m in the last 3500 years at the pre-A.D. 1931 western margin of Ahuriri Lagoon. Most of this subsidence occurred between 3500 years B.P. and 1750 years B.P. at an average rate of 4.6 m/1000 years and was probably matched by the similar rate of peat accumulation. A hiatus in peat growth between c. 1800 years B.P. and c. 500 years B.P. was possibly the result of tectonic uplift.
Maori human bones were buried in the upper 0.5 m of the peat 480–550 years B.P. and their burial was followed immediately by inundation by Ahuriri Lagoon as a result of either further tectonic subsidence or the breaching of a barrier at the western margin of the lagoon. The entire sequence was uplifted 1 m during the A.D. 1931 M7.8 Hawke's Bay earthquake.
If all tectonic movement was co-seismic, as in A.D. 1931, then the four or five earthquake events deduced from the geologic record at Poraiti are a minimum.
