Sponge Bay Uplift, 1931

On the afternoon of the 17th February 1931 an extraordinary event occurred at Sponge Bay, 3.5 kilometres from Gisborne. Men engaged in obtaining stone supplies on the beach of Tuamotu Island stated that a reef “just rose out of the sea without warning.” The 2 acre boulder bank rose an average of 2 metres without perceptible shaking of the ground in the immediate area or surrounding district.

Writing in the New Zealand Department of Scientific and Industrial Research Bulletin No. 43, Sydney Strong stated that he had studied the area two months previously. At that time, 1-2 feet (30-60 cm) of water had covered the reef at low tide. He added, “While the sea-floor rose on the seaward side of the Sponge Bay Beach [during the uplift], the area on the land side was depressed a few feet and broken into segments by numerous small crevices or cracks. Much inflammable gas escaped during the initial period of the movement in these areas. At the head of the Wheatstone Road, one mile [1.6 km] inland on the main fault-line, a renewed escape of inflammable gas and dislocation of the ground was also observed on the extinct mud volcano which is situated there.”

The event happened two weeks to the day after the devastating magnitude 7.8 Hawkes Bay earthquake and only 4 days after the magnitude 7.3 earthquake in the same area. The uplift was part of the regional response to the strain released by the earthquakes and their aftershocks.

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