Geological setting of the 1987 Edgecumbe earthquake, New Zealand

Abstract

The 1987 March M _L 6. 3 Edgecumbe earthquake occurred at shallow depth within the Whakatane Graben at the northeastern end of the onshore Taupo Volcanic Zone. The graben has formed where the north-south-trending North Island Shear Belt is intersected by the northeast-trending rift of the Taupo Volcanic Zone, and has been the site of continuing late Quaternary extension and subsidence. A history of displacement relative to sea level can be deduced from stratigraphy of drillholes on the downfaulted graben floor, and from outcrops of marine sediments on the uplifted graben margins. Subsidence had commenced before 0. 6 Ma (possibly at c. 1 Ma) and has continued to the present day at rates of c. 1–2 mm/year. Uplift of the western margin had commenced prior to 0. 4 Ma and the entire western rim was above sea level by c. 0. 3 Ma. Castlecliffian (mid Quaternary) marine sediments have been elevated to more than 300 m above present sea level, indicating long-term uplift rates of c. 1 mm/year. Uplift of the eastern margin has been at 0. 5 mm/year dunng the last 0. 12 Ma. Holocene faulting within the graben has been dominated by northeast trends. During the last 5500 years at least three distinct fault zones have been active within the graben, at Rotoitipakau, Onepu-Edgecumbe, and Matata. All these faults have moved within the last 800 years, and an earlier displacement of the Edgecumbe Fault occurred between 1850 and 800 years B. P. The 1987 March faulting occurred on the Onepu, Edgecumbe, and Rotoitipakau Faults, and also on new surface traces. The new traces are presumed to have formed on existing faults previously buried by younger (post-1850 and post-800 years B. P.) sediments. The 1987 earthquake and faulting resulted from continuation of tectonic processes active in the Whakatane Graben during the Quaternary.