Earthquakes on the Campbell Plateau

Abstract

Abstract Before the installation of a seismograph on Campbell Island, the Campbell Plateau was regarded as aseismic. In the first six years of the instrument's operation, however, it recorded twelve local earthquakes, with magnitudes between 3½ and 5½. All these earthquakes are within 230 km of Campbell Island, and must have originated on the Campbell Plateau, but only one was large enough for its position to have been determined precisely. The earthquake occurred on 1971 Jun 26, near Cathedral Banks, an area of volcanic outcrops about 60 km south-east of Auckland Island. The best epicentre for this earthquake was obtained by assuming a P-wave velocity of 7.78 km/s across the Campbell Plateau, rather than that of 8.1 km/s which is appropriate beneath New Zealand. This earthquake, and others in the south of New Zealand that have been recorded at Campbell Island, show crustal phases which confirm that the structure of the Campbell Plateau is continental. The arrival times of these phases are consistent with the crustal thickness of about 20 km determined previously from dispersive-wave analysis.