Focal Depth of the Hawke's Bay Earthquake of February 2–3, 1931

Abstract

IN the seismological report of the Hawke's Bay earthquake of February 2–3, 1931¹, an attempt was made to arrive at a value for the focal depth of the shock. The method used was similar to that developed by Jeffreys² for near earthquakes in western Europe, depending on the apparent delay of the surface wave Pg. In the Hawke's Bay earthquake, a phase agreeing reasonably well in velocity with Pg was observed on the seismograms at Arapuni, Takaka and Wellington, and the mean apparent delay led to a focal depth of 13 miles (21 km.). On account of the confused nature of the Wellington and Arapuni records, and the lack of precise time on most of the records, it was considered that a definite value for the depth could not be obtained, but that it probably lay between 10 miles and 15 miles (16–24 km.). In the original interpretation of the Takaka record, the first phase recorded after Pn was considered to be Pg, and the interval between Pn and Pg was 28 seconds.