THE PRECURSORY EARTHQUAKE SWARM IN NEW-ZEALAND - HYPOTHESIS TESTS

Tests have been carried out on the simple precursory swarm hypothesis, which proposes a one-to-one relation between swarms and mainshock eve nts, and tests are in progress on the generalised hypothesis, which al lows for clustering. The performance of the simple hypothesis relative to the usual stationary Poisson model was favourable for several year s after the start of the tests in 1977, then deteriorated, and fell to the proposed rejection level with the occurrence of the Doubtful Soun d earthquake of 1989 May 31, M 6.1. During the tests on the simple hyp othesis, the database changed substantially with the adoption of a rev ised magnitude scale by the New Zealand Seismological Observatory. Thi s resulted in additional swarms appearing in the past catalogue, and a lso in swarm clusters being recognised as far back as 1968-72, includi ng a cluster precursory to the 1989 Doubtful Sound earthquake. These c hanges gave further grounds for rejecting the simple hypothesis. Accor dingly, the generalised hypothesis, which has been under test in Japan since 1983, was formulated for New Zealand conditions, and tests were started in July 1991.