The precursory earthquake swarm in New Zealand: Hypothesis tests

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A series of tests is being carried out with the object of determining whether the relations evident in New Zealand and Japan, between precursory swarms and major earthquakes, are of value for long‐range, synoptic earthquake forecasting. The first New Zealand test (completed in 1990) showed that clustering, both of swarms and of mainshock events, should be allowed for, and the hypothesis was reformulated accordingly. The second New Zealand test, now completed, confirms the importance of clustering. It also reveals that the applicability of the swarm/mainshock relations is strongly affected by large‐scale tectonics. Further, a simulation study shows that, at the end of the test, the performance of the swarm hypothesis relative to the Poisson model lay between what would be expected if the hypothesis and the Poisson model, respectively, were correct in general. These results support a further reformulation of the hypothesis, which is now taken to be strongly applicable in the Hikurangi and Fiordland subduction zones, not at all in the region of Quaternary volcanism, and weakly elsewhere. This change and the provision for clustering made previously bring the present New Zealand formulation close to that under test in the Japan region. The third New Zealand test commenced on 20 December 1995 and is still in progress.