The precursory earthquake swarm in Japan: hypothesis test

A formal performance test of the hypothesis that swarms are long-term precursors to major shallow earthquakes is in progress in the region of Pacific-plate subduction in Japan. The likelihood of the major earthquakes (JMA magnitude greater than or equal to 6.8) that occurred in the region during the test period (1983-1998) was 5.04 times higher under the swarm hypothesis than under the stationary Poisson model; this result is inconclusive in terms of the proposed acceptance level of 20. The earthquakes were the Hokkaido-Toho-Oki earthquake (M8.1) of 4 October 1994, and the Sanriku-Haruka-Oki earthquake (M7.5) of 28 December 1994. The significance of the performance has been evaluated by a Monte Carlo study of the results since mid-1991. This shows that the Poisson model can be rejected at the 1% level, and that the results are consistent with the swarm hypothesis. The test is continuing.