UCLA

[1] McCann et al. [1979] published a widely cited "seismic gap'' model ascribing earthquake potential categories to 125 zones surrounding the Pacific Rim. Nishenko [ 1991] published an updated and revised version including probability estimates of characteristic earthquakes with specified magnitudes within each zone. These forecasts are now more than 20 and 10 years old, respectively, and sufficient data now exist to test them rather conclusively. For the McCann et al. forecast, we count the number of qualifying earthquakes in the several categories of zones. We assume a hypothetical probability consistent with the gap model ( e. g., red zones have twice the probability of green zones) and test against the null hypothesis that all zones have equal probability. The gap hypothesis can be rejected at a high confidence level. Contrary to the forecast of McCann et al., the data suggest that the real seismic potential is lower in the gaps than in other segments, and plate boundary zones are not made safer by recent earthquakes. For the 1991 Nishenko hypothesis, we test the number of filled zones, the likelihood scores of the observed and simulated catalogs, and the likelihood ratio of the gap hypothesis to a Poissonian null hypothesis. For earthquakes equal to or larger than the characteristic magnitude, the new seismic gap hypothesis failed at the 95% confidence level in both the number and ratio tests. If we lower the magnitude threshold by 0.5 for qualifying earthquakes, the new gap hypothesis passes the number test but fails in both the likelihood and likelihood ratio tests at the 95% confidence level.