NEW SEISMIC GAP HYPOTHESIS - 5 YEARS AFTER

We use earthquake data from 1989-1994 to test a forecast by Nishenko b ased on the seismic gap theory. We refer to this forecast as the ''New Seismic Gap'' hypothesis, because it is the first global forecast bas ed on the seismic gap hypothesis that considers the recurrence time an d characteristic earthquake magnitude specific to each plate boundary segment. Nishenko's forecasts gave probabilities that each of about 10 0 zones would be filled by characteristic earthquakes during periods o f 5, 10, and 20 years beginning on the first day of 1989. Only the fir st of these can be tested now. We used three tests based on (I) the to tal number of zones filled by characteristic earthquakes, (2) the like lihood that the observed List of filled zones would result from a proc ess with the probabilities specified in Nishenko's hypothesis, and (3) the Likelihood ratio to that of a Poissonian null hypothesis. The nul l hypothesis uses a smoothed version of seismicity since 1977 and assu mes a Gutenberg-Richter magnitude distribution. We used both the Harva rd Centroid moment tenser and the National Oceanic and Atmospheric Adm inistration preliminary determination of epicenters catalogs in our te st. We also used several different magnitude cutoffs in our tests, bec ause Nishenko's forecast did not specify a clear relationship between the characteristic earthquake magnitude and the threshold magnitude fo r a successful prediction. Using a strict interpretation, that only ea rthquakes equal to or larger than the characteristic magnitude should be counted, both catalogs show only two qualifying earthquakes in the entire area covered by the forecast. The predicted number is 9.2, and the discrepancy is too large to result from chance at the 99% confiden ce level. The new seismic gap hypothesis predicts too many characteris tic earthquakes for three reasons. First, forecasts were made for some zones specifically because they had two or more earthquakes in the pr evious centuries, biasing the estimated earthquake rate. Second, open intervals before the first event and after the last event are excluded in calculation of recurrence rater Third, the forecast assumes that a ll slip in each zone is released in characteristic earthquakes of the same size, while in fact considerable slip is released by both smaller and larger earthquakes. The observed size distribution of earthquakes is inconsistent with the characteristic hypothesis: instead of a defi cit of earthquakes above the characteristic limit, earthquake numbers are distributed according to the standard Gutenberg-Richter relation. By lowering the magnitude threshold for qualifying earthquakes, it is possible to reduce the discrepancy between the observed and predicted number of earthquakes to an acceptable level. However, for every magni tude threshold we tried, the new seismic gap model failed the test on the number of filled