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New Seismic Gap Hypothesis: Five Years After

Yan Y. Kagan, & David D. Jackson

Published 1995, SCEC Contribution #126

We use earthquake data from 1989–1994 to test a forecast by Nishenko based on the seismic gap theory. We refer to this forecast as the “New Seismic Gap” hypothesis, because it is the first global forecast based on the seismic gap hypothesis that considers the recurrence time and characteristic earthquake magnitude specific to each plate boundary segment. Nishenko's forecasts gave probabilities that each of about 100 zones would be filled by characteristic earthquakes during periods of 5, 10, and 20 years beginning on the first day of 1989. Only the first of these can be tested now. We used three tests based on (1) the total number of zones filled by characteristic earthquakes, (2) the likelihood that the observed list of filled zones would result from a process with the probabilities specified in Nishenko's hypothesis, and (3) the likelihood ratio to that of a Poissonian null hypothesis. The null hypothesis uses a smoothed version of seismicity since 1977 and assumes a Gutenberg-Richter magnitude distribution. We used both the Harvard Centroid moment tensor and the National Oceanic and Atmospheric Administration preliminary determination of epicenters catalogs in our test. We also used several different magnitude cutoffs in our tests, because Nishenko's forecast did not specify a clear relationship between the characteristic earthquake magnitude and the threshold magnitude for a successful prediction. Using a strict interpretation, that only earthquakes 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% confidence level. The new seismic gap hypothesis predicts too many characteristic earthquakes for three reasons. First, forecasts were made for some zones specifically because they had two or more earthquakes in the previous centuries, biasing the estimated earthquake rate. Second, open intervals before the first event and after the last event are excluded in calculation of recurrence rate. Third, the forecast assumes that all 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 deficit 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 magnitude threshold we tried, the new seismic gap model failed the test on the number of filled zones, or the likelihood ratio test, or both, at least at the 95% confidence level.

Kagan, Y. Y., & Jackson, D. D. (1995). New Seismic Gap Hypothesis: Five Years After. Journal of Geophysical Research, 100(B3), 3943-3959.