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