JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B13, PAGES 21,419-21,431, DECEMBER 10, 1991 SeismicGap Hypothesis' Ten Years After YAN Y. KAGAN Instituteof Geophysics and Planetary Physics, University of California, LosAngeJes DAVID D. JACKSON Department of Earth and Space Sciences, University of California, LosAngeles The seismic gap hypothesis states that earthquakehazard increases with time sincethe last large earthquake on certainfaults or plate boundaries. One of the earliestand clearest applications of the seismic gap theory to earthquake forecasting was by McCann et al. (1979), who postulated zones ofhigh, medium, and lowseismic potential around thePacific rim. In the 10 years since, there have been over 40 large (M > 7.0) earthquakes, enough to test statistically the earlierforecast. We alsoanalyze another forecast of long-term earthquake risk, that by Kelleher et al. (1973). The hypothesis of increased earthquake potential after a long quiet period can berejected witha large confidence. Thedatasuggest that, contrary to these forecasts, places of recent earthquake activity have larger thanusual seismic hazard, whereas the segments of the circum-Pacific belt with no large earthquakes in recent decades have remained relativelyquiet. The "clustering" of earthquake times does not contradict the platetectonic model, which constrains onlythe long-term average slip rate, not the regularity of earthquakes. INTRODUCTION The seismic gap hypothesis implies that earthquake hazard is smallimmediately following the previous large earthquake and increases with time since the last large eventon certain fault or plate boundaries [Sykes and Nishenko, 1984, p. 5911]. The hypothesis has re- centlybeenused in long-term forecasting of earthquakes around the whole Pacific rim [McCann et al., 1979; Nishenko, 1991], in Cahfornia [Sykes and Nishenko, 1984; Bakun and Lindh, 1985; Working Group on Cali- fornia Earthquake Probabilities, 1988], and in other Pa- cificregions [Sykes, 1971;Kelleher,1972; and Kelleher et al., 1973]. Henceforth we shall refer to McCann et [1991 [1991 to [191 as KSO [1973].Because of the scientific and social im- portanceof theseforecasts, the seismic gap hypothesis deserves rigoroustesting. The times of large earthquakes in a givenregion can be studied by the statistics of "point processes" [Coz and Lewis, 1966].These processes can be characterized well by their coefficient of variation, definedas the ra- tio of the standard deviation of interval times to the mean intervaltime (or "recurrence time") T. At the one extreme a quasi-periodic process has a coefficientof variationlessthan 1, while a clustering process, at the other extreme,hasa coefficient of variationgreaterthan 1. For comparisonthe simple Poissonprocessserves as a useful benchmark' it has a coefficient of varia- Copyright 1991 by the American GeophysicalUnion. Paper number 91JB02210. 0148-0227/91/91JB-02210505.00 tion equal to 1, and predicts a seismic hazard indepen- dent of time and previous seismicactivity. The seismic gap hypothesis assumesthat earthquake occurrenceis a quasi-periodic process,so that earthquake potential is low when the elapsed time sincethe last large earth- quake is lessthan T and is high afterward. Unfortunately, the record is too short in most seis- mic zones to estimate the distribution of event times directly from the data. The palcoseismic record at Palette Creek, California, is the longest rehable record and suggests a clustering process,although a Poisson process cannot be rejected [$ieh et al., 1989]. Given the shortness of the seismicrecord, we test the seismic gap method using an ensemble of seismic zones,rather than an ensemble of times within a singlezone. The basicidea behind the seismic gap hypothesis has enjoyed intuitive appeal since the early work of Reid [1910]. He suggested that a large earthquake releases most of the stress in a given fault segment and that fur- ther earthquakes there would be unhkely until the stress is somehow restored. However, Gilbert [1909], the first to formulate this hypothesis,calling it the "rhythmic recurrencehypothesis," rejected it after analyzing seis- micity in the United States during the nineteenth and early twentieth century: "The hypothesis of rhythmic recurrence has no sure support from observation,and is not in working order for either large or small areas. Its corollary of local immunity after local disaster is more alluring than safe"[Gilbert, 1909, p. 133]. The acceptance of plate tectonics in the 1960s as a behevablemechanismfor resupplyingstressadded in- tuitive argumentsfor the seismic gap hypothesis. The standard explanation for quasi-periodicity is that the 21,419