A COMPARISON OF SEISMICITY RATES NEAR ADAK-ISLAND, ALASKA, SEPTEMBER-1988 THROUGH MAY-1990 WITH RATES BEFORE THE 1982 TO 1986 APPARENT QUIESCENCE

The rates of seismicity in the part of the Aleutian are monitored by t he Central Aleutians Seismic Network and in subregions of that zone du ring September 1988 through June 1990 have been compared with the rate s during 1977 through 1981, 1981 through 1982, and 1983 through 1985. The purpose of the study was to determine whether the rate of earthqua ke occurrence had returned to the level observed before 1982, when a p rolonged decrease in activity began. This decrease had been interprete d as seismic quiescence and was used as the basis of a prediction of a strong earthquake in this area. An M(W) = 8.0 earthquake did occur on 7 May 1986. The comparison with more recent activity is intended as o ne test of whether the observed decrease in activity was a real physic al phenomenon or had been artificially produced by changes in network instrumentation or data-processing procedures. The only change in inst rumentation since 1986 was the installation of a new digitizer, and te sts were made to confirm that this new unit and the installation of ne w software for picking arrival times did not alter the completeness of detections or the magnitude values. The test cannot be completely def initive because aftershocks of the 1986 event were still in progress, according to the modified Omori fit to the data. The interval used is the latest possible because the network has been terminated. The 1988 to 1990 rates are similar to those in the prequiescence interval, but higher by an amount that is in qualitative agreement with the expected rate of continuing aftershocks. The rates in most of the zone are sig nificantly higher than the rates during the period of decreased activi ty. Rates in the sites near the 1986 mainshock epicenter and the weste rn termination of the 1986 rupture are higher than the pre-1982 rates, and indicate a concentration of the late aftershocks in these places. The results of the test support the conclusion that the 1982 to 1985 decrease in cataloged activity represents a real drop in the rate of s eismicity prior to the 1986 earthquake. Examination of many more cases of seismicity changes before and following a strong earthquake is nee ded as the basis for judging the validity of quiescence as a true prec ursor. Another observation from recent seismicity is that an intense s warm in January 1989 and a magnitude m(b) = 6.1 event in March 1990 wi th a well-developed aftershock sequence have filled a spatial gap in t he aftershocks of the 1986 event, near its epicenter.