- Copyright © 1991, by the Seismological Society of America
The earthquake catalog of the Central Aleutian Seismic Network (CASN) was searched systematically for significant rate changes in the period from 1976 through 1985. The magnitude signatures for the nine changes were modeled by synthetic signatures. By this method, six of the rate changes could be identified as artificial, based on the criteria that the rate change was confined to small magnitude events and that magnitude shifts were present. For each of these changes an artificial cause existed that affected the network within one to two months of the time of change as estimated from the catalog analysis. One of these cases was a rate decrease by a factor of 2 associated with a storm that had put out of operation two thirds of the network. Another change coincided with the introduction of digital data processing. The four remaining cases were artificial reporting rate increases due to installation of several new stations or to repair of several stations that had failed. The three rate changes that could not be identified unambiguously as artificial or natural on the basis of magnitude signatures were all rate decreases by factors of approximately 2. Two of these were clearly due to disasters that had put out of operation more than half of the network. The third was the proposed precursory quiescence to the 1986 Andreanof Island (M = 7.9) earthquake. From this we conclude that, without knowledge of the network history, many of the artificial reporting rate changes in earthquake catalogs can be identified as artificial and that their onset time can be estimated to within one to two months.
An earthquake count, independent from the Adak catalog, was obtained from the Helicorder records of the station Adak for the years 1978 through 1985 for the vicinity of Adak island. Of a total of 1831 events, 335 were not in the Adak catalog and 981 were located within the 1986 aftershock area. The Helicorder earthquake count showed a pronounced decrease by about 50 per cent, similar in amount to that in the Adak catalog. However, the decrease started in 1980, at a time when in the central subregion covered by the Adak catalog only M ≧ 2.9 events decreased, while the smaller magnitude bands show a maximum decrease in 1982. We conclude that the possibility could not be ruled out that electronics changes contributed to the reporting rate decrease starting in the years 1980 to 1982. However, it seems very unlikely that a network that was at its healthiest during the precursor time would be reduced by an improvement in the electronics to a detection level approximately equal to that in two short periods when only one third of the stations were operating. Therefore, the seismic quiescence reported and interpreted as a precursor by Kisslinger et al. (1985) was most likely a natural phenomenon.