2000年伊豆諸島の地震で試みられた活動予測 : 直前予測・短期予測の詳細

During the seismic activity in 2000 in the Izu Islands region, Japan, an experimental study on earthquake prediction was carried out based on several empirical rules. For the imminent prediction, hypothesis A was applied from 15 to 28 July 2000: that is, when earthquakes with similar magnitudes were observed at intervals of less than 2 hours, a larger event would occur within about 4 hours. The alarm period was then changed to 12 hours (hypothesis B), and the intervals of a pair of earthquakes and the alarm period were changed to 24 and 48 hours, respectively (hypothesis C). According to these assumptions, 13 cases out of 25 imminent predictions were successful. Shortterm predictions for an alarm period of several days to 21 days were also attempted from 7 July to 13 October 2000, with the following assumptions: when small earthquakes began to occur adjacent to the previously active area, it would be a precursory signal of a forthcoming larger event within several days (hypothesis D). In addition, when earthquakes with similar magnitudes occurred within 7 days, a larger event might be expected within 7 or 21 days (hypothesis E or E', respectively). Among the 13 short-term predictions, 8 were successful. Although the success rates were not high, such information would still be useful to prompt an examination of the available data from various observations or to prepare for the possible execution of a governmental disaster mitigation program. Accumulating such experience will also be useful for finding an effective prediction method and an appropriate expression to announce the forecast