Predictions of the 1997 strong earthquakes in Jiashi, Xinjiang, China

A sequence of strong earthquakes occurred in Jiashi County, Xinjiang, China (40 degrees N, 77 degrees E, population 280,000) in 1997, after a IO-month quiescence following a M-s 6.9 earthquake on 19 March 1996 in the same are a. From 21 January 1997, through 18 October 1997, 15 earthquakes occurred w ith M-s greater than or equal to 5.0, seven of which with M-s greater than or equal to 6.0. Such sequences of strong earthquakes are rare around the w orld, but have occurred six times in the land area of China during this cen tury. Of the 15 M-s greater than or equal to 5.0 earthquakes in the Jiashi sequence, eight fell within the predicted 7-day time windows of a total of 8 short-term and imminent-stage predictions, but the initial shocks of the sequence (January 21, M-s 6.4, 6.3) were not predicted. Three of the eight predictions made were not fulfilled, one of which was announced to the publ ic (a false alarm). Four of the remaining five predictions were acted on by local authorities and the public was notified 2.5 h to 4 days before the e arthquakes, which caused no deaths. One earthquake (April 11, M-s 6.6) stru ck 30 min after the prediction was made but before action could be taken by the local authorities, and it caused nine deaths. These predictions were b ased on composite observations of precursory anomalies in seismicity, elect romagnetic fields, and crustal deformation, which were in turn compiled ove r the past 30 years in a nationwide earthquake monitoring program. Examples of precursory anomalies include: (1) a clear seismicity pattern of "quiesc ence-intensification-main shock(s)" was observed for the April 6 earthquake s (M-s 6.3, 6.4), which was previously applied to the prediction of the wel l-known 1975 Haicheng earthquake in Laoning, China; (2) distinct decreases in b-values preceded the earthquakes on March 1 (M-s 6.0) and April 6 (M-s 6.3, 6.4); (3) anomalous variation of geomagnetic vertical component preced ed the April 6 earthquakes (M-s 6.3, 6.4); (4) sudden decreases in borehole strain preceded 8 of the earthquakes.