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

Abstract

Abstract A sequence of strong earthquakes occurred in Jiashi County, Xinjiang, China (40°N, 77°E, population 280,000) in 1997, after a 10-month quiescence following a Ms 6.9 earthquake on 19 March 1996 in the same area. From 21 January 1997, through 18 October 1997, 15 earthquakes occurred with Ms ≥ 5.0, seven of which with Ms ≥ 6.0. Such sequences of strong earthquakes are rare around the world, but have occurred six times in the land area of China during this century. Of the 15 Ms ≥ 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, Ms 6.4, 6.3) were not predicted. Three of the eight predictions made were not fulfilled, one of which was announced to the public (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 earthquakes, which caused no deaths. One earthquake (April 11, Ms 6.6) struck 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 based on composite observations of precursory anomalies in seismicity, electromagnetic fields, and crustal deformation, which were in turn compiled over the past 30 years in a nationwide earthquake monitoring program. Examples of precursory anomalies include: (1) a clear seismicity pattern of “quiescence—intensification—main shock(s)” was observed for the April 6 earthquakes (Ms 6.3, 6.4), which was previously applied to the prediction of the well-known 1975 Haicheng earthquake in Laoning, China; (2) distinct decreases in b-values preceded the earthquakes on March 1 (Ms 6.0) and April 6 (Ms 6.3, 6.4); (3) anomalous variation of geomagnetic vertical component preceded the April 6 earthquakes (Ms 6.3, 6.4); (4) sudden decreases in borehole strain preceded 8 of the earthquakes.