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5). Preparation gaps build up in a region inside a background gap or its surroundings in a short time interval (afew years) before the main earthquake. The preparation gap is usually surrounded by small precursory earthquakes, even though one or few relatively large earthquakes (still smaller than the main earthquake) may occur in regions on the edge of the gap. Such smaller magnitude earthquake activity has been considered as premonitory phenomena useful for intermediate-term and even short-term earthquake prediction. Three criteria are proposed for the identification of these gaps (Lu and Song , 1989): (1) with the formation of a preparation gap the seismic strain release should accelerate both in the gap and in its vicinity, (2) the ratio of earthquake frequency outside the gap to that within it should reach a maximum value during the formation of the gap, and (3) some moderate earthquakes often occur in the forthcoming seismic source area before formation of the background gap. The former two are the main criteria for identification of the gap, and the third is a subsidiary criterion for determining the location of the forthcoming earthquake.
In this study, based on the history of earthquakes in the Central-East Iran seismotectonic province, we have found a background gap and three preparation gaps. One of these gaps is related to the destructive earthquake of Ms =6.8, which occurred on 26 December2003 in the Bam region of southern Central-East Iran. This earthquake occurred in the edge of the recognized preparation gap in the Bam region. The other gap is related to a large earthquake of Ms =6.0, on February in the Sefidabeh region of south-eastern Iran. This earthquake also occurred in the edge of the recognized preparation gap in the Sefidabeh region.The strain release curve, the ratio of the earthquake frequency outside the gap to that within it, and the cumulative number-time curve, have good correlation with the earthquakes happened. In addition, recognition of a preparation gap in the Dasht-e-Bayaz region, eastern Central-East Iran, implies accumulating seismic strain and a large earthquake may occur in that region in the future. The well- known Dasht-e-Bayaz and Abiz earthquake faults are located in this preparation gap.]]>
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