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山东省成武4.8级地震前二年,在其一侧小地震形成椭圆形围空区,空区长半轴达120公里。同样主震前二年,在以震中为园心,200公里为半径的园面积内小地震分布基本均匀(地震空白区除外);而在主震爆发前半年,小震集中在未来主震位置附近及其外围局部地区,这可能是震前应力急剧集中、调整的短期震兆。相应地在围绕主震震中较大区域内出现约十个月的“缺震”异常,当异常趋近恢复时发震。
For the first two years after the Chengwu earthquake of magnitude 4.8, Shandong Province, a small earthquake on its side formed an oval-shaped enclosed area with a length of 120 km. In the same two years before the main shock, the distribution of small earthquakes within the area centered on the epicenter and within a radius of 200 km is basically uniform (except for the blank area); in the first half of the mainshock, the small earthquakes concentrate on the location of the future main shock In the vicinity and in some areas outside its periphery, this may be a short-term earthquake with a sharp concentration and adjustment of pre-earthquake stress. Correspondingly, an “absent” anomaly occurred for about ten months in a large area around the epicenter of the mainshock, and an anomaly occurred when the anomaly approached recovery.