华北地区前兆异常中期演化特征

Evolutional characteristics of medium-term precursory anomaly in North China Region

对 1 98 6- 1 999年华北地区前兆异常群体时空演化特征和 1 989年大同 6.1级地震及 1 998年张北 6.2级地震的前兆异常阶段性发展过程进行了分析 ,探讨了前兆异常群体演化与强震成组活动的关系及中期前兆异常演化特征。结果表明 ,华北前兆异常主要集中分布于强震成组活动主体地区及其相关的活动断裂构造带上 ,并呈起伏发展 ,强震前约 3、4年 ,区域异常台项比由正常增加至 1 0 %以上 ,是进入中期阶段的标志 ,震前 1年左右可能再次增加 ,为向中短期过渡的迹象。中期阶段异常种类以地壳形变类为主 ,并主要分布于震中外围 1 0 0～ 2 50 km范围内 ,震中区附近则为少异常的“异常空区”,显示震源区介质处于硬化阶段。综合分析中小地震活动和前兆异常群体演化特征 ,对于判定中期地震危险区是十分重要的。

Spatio-temporal evolution characteristics of medium-term an omaly in No rth China from 1986-1999, as well as the stage-by-stage evolutional processes of precursory anomaly before and after the Datong earthquake with M 6.1 in 198 9 an d Zhangbei earthquake with M 6.2 in 1998 have been studied in this paper. An d th e relationship between the precursory anomaly evolution in group and group activ ity of moderately strong earthquakes is also studied. The results show that, in North China, the precursory anomaly is mainly distributed in the main areas of e arthquake group activity and along the relevant active faults. Ratio of the anom aly observing items to all observing items in this area increases about 10% from normal level about 3 or 4 year before the strong earthquakes, maybe this is the obvious sign from long-term stage into middle-term stage. The ratio may incre as e again and this means from medium-term stage into the short-term stage. Crust al deformation is the major type of precursory anomaly and distributed in the area of 100-250 km range around the epicenter. There are few anomalies near the epi c entral area, indicating the medium strengthening in seismic source area. It is h elpful for judging the emphatic area of medium-term earthquake risk by syntheti c al analysis of seismic activity and the features of precursory anomaly evolution in group.