渤海地震震中附近地区浅层地质构造特征

THE CHARACTERISTICS OF THE SHALLOW GEOLOLGICAL STRUCTURE IN THE ADJACENT REGION OF THE EPICENTER OF THE BOHAI SEA EARTHQUAKE

本文利用浅层地震勘探剖面分析了1969年渤海7.4级地震震中附近地区浅部地质构造特征。震中及余震区范围内,海底地形平缓,相对上隆。全新统(海相)厚度7—10米,较周围地区要小,上更新统最上部(陆相)厚度42—45米,较周围地区稍大,二者等厚线形态、走向都是 NE 向,与7.4级地震的余震区长轴方向(N45°E)一致。断裂较发育且数量多(6条),并多集中在震中西南4—6公里处,皆是高角度(80°)的正断层。较老的断裂在22000年前生成与活动,继承性多期活动的(NW 向,一条)、新生的(NE 向,一条)断裂在22000年之后,甚至在全新世时期还在活动、生成。断裂每次活动的垂直滑动量约3米。7.4级地震的发生主要与 NE 方向的构造有关。

The characteristics of the shallow geological structure in the adjacent region of the epicent-er of the 1969 Bohai Sea earthquake (7.4M_S) are analysed in the paper by use of the shallowseismic prospecting profiles.In the range of the epicenter and aftershock area,the submarinetopography is smooth with relative swell.The thickness of Holocene series (marine facies) is7-10m,which is smaller than that of the peripheral area.The thickness of upmost part ofEpipleistocene is 42-45m,which is larger than that of the peripheral area.The isopach formsand strikes of them are both in NE orientations,which are consistent with the direction of longaxis (NE) of the aftershock area of the 7.4M_S earthquake.Faults are well-developed with a largenumber (six) and all of them are high-angle normal faults clutered in 4-6km southwest of theepicenter.The older faults generated and were active 22000 years ago.The inherited multiphaseactive fault (NW) and the palingenetic faults (NE) generated have been activated since thattime.The vertical sliding of faults amounts to 3m every times.The occurrence of the 7.4M_S earthquake is mainly associated with the structures in the NE orientations.