南海北部滨海断裂带的深部地球物理探测及其发震构造研究

Studies on the Binhai Fault Zone in the Northern South China Sea by the Deep Geophysical Exploration and its Seismogenic Structure

滨海断裂带为华南亚板块与南海亚板块的分界断裂,是南海北部陆缘的重要控震构造和发震构造。为了探明南海北部海陆过渡带特别是滨海断裂带两侧的深部地壳结构变化特征,在南海北部进行了一系列的海陆地震联测的实验,根据海陆联合深地震探测的结果,南海北部以滨海断裂带为界,断裂带西北部为华南亚板块的典型陆壳,地壳厚30km,上地壳下部存在一层速度为5.5～5.9km.s-1、厚度为3.0～4.0km的低速层,埋深约10～18km;断裂带东南部为南海亚板块减薄型陆壳,厚25～28km,上地壳下部的低速层逐渐减薄并最后尖灭。滨海断裂带为一个上下连续倾向SE的低速破碎带,宽度6～10km。滨海断裂带与上地壳下部的低速层的交接构造部位形成南海北部的重要应力集中带和应变能积聚带,是地震孕育、发生的深部动力学条件。滨海断裂带的发震构造属NEE向与NW向相交切的断裂构造型式,以NEE向的滨海断裂带为主,NW向断裂带为辅。

The Binhai Fault Zone(BFZ) is the boundary fault of the South China subplate and South China Sea subplate,and is an important earthquake -controlling and -triggering structure in the northern margin of South China Sea.To investigate the deep crustal structure across the onshore-offshore transitional zone and BFZ of northern South China Sea,onshore-offshore seismic explorations were carried out in this area.According to the exploration results,the northern South China Sea is divided into two parts by the BFZ.The northwest side of the BFZ is a typical continental crust that belongs to the South China subplate.Its thickness is 30 km.A low velocity layer is found in the lower part of the upper crust.Its velocity is 5.5～5.9 km/s and its thickness is 3.0～4.0 km.The layer is buried at a depth of 10～18 km.The southeast side of the BFZ is a thinning continental crust,which has a thickness of about 25～28 km.The low velocity layer thins gradually and eventually wedges out to the seaside.The BFZ is an up-to-down-continuous low velocity zone,which dips SE and has a width of 6～10 km.The intersection part of the BFZ and the low velocity layer forms an important stress concentrating and strain energy accumulating zone,which serves as a deep dynamical condition for earthquake's pregnancy and occurrence.The pattern of BFZ seismogenic structure is constituted by mutually cutting faults of trending NEE and NW,respectively,where the NEE-trending BFZ is dominant and the NW fault subdominant.