东亚与全球地震分布分析

Analysis of earthquake distributions in East Asia and in the world.

现代地震学是一门年轻的学科,但人类对地震现象的观察、记录和思索已有数千年历史。特别是在中国,很早就有灾害性地震的详细历史记载。最近40多年,我国、东亚及世界其他大陆地区的多次大地震的发生,推动了地震构造研究的发展,加深了对地震分布规律性及其地球动力环境的认识。从地质背景和发生机制看,全球地震构造可分为三大类:第一是太平洋海底地壳与陆缘地壳浅、中、深俯冲构成的地震带;第二是南半球离散地壳块体对北半球大地块边缘碰撞浅俯冲带;第三是全球三大洋脊张裂转换构造带。地球北半部内陆的中纬度地带有4个多震密布区,它们都处于N25°～55°的纬向大陆带内,与大陆会聚所造成的陆内变形有关。4个多震密布区的东半部则是相对少震区,显示出相对稳定的地壳结构。全球表壳GPS矢量场和北南两个极区各有不同,北半球欧亚大陆是向北呈弓字形运动;南半球是南美、非洲、阿拉伯、印度、澳大利亚等5块离散的大陆块,除南美大陆,其他4块大陆都是向NE和NNE方向运动。这4块大陆都是依次运动加速,澳大利亚陆块运动最快,向NNE方向约10 cm/a;同时从南太平洋南部沿NWW方向左型转换断层的运动也是高速的,这两个方向运动的交叉相碰,现已处在全球最为强烈的地震活动区域。北冰洋内群岛GPS站点向阿留申岛弧推进;南极冰陆的9个GPS站点则呈现旋扭状彼此相差约90°,可能表明北/南半球彼此有明显的1/4左右表壳的扭动。从卫星重力数据推测的地球的形状、全球热流的和地内热散失量的分布、地球磁场的西漂以及大地震引起的地球振荡特征等证据推测,地球的内部结构具有一定程度的非对称性和非均匀性,它们对全球板块运动、板块变形以及大地震的空间分布可能有一定的控制作用。

Although seismology is a young science with respect to other disciplines such as physics and chemistry,human activities of observing,recording and speculating on earthquakes have already had a long history for thousands of years.In particular,destructive quakes were recorded in detail as early as 780 B.C.In last 40 years,many major and great quakes hit continental areas of China,East Asia and the rest of the world successively,which have promoted the research of seismotectonics and the understanding of rules of earthquake distribution and its dynamic settings.From geological backgrounds and mechanisms,global seismotectonics can be classified into three types.The first is seismic belts with shallow,intermediate and deep focal events associated with subduction of oceanic plates,such as in the circum-Pacific.The second is along continental collision zones where shallow quakes are dominant,for example,the Himalayas.And the third is the mid-ocean ridges and transform faults in oceans.In the North Hemisphere,there are four land regions of middle latitudes（25°55°N） with dense quake distributions,which are related with continental convergence,while the crust on their east side seems to be stable with low seismicity.GPS measurements in recent years show that the Eurasia continent is moving eastwards forming a trace convex to north.In the South Hemisphere,except the South America,all other continents,i.e.Africa,Arabia,India and Australia move toward NE or NNE.Among them,Australia moves at a rate as large as 10 cm/a.Meanwhile the southern Pacific moves along a left-slip transform fault at a high speed,converging with the Australia plate around Indonesia where seismicity is extremely high.In the Arctic Ocean,GPS sites move to the Aleutian Islands,while in the Antarctic ice land motions of 9 GPS sites exhibit a rotation style with difference about 90°,probably implying a shear between the North and South Hemisphere.From a series of evidence,such as the earth＇s shape derived from satellite gravity,distribution of global heat flow and heat loss from the interiors of the earth,westward drift of the geomagnetic field,and oscillation of the whole earth caused by great quakes,it is inferred that the inner structure of the earth is asymmetric and heterogeneous to some extent.Such a characteristic might govern the global plate motion,plate deformation and distribution of great earthquakes.