中蒙弧地区地震活动性及其地球动力学成因研究

Study on seismicity and its geodynamic genesis in Sino-Mongolia Arc Areas

应用中国地震局地球物理研究所和蒙古科学院天体和地球物理研究中心合作编制的蒙古国及邻区M≥3.5的地震目录,研究了中蒙弧地区的地震活动性特征.结果表明,与中国大陆的"南北地震带"相对应,研究区地震总体分布大致以107°E为界,呈现西强东弱的特点,7级以上的强震集中于贝加尔湖、萨彦、阿尔泰以及天山地区,107°E以东,除纬度40°线附近(燕山地震带)地震较集中且强度大之外,其它地区地震稀少,强度也低.通过断层的野外调查和本地区震源机制解,进一步研究了该地区地震活动性特征的构造应力场及地球动力学成因.研究区大部份地震都是走滑型断层活动的地震.逆断层活动的地震主要分布于中国的天山地区和中蒙边境一带的阿尔泰山地区,正断层活动的地震主要分布于俄罗斯的贝加尔湖裂谷带,走滑兼倾滑断层活动的地震主要分布在研究区域的西部地区.研究区域内的大部份地区主压应力轴(P)的倾角都小于30°,为水平或近水平的构造应力场,自西向东主压应力轴的走向从近南一北方向逐渐转为北东一南西方向.断层的野外调查、震源机制解和区域构造应力场的方向表明,中蒙弧地区主要来自西南面的印度洋板块向北偏东方向的碰撞挤压,通过青藏高原传递到本区,来自东面太平洋板块的影响已较微弱,这是研究区地震活动西强东弱、8级以上强震都发生在西部的主要原因.

Seismic characteristics of Sino-Mongolia Arc areas are studied using the uniformM≥3. 5 earthquake catalogue established by Institute of Geophysics, China Earthquake Administration and Research Center of Astronomy and Geophysics, Mongolian Academy of Science. The results show that the epicenter distribution can be roughly divided into two parts, bounded by the longitude line 107°E, in accordance with the ＂North-South Seismic Belt＂ in the mainland of China. The seismicity is of a high level with many major earthquakes in the west and a low level with few events in the east. Earthquakes of M≥7 mainly occur in the Lake Baikal, Sayan, Altayand Tian Shan Mountains. Very few major earthquakes are documented in the east except the area around the latitude line 40°N, i.e. the Yanshan seismic belt of China. Furthermore, the causes of these seismieity characteristics and their origin of the tectonic stress field are also studied according to the focal mechanism solutions and field investigations on faults. Most of these earthquakes are of strike-slip faulting especially in the eastern region of the study area. Reverse faulting earthquakes occur in the Tian Shan Mountains and the Altay Mountains regions. Normal faulting events are mainly distributed in the Lake Baikal region, and the quakes of dominant strike-slip with dip component are recorded in the western region of the study area. From west to east, the directions of principal compressive axes （P axes） turn gradually from nearly NS to NE--SW and their dips are less than 30°. Field investigations on faults, focal mechanism solutions and study of the tectonic stress field indicate that the study area is affected by the NE directed compression from the India-Eurasia collision, which is transferred through the Tibetan plateau. While the influence from the Pacific plate to the east is very weak. This is the reason why the seismicity is higher in the west than in the east, and all of earthquakes with magnitude 8 or greater occurred in the west.