中国东部及其邻区上地幔顶部Pn波速度结构及各向异性

Pn velocity and anisotropy of uppermost mantle beneath East China and adjacent regions

中国东部地处欧亚板块东南部,紧邻西太平洋板块俯冲带,有着复杂的地质构造和深部结构,是国内外学者研究的重点区域.本文采用Pn波层析成像方法反演得到了中国东部及其邻区上地幔顶部Pn波速度结构及各向异性.研究中应用的Pn到时数据主要来自多种地震观测报告,并特别补充了东北流动台阵、华北流动台阵以及区域地震台网记录的地震事件,拾取了大量高精度的Pn到时数据,最终挑选出2049个台站记录的24072次地震的240814条Pn波到时数据.结果显示,中国东部地区上地幔顶部平均速度为8.06km·s-1,速度变化范围为7.81~8.32km·s-1.东北地区东部表现为显著的低速异常,华北克拉通中、东部Pn呈现低速,而西部地区则表现为高速异常,华南地块主体表现为高速.反演结果还揭示江汉盆地、下辽河盆地、二连盆地及海拉尔盆地都显示出高速,而四川盆地和松辽盆地内部则呈现出不均匀的结构特征.四川盆地的高速异常显示出明显的分块现象,这可能是该盆地在沉积前具有不同的基底物质;松辽盆地的北部呈现为高速,而南部却表现为低速异常,这一特征与松辽盆地南、北部分别为高、低热流相对应,暗示盆地南部的岩石圈已经历了改造.研究进一步揭示Pn波低速区和高、低速过渡带的各向异性也较为强烈,而大部分强震都发生在这些区域之上的地壳内,说明这些部位容易发生变形而应力集中或产生应力差.

East China is located in the southeastern portion of the Eurasian plate and to the western Pacific subduction zone.With the complex geological structure and deep structural characteristics,this area attracts much attention of geoscientists to study.In this study,we determine the Pn velocity and anisotropy of uppermost mantle beneath East China and adjacent regions by using Pn anisotropic tomographic method.We use the data from a variety of seismological observation bulletins,especially we add earthquake events recorded by temporal seismic stations in Northeast China,North China,regional permanent seismic stations in East China and pick up a large number of Pn arrival times.Finally,we use 240814 Pn arrival time from 24072 earthquakes recorded by 2049 stations.Theresults show that the average Pn velocity of uppermost mantle in East China is 8.06 km·s^-1 and the variation range is 7.81-8.32 km·s^-1.The eastern portion of Northeast China is characterized by low velocity anomalies.The eastern and centre portion of North China Craton（NCC）exhibit low velocity and the west NCC shows high velocity.The South China block is mainly characterized by high velocity anomalies.The results also show that Jianghan basin,Xialiaohe basin,Erlian basin and Hailar basin all exhibit high velocity,while Sichuan basin and Songliao basin show a heterogeneous structure.The high velocity anomalies in the Sichuan basin show obvious partitioned phenomenon,which may be caused by different base materials before deposit.The Songliao basin exhibits high velocity in the north and low velocity in the south,which is consistent with the distribution of the thermal flow.It may imply that the lithosphere of the southern part of the basin has been modified.This study further reveals that Pn anisotropy is strong in low velocity region and highlow velocity transition zone,and most earthquakes occur in the crust above these regions.It may indicate that these parts are prone to deform and stress concentration or engender stress difference.