中国东南部地区背景地震重新定位及隐伏活动构造初步研究

摘要

中国东南部地区或分布崇山峻岭,或为新生代盆地覆盖,因而活动断裂的发现与定位极为困难.沿断裂的地震活动可以成为断裂现今活动的直接证据,但需要地震的精确定位.文中通过精定位背景地震活动性,初步分析了区域构造活动与背景地震活动的关系,解译了研究区存在的隐伏活动断裂.利用双差地震定位法对中国东南部地区大致22° ～ 35°N,106°～ 122°E范围内1990-2014年发生的背景地震开展了重新定位工作,重新定位了51 000多个小震的震源参数.重新定位的背景地震活动表现出与区域构造活动相一致的分布特征,具有分区活动性,不同构造区出现不同的分布样式.现今活动较弱的构造区,地震活动分布离散或稀疏,没有指示出隐伏活动断裂的存在.在现今活动较强的构造区,地震活动明显集中成线性条带状,线性条带的展布样式和方向与区域构造格局和主要断裂走向一致.以线性条带分布的背景地震活动性揭示出的隐伏活动断裂主要分布在东南沿海一带、桂西北地区、四川盆地东南缘以及霍山一带,多数为以往研究所未发现的断裂.重新定位地震在不同构造区的震源深度分布差异明显,在块体中部区域震源深度最浅,主要分布在0～15km,表明脆性变形主要发生在上地壳,中下地壳为半塑性、塑性变形状态;在东南沿海一带,有下地壳地震发生.震源深度最深的区域为四川盆地的东南缘一带,有地震震源深度分布在40 ～ 44km,反映了地壳厚度大且下地壳为脆性变形特征,与较低的大地热流值和较硬的流变强度相一致.%Most of the regions in southeastern China are covered by thick Cenozoic sediments,or are the mountainous areas,so it is difficult to find and locate the active faults using the conventional geologic methods.The precisely relocated background seismicity in the seismically active region can be used to identify the buried active structure.In this paper,we investigated the relationship between regional tectonics and background seismicity,and interpreted the possible buried active faults in southeastern Chinausing the relocated background seismicity.We relocated the background seismicity occurring in the region from 106°E to 122°E and from 22°N to 35°N between 1990 and 2014 using the doubble difference earthquake location algorithm.More than 51 000 small earthquakes were relocated.In general,the relocated background seismicity corresponds well to the tectonics,showing the zonation features with typical seismicity pattern in each tectonic regime.It is observed that in the weakly active tectonic regime,the seismicity distributes dispersely or even scarcely,while in the strongly active tectonic region,the seismicity is highly clustered and organized to lineation pattern showing the same direction as the strike of the dominating fault zone.We interpreted the buried active faults using the lineation of seismicity.The inferred active faults are observed in the southeast coast region,the northwest Guangxi Province,the southeast boundary region of the Sichian Basin,and around the Huoshan Fault,many of which were not found by previous studies.The relocated hypocentral depth varies greatly in southeastern China.The shallowest earthquakes between 0 and 15km mainly distribute in the central region,indicating that the brittle deformation process only occurred in the upper crust,while the middle and lower crust are to be half-ductile and ductile deformation.There are earthquakes occurred in lower crust in the southeast coast region.The maximum depths distribute in the southeast boundary region of the Sichuan Basin,some are greater than 40km,indicating that the crust depth is larger than other places and the lower crust still sustains brittle deformation,which corresponds to the lower geothermal value and high crustal strength.