【摘要】 2014年11月22日16时55分在四川省甘孜藏族自治州康定县发生的6.3级地震,结束了鲜水河断裂带近30多年以来没有较大地震发生的历史,其潜在的地震危险性再次引起国内外地学工作者的关注.为了研究鲜水河断裂带南东段深部孕震环境和探求康定MS6.3地震的成因,本文先利用四川区域数字地震台网和康定地区及周边所布设的流动地震台阵在2009年1月1日至2014年12月5日期间所记录到7397次区域地震事件的99287条P波到时资料,反演得到了鲜水河断裂带南东段上地壳范围内不同深度的三维P波速度结构特征;再对康定震区及周边的重力、航磁数据进行视密度、视磁化强度反演,得到了壳内不同深度密度的横向变化信息和视磁化强度的分布特征;在此基础上综合研究鲜水河断裂带南东段的深部孕震环境.研究结果表明,雅江—九龙一带的低速区与泸定—宝兴高速区的速度结构特征表明了鲜水河断裂带南东段两侧壳内物质存在显著的横向介质差异,康定MS6.3地震发生在该高低速异常区的分界线上;结合康定MS6.3地震的1028个余震序列的精确定位结果可以看出,重新定位后的余震沿着鲜水河断裂带南东段呈条带状分布,且震源深度优势分布层位深度为8~15km,该余震序列的空间分布特征与鲜水河断裂南东段的深部介质条件密切相关.鲜水河断裂带南东段特有的视密度和视磁化强度异常分布特征反映了康定地区东西两侧块体的基底性质存在明显差异,康定—石棉及其以东地区所表现出的磁异常高和重力高的位场特征,反映该区域由强磁性、高密度物质组成,而康定MS6.3地震就发生在康定—石棉重力梯度变化带上、雅安—泸定磁性穹窿区的西边界线上.随着川青块体向南东方向滑移,受到四川盆地西缘边界刚性基底对川青块体的强烈阻挡,加剧了康定—石棉及其以东地区基底岩层的褶皱变形并产生了强烈的应力积累,所积累的应力突然释放导致了康定MS6.3地震的发生,这正是此次鲜水河断裂带南东段康定地区强震孕育和发生的深部构造环境和介质特征.根据本文对鲜水河断裂带南东段深部孕震环境的综合研究成果可知,石棉段处于重磁异常梯级带上且其北东侧表现出的高密度、强磁性和高波速等物性特征有利于区域应力的相对集中,因此,鲜水河断裂带南东段石棉地区的地震活动趋势和地震危险性背景值得进一步关注和研究.更多还原

【Abstract】 At 16∶55,November 22,2014,an MS6.3earthquake occurred in Kangding country,Garzê,a Tibetan autonomous prefecture in Sichuan Province.This event ended a 30-year interval without any large earthquake in the Xianshuihe fault zone and motivated scholars in China and elsewhere in the world to assess the seismic risk of this region.To study the deep seismogenic environment of the southeastern section of the Xianshuihe fault zone and the genesis of the Kangding MS6.3earthquake,we conducted a seismic tomography study.Using 99287P-wave travel time data of 7397 regional seismic events,recorded by a digital seismic network and by transportable seismic stations in Sichuan Province,the structure of the upper crustal threedimensional P-wave velocity was ascertained.Through field separation and inversion imaging of the gravity and aeromagnetic data of the Kangding seismic zone,the apparent density and apparent magnetization inversion results were obtained at different depths in the crust.The low velocity zone of the Yajiang-Jiulong area and the velocity structure characteristics of the LudingBaoxing high-speed zone indicate the existence of a transverse medium that is significantly different from the crustal material on either side of the southeastern section of the Xianshuihe fault zone.The Kangding MS6.3earthquake took place at the boundary of the high velocity anomaly zone and the low velocity anomaly zone.Accurate locating for the sequence of 1028 aftershocks of the Kangding MS6.3 earthquake reveals that these aftershocks show a zonal distribution along the southeastern Xianshuihe fault zone with focal depths 8~15km.Such spatial distribution characteristics of the aftershocks are closely related to the deep medium conditions of the southeastern segment of the Xianshuihe fault.The unique anomaly distribution of apparent density and apparent magnetization intensity reflect the existence of obvious differences in basal properties on both sides of the Kangding area.The high magnetic anomalies and high gravity potential field characteristics of the Kangding-Shimian′s eastern area indicate that this region is comprised of strong magnetism and high-density material.The Kangding earthquake occurred in the Kangding-Shimian gravity gradient belt,and its epicenter is located on the magnetic dome′s western boundary of the Ya′an-Luding area.When moving southeastward,the Sichuan-Qinghai block is strongly hampered by the rigid basement of the Sichuan basin on its western margin.Such a process has strengthened folding deformation and the accumulation of stress within the Kangding-Shimian area and its east.The sudden release of accumulated stress led to the occurrence of the Kangding MS6.3 earthquake.These are the deep seismogenic environment of the southeastern Xianshuihe fault zone.The Shimian segment of the southeastern Xianshuihe fault zone is also located in a gravitational and magnetic anomaly zone,with high density,strong magnetism,and high velocity media on its northeastern side.Therefore,this segment also deserves further risk assessment and investigation into its seismicity trend.更多还原