近震体波走时与重力联合反演华北地区地壳上地幔顶部P波速度结构  

Crust and uppermost mantle P-wave velocity structure of the North China:Constraints from joint inversion of local earthquake body-wave traveltimes and gravity anomalies

作　　者：李鑫月 石磊[1,2] 李永华 LI XinYue;SHI Lei;LI YongHua(Institute of Geophysics,China Earthquake Adminstration,Beijing 100081,China;Key Laboratory of Earthquake Source Physics,China EarthquakeAdministration,Beijing 100081,China)

机构地区：[1]中国地震局地球物理研究所,北京100081 [2]中国地震局震源物理重点实验室,北京100081

出　　处：《地球物理学报》2024年第4期1439-1453,共15页Chinese Journal of Geophysics

基　　金：国家自然科学基金联合项目(U1839210);中国地震局地球物理研究所基本科研业务费专项(DQJB20R35,DQJB22X16);国家自然科学基金项目(42274118,42274080)联合资助

摘　　要：华北是克拉通破坏的典型案例,也是陆内强震活动最为强烈、频繁的地区之一,对该区壳幔速度结构进行研究,可为深化认识克拉通的形成演化、板内强震与火山发生机理等研究提供重要的深部约束.利用202个固定台站、360个流动台站所记录到的天然地震事件P波资料和实测布格重力异常,采用近震体波走时与重力联合反演方法获得了地壳、上地幔顶部P波速度结构.重力资料的引入不仅对壳幔结构进行了更合理的约束,而且有效提高了浅层和边缘射线分布稀疏区的成像能力.联合成像结果显示,鄂尔多斯地块和华北平原浅层表现为明显的低速异常,可能与其存在厚的松软沉积层有关;鄂尔多斯东部上地壳存在明显的P波高速异常,推测该高速异常与古元古代孔兹岩带的形成有关.大同火山区15~25 km和25~40 km表现为明显的低速异常,且这两个低速层是连通的,推测与地幔热物质上涌导致的多期次岩浆侵位有关.三河—平谷8.0级地震和邢台7.2级地震震源都位于脆性的上地壳,均下伏低速异常,推测在区域动力环境加载作用下,韧性的壳内蠕变导致脆性上地壳岩石中弹性应变能的局部积累,从而引起地震的发生.North China is a typical case of studying the destruction of craton, and it is also one of the regions with the most intense and frequent strong continental earthquake activity. Studying the crust and mantle velocity structure in this area can provide important deep constraints on the understanding of the formation and evolution of cratons and the mechanism of occurrence strong intraplate earthquakes and volcanoes. Using P-wave traveltime data recorded by 202 permanent seismic stations and 360 temporary stations employed in the North China and Bouguer gravity anomalies, we obtain the crust and uppermost mantle velocity structure of the North China by joint inversion method. The introduction of gravity data can not only restrict the crust and mantle structure more reasonably, but also effectively improve the imaging ability of shallow and marginal regions with sparse ray distribution. The results show that the shallow layers of the Ordos block and the North China Plain show obvious low-velocity anomalies, which may be related to thick and soft sedimentary layers. An obvious P-wave high-velocity anomaly in the upper crust of eastern Ordos is observed, which is speculated to be correlated with the Paleoproterozoic khondalite belt. Below Datong volcano, there are obvious low-velocity anomalies at depth range of 15~25 km and 25~40 km, which are connected and presumed to be related to the multi-stage magmatic emplacement caused by the upwelling of mantle hot material. The Sanhe-Pinggu 8.0 earthquake and the Xingtai M 7.2 earthquake are both occurred in the brittle upper crust with underlying low-velocity anomalies. Under the regional dynamic loading effect, the ductile intra-crust creep will lead to the local accumulation of elastic strain energy in brittle upper crustal rocks, which will cause earthquakes.

关 键 词：华北地区 重力异常 近震走时 联合成像 P波速度 

分 类 号：P315[天文地球—地震学]