四川长宁MS6.0地震震区上地壳速度结构特征与孕震环境

Upper crustal velocity and seismogenic environment of the Changning MS6.0 earthquake region in Sichuan,China

北京时间2019年6月17日22时55分,四川省宜宾市长宁县发生了MS6.0地震(28.34°N,104.90°E),四川盆地内部及边缘地带的深部孕震环境和潜在地震危险性再次引起了国内外地震专家和学者们的密切关注.为了揭示长宁MS6.0震区的深部介质结构特征和孕震环境,综合解译地震活动的构造背景和展布特征,本文充分收集川东南宜宾长宁地震震区及其周边范围内由四川省数字测震台网、宜宾市地方测震台网以及2016年以后宜宾长宁地区新增小孔径流动地震台阵等共计35套观测地震设备2013年1月—2019年7月记录到的17305次地震的P波到时资料的数据,应用双差地震层析成像方法,反演得到了长宁震区及周边上地壳三维P波速度结构特征,并结合此次震后科考组在震区获取的三维大地电磁阵列测深和重力密集测量等最新观测资料,综合分析讨论了长宁震区速度结构特征与地震活动关系、孕震环境及其地震危险性等科学问题.研究结果表明:长宁震区及周边上地壳P波速度结构呈现出明显的横向不均匀性,震区沉积盖层的物性特征分异明显,双河场背斜褶皱北西侧的波速结构与其东部存在明显的差异性且浅层P波速度结构分布特征与地表地质构造和地层岩性密切相关.重新定位后的长宁MS6.0地震序列空间分布特征与震区上地壳介质速度结构存在密切关系,序列大体上沿着高低速异常分界线呈NW-SE向展布,并终止于白象岩—狮子滩背斜构造东段附近,长宁震区及周边介质速度结构的非均匀变化是控制主震及其序列空间展布的深部构造因素.三维P波速度结构还表明了长宁MS6.0震区双河场褶皱附近存在不一样的深浅构造背景,震区褶皱构造伴生断裂的复杂性可能破坏了盖层地层成层性,造成了介质物性界面的变化多样,从而导致深浅构造耦合存在明显的差异.长宁MS6.0地震震中位于速度结构发生变化的边界带附近,这种介质物性变化的边界带可能是中强地震孕育和发生的有利部位.长宁MS6.0地震及其序列绝大部分发生在基底滑脱带之上,由于受到区域NE-SW向主压应力和经华蓥山构造带传递而来的NW-SE向的现今应力场的共同作用,导致了此次长宁6.0级地震的发生,而随后发生的珙县MS5.1、长宁MS5.3、珙县MS5.4和MS5.6地震以及大量中小地震事件均为长宁6.0级地震触发作用所致.P波速度结构还揭示了震区双河场褶皱以及该褶皱构造地表出露伴生的大地湾断层和NW向大佛崖断层两侧浅层速度结构特征各异,结合长宁—双河背斜与轴线方向一致的NW向伴生断裂构造比较发育,而褶皱东侧的伴生断裂走向表现出多样性和复杂性,由此推断除了受区域性构造运动的影响之外,长宁震区局部构造的差异性活动也较为突出,长宁—双河背斜构造区轴部构造及其伴生的断裂具备一定的发震能力和深部孕震背景,这可能也是长宁地震余震强度较大、活动持续时间较长的主要原因,川东南地区地震活动趋势和潜在地震危险性仍值得进一步关注.

22∶55 on June 17,2019,a magnitude 6.0 earthquake(28.34°N,104.90°E)occurred in Changning County,Yibin City,Sichuan Province,China.The deep seismogenic environment and potential earthquake risk in the inner and marginal areas of Sichuan Basin have once again attracted the close attention of seismologists and scholars at home and abroad.In order to reveal the characteristics of the deep medium structure and the seismogenic environment of the Changning MS6.0 earthquake area,and to interpret the tectonic background and genesis of the earthquake activities comprehensively,In this paper,we fully collected a total of 35 sets of observation seismic equipment which deployed in the Changning earthquake area in Southeast Sichuan and its surrounding areas,including Sichuan digital seismological network,Yibin local seismological network and new mobile seismological stations in Yibin after 2016,P-wave arrival data of 17305 earthquakes recorded from January 2013 to July 2019.Double-difference seismic tomography method was used to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Changning earthquake area,combined with the three-dimensional magnetotelluric array sounding and gravity intensive measurements obtained in the earthquake region by the post-earthquake scientific research,we comprehensively analyzed and discussed the scientific issues such as the relationship between the velocity structure characteristics and seismic activity in the Changning earthquake area,the deep tectonic environment and the seismic risk.The results show that the P-wave velocity structure of the upper crust around the Changning earthquake zone shows obvious lateral inhomogeneity,the physical properties of the sedimentary layer in the earthquake zone are obviously different,and there is a high-velocity anomaly distribution in the sedimentary layer where the Changning anticline is located,There is a clear difference between the velocity structure of the periphery around Shuanghe anticline and its east.The spatial distribution characteristics of the relocated Changning MS6.0 earthquake sequence are closely related to the velocity structure of the upper crust.The sequence generally spread in the NW-SE direction along the boundary of high-low velocity anomaly,and terminate near the east section of the Baixiangyan-Shizitan anticline,The inhomogeneous variation of the velocity structure of the Changning earthquake area and its surrounding medium is the deep structural factor controlling the spatial distribution of the mainshock and its sequence.The three-dimensional P-wave velocity structure also shows that there are different deep-shallow structural backgrounds near the Shuanghe folds in Changning MS6.0 earthquake.The different activities of local structures in the earthquake region not only cause the variety of fold structure axial and associated fault strike,but also cause the obvious differences between coupling relationship of deep and shallow structures and physical characteristics of sedimentary cover in the earthquake area.The epicenter of the Changning MS6.0 earthquake was located near the boundary zone where the velocity structure changes.This boundary zone of physical property change may be a favorable location for earthquake preparation and occurrence of moderate-strong earthquakes.The Changning MS6.0 earthquake and most of its sequence occurred above the basement detachment zone,due to the joint action of the regional NE-SW principal compressive stress and the NW-SE present stress field transmitted through the Huayingshan structural belt,the following earthquakes with magnitude 5.1 in Gongxian,magnitude 5.3 in Changning,magnitude 5.4 and magnitude 5.6 in Gongxian,as well as a large number of small and medium-sized earthquake events,were triggered by the Changning MS6.0 earthquake.The P-wave velocity structure also reveals that there are different shallow velocity structures on both sides of Shuanghe fold,Dadiwan fault and Dafuya fault in NW direction.Combined with the ground geological survey results,such as the relatively developed NW fault structure associated with the axis of Changning-Shuanghe anticline,and the strike of associated faults of the east fold structure also shows diversity and complexity.The results show that in addition to the influence of regional tectonic movement,the differential activities of local structures in the Changning earthquake area are also prominent.The axial structures and associated faults in the Changning-Shuanghe anticline structural area have certain earthquake generating capacity and deep seismogenic background,this may also be the main reason for the large aftershock intensity and long duration of Changning earthquake.The seismicity trend and potential seismic risk in southeast Sichuan still deserve further attention.