川滇交界东段昭通、莲峰断裂带的深部结构特征与2014年鲁甸M_S6.5地震

2014年鲁甸M_S6.5地震位于川滇菱形块体向东突出的过渡变形区大凉山次级块体南东缘的昭通、莲峰断裂带内部,属于青藏高原东南缘南北地震带的中南段,近十多年来,该断裂带及其周边中强地震的发生频次明显增多,昭通、莲峰断裂带是否具备孕育和发生强震的深部构造背景成为一个亟待研究的问题.为了研究昭通、莲峰断裂带的深部结构特征及孕震背景,探求2014年鲁甸M_S6.5地震的成因的深部动力机制,本文充分收集了四川、云南等区域数字地震台网和"中国地震科学台阵探测-南北地震带南段"("喜马拉雅"项目Ⅰ期)流动地震台阵的观测数据,应用区域震和远震联合反演的方法得到川滇地区三维速度结构图像,在此基础上重点剖析和研究了昭通、莲峰断裂带P波速度结构;再对昭通、莲峰断裂带及周边区域的重力、航磁数据进行三维视密度和视磁化强度反演,得到了壳内不同深度层视密度的横向变化特征和反映壳内磁性物质的分布范围以及结晶基底的视磁化强度异常分布情况,综合分析研究昭通、莲峰断裂带的深部结构特征及孕震动力环境.研究结果表明:川滇交界东部昭通、莲峰断裂带及其周边地区上地壳物质存在显著的横向介质差异,中下地壳深度范围大凉山次级块体西南缘存在低速异常分布,并呈现出近SN向的展布特征,2014年鲁甸M_S6.5地震位于该高低速异常的分界线附近略偏向高速体一侧.P波速度结构还揭示了鲁甸M_S6.5主震震源体下方中下地壳存在大范围低速异常分布,P波速度异常扰动与重磁异常的展布特征、梯度变化在深度和分区特征上均具有较好的联系和可比性,结合昭通、莲峰断裂带中下地壳范围内存在大范围的低密度弱磁性异常分布,综合表明了该区中下地壳物质相对较为软弱,这种特有的深部物性结构特征有利于应力在脆性的上地壳内积累和集中.研究结果还揭示了共轭断裂的深部构造形态,高低航磁异常边界与NW向的苞谷脑—小河断裂的深部展布形态相一致,苞谷脑—小河断裂处于航磁异常突变带附近,昭通断裂北段(昭通—鲁甸段)位于上地壳强磁性、高波速异常区内且具有深大断裂的深部地球物理场响应特征,因此该断裂段(昭通—鲁甸段)具备发生7级及以上强震的深部构造背景.当大凉山次级块体内部的中下地壳低速管流层自NW向SE方向运动到昭通、莲峰断裂带附近时,受到华南块体的强烈阻挡,应力在昭通、莲峰断裂附近基底性质存在差异处集中,脆性上地壳中低强度区域在横向挤压的构造应力场作用下易于破裂从而引发强震,这也正是昭通、莲峰断裂带内部鲁甸M_S6.5地震孕育和发生的深部构造环境.

Deep structure of the Zhaotong and Lianfeng fault zones in the eastern segment of the Sichuan-Yunnan border and the 2014 Ludian MS6.5 earthquake

The 2014 Ludian M_S6.5 earthquake occurred in the Zhaotong and Lianfeng fault zones on the southeastern margin of the Daliangshan secondary block in the eastward-protruding transitional deformation zone of the Chuandian diamond block, part of the north-south seismic belt in the southeastern margin of the Tibetan plateau. Over the past ten years, moderate-sized earthquakes on this fault zone and its surrounding areas have increased notably. Whether the deep structure of this fault zone can generate major earthquakes is an urgent issue to be addressed. This work attempted to answer this question and examines the deep dynamic mechanism of the 2014 Ludian M_S6.5 earthquake. We collected data from seismic stations of the regional digital seismic networks of Yunnan and Sichuan and mobile seismic arrays of Chinese seismic array detection-the southern section of North-South seismic belt ("Himalaya" project phase 1). Then we imaged the three-dimensional velocity structure beneath the Sichuan-Yunnan area using joint inversion of regional and teleseismic data. On this basis, the P-wave velocity structure of the Lianfeng and Zhaotong fault zones was studied exhaustively. And then we performed inversion gravity and aeromagnetic data to obtain apparent density, apparent magnetization intensity in the study area, which permits to reveal the deep dynamic environment of earthquakes. The results show that there are significant lateral medium differences in the upper crust of beneath the Zhaotong and Lianfeng fault zones and adjacent areas. The low velocity anomalies are present in the middle-lower crust below the southwestern edge of the Daliang Shan secondary block, showing a nearly NS trend. The 2014 M_S6.5 earthquake is located near the boundary between the high and low velocity anomalies and slightly tend toward the side of the high-velocity body. The P-wave velocity structure also reveals a large-scale low velocity anomaly distribution in the middle-lower crust below the M_S6.5 mainshock source. The distribution of P-wave velocity perturbation and gravitational and magnetic anomalies have better connection and comparability in both the depth and the partition. In combination with the distribution of low-density and weak magnetic anomalies in the middle-lower crust of the Zhaotong-Lianfeng fault zone, it is inferred that the middle-lower crust in this area is relatively weak. Such a deep structure is easy to accumulate stress in the brittle upper crust. The results also reveal deep structure of conjugate faults, and the boundary between high and low aeromagnetic magnetic anomalies is consistent with the NW-trending Baogunao-Xiaohe fault. The Zhaotong-Ludian section of the Zhaotong fault zone is located in the strong magnetic and high-velocity anomaly zone of the upper crust and has response to deep geophysical fields, so the fault segment (Zhaotong-Ludian section) has deep structural background of major earthquakes. Research suggests that the low-velocity channel flow in the middle-lower crust of the Daliang Shan block moves from NW to SE, and is strongly blocked by the South China block in the vicinity of the Lianfeng and Zhaotong fault zones, thus resulting in stress accumulation near the basement of the Zhaotong and Lianfeng faults where there are significant differences of properties. Consequently the low-strength regions of the brittle upper crust tend to rupture under the lateral compressive tectonic stress field leading to strong earthquakes. This is just the deep tectonic setting of the 2014 Ludian M_S6.5 earthquake in the Zhaotong and Lianfeng fault zones.