2014年云南鲁甸MS6.5地震发震构造特征及动力源分析

Analysis of the Dynamic Source and Characteristics of the Seismogenic Structure of the Ludian,Yunnan MS6.5 Earthquake of 2014

摘要:

通过对2014年8月3日云南省昭通市鲁甸县发生的M_S6.5地震的震源机制解、余震空间分布、活动断裂组合样式和区域构造背景等特征的综合分析表明:(1)根据主震及4级以上强余震的震源机制解、余震空间分布、烈度长轴方向,判断本次地震的发震断裂为NW向的包谷垴-小河断裂;(2)根据地表GPS水平运动速率及水平缩短速率的差异性、断裂组合样式和历史余震深度,判断发震断裂具有薄皮-同向差异逆冲型捩断层的特征;(3)包谷垴-小河断裂活动可能主要受深部的"管道流"控制,"管道流"自NW向SE方向运动,在昭通断裂带处受到华南板块的差异阻挡,造成包谷垴-小河断裂西侧管道流运动速率大于东侧管道,从而驱动包谷垴-小河捩断层的左旋滑动,导致了鲁甸地震的发生。

Abstract:

According to the China Seismic Network, on August 3, 2014(Beijing time), an M_S6.5 earthquake occurred in Ludian County, Zhaotong City, Yunnan Province, China. A comprehensive analysis of the regional tectonic activity, aftershock distribution, focal mechanism solutions, and other features obtained the following results.(1) The focal mechanism solutions for the main shock and aftershocks that exceeded M_S4 indicate that the Ludian earthquake had two directions:NE and NW. Furthermore, the main direction of the spatial distribution of aftershocks was NW, which was also the direction of the long axis of the intensity distribution. All of this evidence indicates that the triggering seismic fault was the Baogunao-Xiaohe fault, which has a NW strike.(2) On the basis of GPS measurements of the horizontal movement rate of the earth's surface of the Ludian earthquake area, during the period 1999-2007 and the shortening rate of the Zhaotong fault for 1999-2013, we determined that the western block of the Baogunao-Xiaohe fault moved faster than the eastern block. This finding indicates that after colliding with the South China Block, the two fault blocks have differential thrust. The Zhaotong-Lianfeng fault consists of two thrust dextral strike-slip fault zones that strike NE:in this fault zone, the Baogunao-Xiaohe fault cuts the Zhaotong-Lianfeng fault. On the other hand, the Baogunao-Xiaohe fault has a NW strike direction, high inclination, sinistral strike-slip, and a short extensional length. From this evidence, it is apparent that this is a typical tear fault. In addition, the seismic depth of the Zhaotong-Lianfeng fault during 1980-2011 was about 0~20 km, indicating that the Baogunao-Xiaohe fault is at shallow depth. In general, the Baogunao-Xiaohe fault is a thin-skinned, constant-direction, differential-thrust type of tear fault.(3) The Zhaotong-Lianfeng fault is in accordance with the "conduit flow" theory in its dynamic source direction(NE), style of fault combination, and depth of the main shock(about 15 km). In addition, the basal slip of the Zhaotong fault is deeper than that of the Lianfeng fault, which is on the northwest side of the Zhaotong fault. Therefore, the movement of the Zhaotong-Lianfeng fault is closely related to conduit flow, and the Baogunao-Xiaohe fault is probably controlled by deep conduit flow. The northern section of the Zhaotong-Lianfeng fault has a stronger resistance than the southern section, and the high-speed conduit flow drags the brittle upper crust. The conduit flow moves from NW to SE. When the flow meets the Zhaotong-Lianfeng fault, which is the boundary between the Daliangshan secondary block and the relatively stable South China block, it is obstructed differently(the resistance to the north is stronger) by the South China Block. This causes the conduit flow west of the Baogunao-Xiaohe fault to move faster than the flow to the east of the fault. However, drag from the conduit flow can cause the block to move, because the speed of the conduit flow is far greater than that of the upper crust. Thus, the western block moves faster than the eastern block, which left-lateral slip on the Baogunao-Xiaohe tear fault in order to adjust for the different thrust in two blocks. This was the mechanism that caused the Ludian M_S6.5 earthquake.