以精定位背景地震活动与震源机制解研究郯庐断裂带中南段现今活动习性

郯庐断裂带是中国东部板内一条规模最大的强构造变形带与地震活动断裂带，其断裂结构与历史地震活动性具明显的分段活动性。文中通过沿郯庐断裂带中南段的历史地震活动性、精定位背景地震活动性与震源机制解分析，讨论了断裂带的深部几何结构与现今活动习性。现今地震活动在中段主要沿1668年郯城MS 8地震破裂带线性分布，线性条带在泗洪-诸城间延伸约340 km长,为1668年地震长期缓慢衰减的余震序列活动。大震地表破裂遗迹与精定位地震分布都揭示出郯庐断裂带中段的两条全新世活动断裂昌邑-大店断裂与安丘-莒县断裂以右阶斜列的形式共同参与了1668年郯城MS 8地震破裂。精定位震源剖面刻画出两条断裂结构面呈高角度相背而倾，其中昌邑-大店断裂倾向SE，安丘-莒县断裂倾向NW，两条断裂在深部没有合并汇聚。余震活动所代表的1668年地震震源破裂带是郯庐断裂带中现今尚未闭锁的安全段落，对应于高b值段。而未发生破裂的安丘以北段，小震活动不活跃，b值低，现今可能已成为应力积累的闭锁段。震源机制解揭示的断裂应力状态在中段以NE向主压应力为主，表现为右旋走滑活动性质，且存在少量正断分量，南段转为以NEE至近EW向为主，存在少量的逆冲分量。在中段与南段的转折处宿迁-嘉山段，主压应力方向垂直断裂带走向呈NWW向，反映出局部以挤压为主的应力特征，其中泗洪-嘉山段也是历史地震未破裂段，现今小震活动不活跃，因此该段可能更易于应力积累。精定位小震活动在郯庐断裂与北西向断裂相交汇处聚集，反映出北西向断裂的新活动性，以及郯庐断裂带现今的逆冲作用。在断裂带南端，精定位背景地震活动沿与其相交汇的襄樊-广济断裂带东段呈北西向线性分布，表明了该段的现今活动性。

The present activity of the central and southern segments of the Tancheng-Lujiang fault zone evidenced from relocated microseismicity and focal mechanisms

The TanchengLujiang Fault Zone (TLFZ) is the largest intraplate active tectonic deformation and strong historical seismicity zone in eastern China. It shows the segmentation feature in fault geometry and historical seismicity. We investigated the present activity of the central and southern segments of the TLFZ by analyzing the historical seismicity, relocated background seismicity and focal mechanism solutions. The background seismicity, resulting from the lingering aftershocks of the 1668 Tancheng earthquake sequence, aligns along the surface rupture zone of the 1668 Tancheng MS 8 earthquake extending roughly 340 km long from Sihong to Zhucheng. Both the earthquake rupture relic belt and the relocated background seismicity reveal that, the two Holocene active faults in the central segment, namely the ChangyiDadian fault and the Anqiu Juxian fault, involved in the rupture process during the 1668 Tancheng earthquake in the pattern of en echelon. The cross sectional profiles of the hypocenters delineate that the planes of the ChangyiDadian fault and AnqiuJuxian fault dipped to the southeast and northwest, respectively, with high dipping angles thus avoided merging in lower crust. The microseismicity belt represents the seismic rupture zone of the 1668 Tancheng earthquake, has high b values indicating this segment is not locked at present; in contrast, the north segment to Anqiu was not involved in the rupture and has low b values and low microseismicity rate, thus this segment may be locked. The focal mechanism solutions indicate that the maximum stress orientation in the central segment of the TLFZ is in the direction of NE; and the nodal plane of the mean solution with the same direction as the fault strike shows right lateral strike slip with some normal components. While in the southern segment, the maximum stress orientation turns to NEE near EW and shows right lateral strike slip with some thrust components. In the transition part of the central and southern segments from Suqian to Jiashan, as the focal mechanism solutions show, the maximum stress orientation is in the direction of NWW, which reflects the local stress state to be dominated by thrusting. Among these segments, the SihongJiashan segment experienced few historical earthquake and has a low microseismicity rate and is therefore more likely prone to accumulate stress at present. The relocated microseismicity concentrates at the intersections of the TLFZ and the northwest trending faults, illustrating the present activity of the NW trending faults and the thrust faulting on the TLFZ. Likewise at the southern end of the TLFZ, the relocated microseismicity aligns along the eastern segment of the NWW trending XiangfanGuangji fault, demonstrating the present activity of this segment.