关键词: 郯庐断裂带, 背景地震活动性, 地震重新定位, 震源机制, 断裂现今活动习性

Abstract: 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.

Key words: TanchengLujiang fault zone, background seismicity, earthquake relocation, focal mechanism, present activity of fault