Abstract: In 1631, an earthquake of M_S6 3/4 occurred in the Taiyangshan uplift about 10km north of Changde City, Hunan Province, which is the largest destructive temblor documented in history of South China. With the economic and social development of Changde City and the expansion of the urban, it is necessary to conduct assessment of seismic hazard, including probing the deep structure beneath the region around this historical event. To this end, three magnetotelluric(MT) profiles have been carried out across the Taiyangshan area with 76 sites in 2014. Remote reference, "robust", and phase tensor decomposition techniques were used to process the acquired MT data, and the NLCG two-dimensional inversion was made to image the deep electrical structure in combination with relevant geological and geophysical data available. The images of 3 MT profiles permit to delineate the deep extension of major faults and the deep structural features of the tectonic units in the study area. The largest fault, the Xiaowupu fault shows a steep southwest-dipping with extension of tens of kilometers from the surface to the subsurface. The Shichaipo Fault presents a low-resistivity body around a depth of about 5km. The Huanxian and Dongting Lake Basins show a low-resistivity characteristic from the ground to a depth more than 10km, good-electricity layering, meaning tectonic stability, and corresponding to extensive Cretaceous and Cenozoic strata. The electrical structure of Taiyangshan uplift overall presents a high-resistivity characteristic from the surface to a depth of about 20km, which is the widest in the central Taiyang Mountains. The deep electrical structure of 3 profiles together reveal that the contact between the Dongting Lake Basin and Taiyang Mountains is obviously segmented in NS direction. It is inferred that the Xiaowupu fault is probably the causative feature of the 1631 Changde M_S6 3/4 earthquake. The deep electrical structure nearby the epicenter appears to be complex with alternating high and low resistivity, and the epicenter is located in the high resistivity zone. The low-resistance decoupling in proximity of the fault is likely responsible for the earthquake generation. The Taiyangshan uplift resides in the southwest corner of the Jianghan-Dongtinghu Basin, where differential up and down activity during Quaternary was most intense resulting in big landform contrast, forming the tectonic setting of medium-sized earthquakes in this region.

Key words: Magnetotelluric, 1631 Changde earthquake, Taiyangshan Mountains, Xiaowupu fault

摘要： 1631年在湖南省常德市以北约10km的太阳山凸起区发生的6 3/4级地震是华南地块内部影响范围最大的1次破坏性地震；随着常德市经济社会发展和城区规模的扩大，需要进一步探查该次历史地震区的深部孕震环境和发震构造。2014年跨过太阳山凸起沿3条剖面开展了76个测点的大地电磁观测，资料处理采用了远参考道、"Robust"、阻抗张量分解等技术。依据3条剖面的二维反演结果，结合其他地质与地球物理资料对深部电性结构特征等进行了解译。3条剖面的结果揭示了研究区内主要断层的纵向延展状况和各构造单元的深部结构特征：研究区规模最大的肖伍铺断层表现为明显陡立略向SE倾斜的电性变化带，发育深度可达十几km；拾柴坡断层为向W倾斜的低阻带，延展深度约5km；澧县凹陷、洞庭湖沉降区自浅部至十几km深度具有稳定成层结构，电阻率值较低，对应于该区广泛发育的白垩系和新生界，而两者之间的太阳山凸起自浅表至20km深度均表现为高阻特征，高阻体范围在太阳山凸起的中段横向分布最宽，太阳山凸起和洞庭湖沉降区接触边界自南向北具有明显的分段性。通过大地电磁探测结果推断肖伍铺断层为1631年常德6 3/4级地震发震断层，震源区附近表现出高、低阻电性过渡带的特征，震源位于高、低阻介质接触区且偏向于高阻体内，断层附近低阻解耦作用的存在是该地区孕育地震的深部条件。太阳山凸起位于江汉-洞庭湖断陷盆地的西南隅，是江汉-洞庭湖盆地第四纪断块差异升降活动最剧烈和构造地貌反差最强烈的地区，这是该区发生中强地震的外在动力。

关键词: 大地电磁, 1631年常德地震, 太阳山凸起, 肖伍铺断层