Abstract: In order to understand the mechanism of the 1668 M_S8.5 earthquake occurred in Tancheng, it is important to probe the fine deep geological structure beneath the epicenter. A MT profile 20km south of the epicenter has been deployed. There are 17 sites along the profile, with a 3km average separation. Signals in Ex, Ey, Hx and Hy were measured in a cross manner, with x-axis orientated to the north. Record length for each site was at least 20h. The impedance and phase at sites in high cultural noisy environment were estimated by remote reference technique. As the Tanlu Fault Zone(TLFZ)is in NNE, nearly northerly, thus YX mode was considered as TM mode. Gauss-Newton inversion was done in 2-D mode with only the TM impedance and phase as input data. The electrical sections of 10km and 40km depth were respectively obtained after 8 iterations. The both initial models were created by Bostic approximation. The sections reveal the following features.
The TLFZ consists of five faults, from east to west numbered as F₀ to F₄. F₁ is the primary fault, steeply dipping west down to mantle, which has turned into a buried one overthrust by the east dipping Fault F₀. F₂ and F₃ dip east at 45 degrees, parallel to F₄, truncated by F₁ at depth. F₄ dips east in the shallow subsurface and gradually dips to west toward depth through the entire crust merging with F₁ to form a bigger one. These four faults constitute a flower-shaped structure, showing the nature of strike-slip of the TLFZ, associated with normal faulting in the late Yanshanian to early Himalayan. F₁ dips west, overthrust by east-dipping F₀, implying the compression from the westward subduction of the Pacific plate, thus present-day compression is superposed on the early tensile and strike-slip feature.
Based on MT data, it is inferred that the 1668 Tancheng M8.5 earthquake occurred at the junction of F₁ and F₃ about 15km deep. Thus it was likely resulted from westward compression of the Pacific plate, leading to thrust of the Sulu uplift along F₀, inducing activity of F₁ at depth, reactivated F₃, and adjusting the stress distribution in the region.

Key words: Tancheng earthquake, Tanlu fault zone, magnetotelluric, fine crust structure, cause of M_S8.5 earthquake

摘要： 了解1668年郯城8.5级大地震震区深部电性结构，对获得郯城大地震的发震原因有重要的作用。在距离该震中20km远的南侧位置，布置了1条大地电磁剖面。剖面长度约50km，由17个测点组成，点距平均3km。观测采用十字形布极，测量时间为20h。在对数据进行处理后，利用高斯-牛顿反演理论对TE+TM模式数据进行反演，获得了剖面地壳范围内的电性结构。分析认为：剖面域郯庐断裂带由5条断裂组成，自东向西依次编号为F₀到F₄；其中F₁为主控断层，W倾，高角度，穿透岩石圈，但该断层已经转化为隐伏断层，为E倾的断层F₀所推覆；F₂、F₃ E倾，倾角在45°左右，深部被F₁截断；F₄浅部E倾，但深部逐渐转为W倾，并断穿整个地壳，在莫霍面与F₁会合为1条深大断裂。F₂、F₃、F₄与F₁形成的整个断裂呈现花状构造特征，反映了郯庐断裂带的走滑性质，表明自燕山晚期—喜山早期郯庐断裂带发生了强烈拉张正断活动；而主控断层F₁ W倾，并为E倾F₀断层所推覆，预示扬子地块受太平洋板块W向俯冲挤压而俯冲到华北板块下，郯庐断裂带由原先的拉张走滑特征叠加了现今的挤压特征。结合本次研究成果，推测1668年8.5级地震发生在断裂F₁和F₃相交位置，震源深度15km。正是由于上述断层结构及构造应力场的变化和转换，特别是太平洋板块向W俯冲挤压，导致苏鲁隆起沿F₀逆冲，诱发深部F₁的活动，连带造成F₃活化形成1668年大地震，并调整应力分布状态的过程。

关键词: 郯城大地震, 郯庐断裂带, 大地电磁, 地壳精细结构, 地震原因