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

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