关键词: 地震地表破裂, 浅层地震探测, 地壳结构, 深浅构造关系, 唐山地震

Abstract: In this paper, field observation on earthquake surface rupture zone and high-resolution seismic reflection profiling of faults in combination with related geological data have led to some new insights about shallow Quaternary geology and underground fault structure in the 1976 Tangshan earthquake area. On this basis, we analyze prospecting results of deep structure and then study the relationship between deep and shallow structures in the 1976 Tangshan earthquake area, as well discuss tectonic setting and seismogenic model for this earthquake. Tanshan fault zone is a complex structure zone associated with faults and folds in this area. It is mainly developed on southeastern flank of an anticline and northwest flank of the Kaiping syncline, and consists of a series of NNE-NE-trending parallel faults. This fault zone can be divided into southern and northern segments and/or bifurcated into eastern and western branches, controlling seismic activity and geological motion in this area. The eastern branch of southern segment of this fault zone is Tangshan Fault, which strikes to N30°E and consists of two parallel faults. The eastern fault dips to WNW at an angle of 70°～80° and represents the main fault with reverse-strike-slipping. The 1976 Tangshan M 7.8 earthquake produced four surface rupture zones. The main surface rupture zone is ～8km long and generally strikes to N30°E. It consists of many secondary right-lateral en echelon ground fissure zones on both southern and northern segments, distributed along Tangshan Fault. This surface rupture zone underwent horizontal and vertical dislocations, with horizontal separation of 1.53m in maximum and vertical offset of 0.2～0.7m. The western side of this zone has been uplifted and the eastern side subsided. The high-resolution seismic reflection profiling revealed the following relations between Tangshan Fault and earthquake surface rupture zone:(1)their positions are consistent;(2)their geometric attitudes are consistent, dips to west at angle of 70°～80°;(3)their movements are of reverse-strike-slip faulting, while strong-reflection interfaces show a compressive bending and strike-slip motion characters. These facts indicate that there exists a high-angle reverse strike-slip fault, i.e. the NNE-trending and WNW-dipping Tangshan Fault, the sudden motion of which produced the 1976 Tangshan M 7.8 earthquake. The high-resolution seismic reflection profiling also revealed a SE-dipping reverse fault with a dip angle of about 70°. It occurs on the western steep limb of the Kaiping syncline east of the Tangshan Fault, and may be a shallow bending-slip or bending-moment fault, on which a ground fissure zone produced by the 1976 Tangshan earthquake. Analysis on deep seismic sounding result indicates that the relation between shallow and deep structures is close in spatial location, geometric structure and movement nature. There exist W-dipping slope of the Moho discontinuity, crustal "anticline", horizontal detachment and propagation fault in the middle crust, high-angle reverse-strike-slip fault and fold structure in the upper crust. They form a structural pattern of multi-order and multi-step composite fault-propagation fold, and controll the tectonic deformation and seismic activity in this area. The 1976 Tangshan earthquake is a result of stick-slipping along Tangshan Fault zone caused by the detachment and fault-propagation in the middle crust and by the stress concentration on the base of upper crust. The cumulative vertical offset has reached 15m along the Tangshan Fault since the late Pleistocene. The vertical offset of ground is 0.3m caused by the Tangshan earthquake and 0.5m obtained by inversion of seismic wave data. If creepslip is not taken into account, the recurrence interval of earthquakes on the Tangshan Fault is 2.4～4.0ka, which is comparable with the result of regional paleo-seismological research.

Key words: earthquake surface rupture, high-resolution seismic reflection profiling, crustal structure, relation between deep and shallow structures, Tangshan earthquake