关键词: 正断层, 断层位移, 演化, 断层分段

Abstract: When normal faults overlap each other in extentional direction the overlapping faults and the overlapping area will compose a fault overlap zone. Fault overlap zones with a separation more than several kilometers are usually considered as segment boundaries in studying earthquake rupture segmtntation. The Jiugongkou fault overlap zone in Weixian, Hebei, however, presents a typical opposite example which can not be regarded as a segment boundary. The Jiugongkou fault overlap zone is located on the southern marginal fault zone of the Guangling Weixian Basin, with 7km of an overlapping length and 2.2～5.7km of an overlapping separation. The latest Pleistocene alluvial fan developed within the overlapping area. Geological and geomorphological investigations provide evidences for the fault growth: abandonment of the latest Pleistocene alluvial fan, offset of the fan gravels, doposition of the fault scarp derived colluvial deposits, instrusion of docite magma along the fault, occurrence of the antithetic faults, and fracture clevages developed in loesses. Throw analysis indicates that no displacement deficit has happened within the overlap zone, but interaction and displacement transfer have occurred between the two overlapping faults. They manifeste as asymmetric throw profiles for both the main overlapping fautls, symmetry of a composite throw distribution for all the fault traces characterized by a single fault, and a significant increasment of the throw within the overlap zone. Moreover, dating of the faulted materials shows that the fault growth and the displacement transfer occurred in a same time period, starting between (27400±2200) and (26000±2100)a B.P. and continuing to after (18700±1500)～(1300±1000)a B.P. For the overlap zone the mechanisms of displacement transfer may be associated with magmatic activity and tectonic tilting. The overlap zone experienced evolutionary stages offset and overlap, however, has not been breached or linked yet. The Jiugongkou fault overlap zone has evoluted and developed into a geometric and kinematic coherent unit, characterized by a single fault, in terms of fault growth, interaction and displacement between overlapping faults. In this way, a fault overlap zone on fault zones often occurs only as a temporary feature. These results can be of great importance to seismic hazards assessment, especially magnitudes of potential earthquakes, as well as earthquake probability predicition.

Key words: Normal fault, Fault displacement, Evolution, Fault segmentation