【摘要】 1654年7月21日甘肃省礼县8级地震发生在南北地震带的中北段,该地区的构造变形和构造活动与青藏高原向北东方向的扩展密切相关,复杂的构造几何特征主要受控于东昆仑断裂、西秦岭北缘断裂和一系列北东向断裂.礼县—罗家堡断裂为一条北东东向的左旋走滑活动断裂,错断了含有仰韶文化红色陶瓷片的一级阶地堆积物,阶地面上断层陡坎高约1.5m.沿断裂带发现冲沟的左旋位错量为3~10m,晚更新世黄土中残留的断层陡坎高4.5~8m.其中两条冲沟中发现裂点,高3.5~3.9m,距断层陡坎的距离为16m.礼县—盐关—罗家堡—天水镇一带发育大量的滑坡,长轴走向与礼县—罗家堡断裂一致,滑坡体后缘、侧壁陡峭,出露晚第四纪黄土,鲜有植被覆盖.礼县—罗家堡断裂为1654年8级地震的发震断裂.综合分析认为,受青藏高原向北东方向的扩展,被西秦岭北缘断裂、礼县—罗家堡断裂和岷县—宕昌断裂围限的礼县次级地块向南东滑动可能导致了1654年礼县8级地震的发生.断裂北、南两侧地壳电性结构的差异为强震的孕育提供了深部构造条件.更多还原

【Abstract】 We have studied the earthquake occurred in vicinity of Luojiabu,southwest of Tianshui city,Gansu Province,where the M =8 Lixian earthquake occurred on July 21,1654.In geotectonic setting,this earthquake happened in the middle-northern segment of South-North Seismic Belt,which generates numerous strong earthquakes and consequently makes this area one of the most seismically hazardous regions in China.However,causative structure and mechanism of Lixian earthquake is still in debate.Combined with active fault mapping,deep geophysicaldata,globe position system vector field and the distribution of landslides along the fault,we discuss that the mechanism of Lixian earthquake and try to estimate the seismically hazardous in this region.Fault descriptions are based on field observations aided by satellite image from Google earth collected along the fault zone.Our primary purpose is to document features bearing on the recency and kinematics of Quaternary movement along the fault.The observations are largely synthesized on a set of maps.The maps show active fault traces and are annotated to indicate the location of features indicative of the sense,style and amount of offset of geomorphic features.We also map the distribution of Quaternary deposits in the vicinity of fault.Combined with the electrical structure profile derived by 2D inversion of the MT data,we analyzed seismogenic structures of Lixian 1654 earthquake.On July 21,1654,the Lixian M=8earthquake occurred at the east part of Lixian County,Gansu Province,China,in the middle-northern segment of the South-North Seismic Belt.In this region the tectonic activities and deformation are closely related to the north-eastward growth of Tibetan Plateau.Their complicated structural geometries were controlled by the Eastern Kunlun fault,Northern Marginal fault of West-Qinling,and a series of north-east trending faults.The Lixian-Luojiabu fault is a northeast trending sinistral strike-slip active fault and is 150 km long consisted by three sub-segments aligned in right-step-echelon.The fault cuts the T1 terrace(lowest terrace in this area)deposits which contain the red ceramic fragments marking the Yangshao layer.On the terrace surface,the height of fault scarp is about 1.5 m.Along the Lixian-Luojiabu fault,the sinistral displacement of gullies about 3~10mand the height of fault scarps about 4.5~8 mare found in a number of locations.And the Knickpoints about 3.5~3.9mhigh are found 16 maway from the fault scarps.According to knickpoint recession rates at the same environment,these knickpoints may be formed as a 1654 earthquake in Lixian.A large number of landslides are distributed in a belt along Lixian-Yanguan-Luojiabu-Tianshui towns line,and its major axis direction is consistent with the trending of the Lixian-Luojiabu fault.Rear walls and side walls of the landslides are steep,with the outcrops of Late Quaternary loess and little vegetation cover.The Lixian-Luojiabu active fault is the seismogenic fault of the Lixian M=8earthquake in 1654.The south-eastward sliding of the Lixian Sub-block,which is surrounded by the Northern Marginal fault of West-Qinling,Lixian-Luojiabu fault,and the Minxian-Dangchang fault,led to the occurring of the Lixian M=8earthquake in 1654.Differences in electrical structure for the crust on the two sides of Lixian—Luojiabu fault reveals the structural condition for strong earthquake generation.Lixian-Luojiabu fault is a sinistral strike-slip active fault,which generated M=8Lixian earthquake on July 21,1654.This earthquake produced 7~10mhorizontal offset and with 3~4mfault scarps along the Lixian-Luojiabu fault.Tectonically,it took place on the middle-northern segment of the South-North Seismic Belt where a series of M >5had already occurred.Based on analysis of activity of other faults,GPS vector field and deep structural condition,the Lixian-Luojiabu fault activities and deformation are closely related to the northeastward growth of Tibetan Plateau.更多还原