Surface Rupture And Its Seismogenic Faults Of 1556 Huaxian M8? Earthquake

Weihe basin has experienced many episodes of tectonic activity during the past 65 million years.Being one of the active fault basins around the Ordos,there are a series of steep normal faults and their reverse faults in Weihe basin.These faults jointly control the sedimentation and geomorphology of asymmetric secondary basins dominated by basin-mountain boundary faults.With high frequent seismicity,its historical record of occasional great earthquakes amply demonstrates that it poses a major natural hazard to inhabitants of this region.For instance,an M8¹/₄ earthquake occurred at Huaxian County on 23th Jan.1556 in eastern Weihe basin,which caused serious casualties and economic losses.The surface faulting accompanying the earthquake has been reported that the range-bounding normal faults-Huashan front fault was responsible for the seismic slip.A debate on the distribution of surface rupture of the earthquake and the coseismic offsets were still on because a 40-kilometer-long seismic rupture zone along the Huashan front fault does not insufficiently explain the intensity of the earthquake.In this paper,we present and discuss the faulted micro geomorphology and geometry of faults at southern Gushi graben,where covered by thicken vegetation and rebuilt farmland using by high-resolution imagery interpretation,field investigation,drilling explore,trench,combined with the dating method of AMS ¹⁴C,OSL,and ¹⁰Be.For the purpose of determining slip distribution and rupture offsets in 1556 event.Now we have these results:Firstly,the coseismic surface rupture of the Huaxian earthquake was distributed along the south-southeast margin of Gushi graben,namely Weinan plateau front fault and Huashan front fault with trending E,and it was turning to NE at Huayin?Duyu?,along the Tongguan plateau-Zhongtiaoshan western margin fault.The total rupture length was found approximately 120 km,far more exceeding than the length of 40 km in the previous view.Secondly,the coseismic surface rupture was manifested as fault scarp according to the normal fault,and the profile of the scarps was involving collapse wedge.The maximum coseismic offset was at Xianyu of Huashan front fault with 9-m-height.Then it gradually decrease to the west,and the westmost end-point was at Linghe of Weinan plateau front fault with 0.1-m-height offsets,and the eastmost end-point was at Xiyaowen village of Tongguan plateau-zhongtiaoshan west margin fault with near 1-m-height offsets.Thirdly,the interseismic deformation in Weihe Basin and its eastern part-Gushi graben-takes places in a left-lateral shear zone of roughly east-west orientation of the North China block since Cenozoic.The faults at the basin and its boundary generally exhibit the kinematic characteristics of the left-lateral strike-slip component and are characterized by negative flower structure in middle-deep seismic profiles.The upper uplift of the upper mantle caused the lithosphere of the Weihe Basin to be in a tensile stress environment,and the extension of the upper crust formed a graben?basin?controlled by a series of normal faults—the Weihe Basin.Therefore,it is estimated that there may have a steep deep fault that cuts the crust and the Moho under the Gushi graben with the characteristics of left-lateral shear under the regional stress field.Then the Huaxian earthquake occurred in 1556 caused by the slip of the fault.In the deep steep strike and slip.The middle-upper crust of the Gushi fault depression is in the extension of NNW-SSE,and it accumulated thick Cenozoic sediment in the basin,which leads to the left-lateral shear slip displacement of the focal energy cannot propagate to the surface and been shown at seismic surface rupture zone.The extension of the basin makes the coseismic offsets in the southern margin faults?including Huashan front fault,the Weinan plateau front fault?and the western margin fault?including the west margin of Tongguan plateau-Zhongtiaoshan fault?of the Gushi graben manifest the characteristics of normal faulting near the surface as normal faulting surface rupture zone with vertical component only.