| The Tanlu fault zone is the largest strike-slip fault system in eastern China,with a total length of over 2000 km.Since the Late Pleistocene,the activity of this fault system in Shandong and Jiangsu provinces has been mainly along the Anqiu-Juxian fault(AJF).The Anqiu Ms 7.0 earthquake in 70 BC and the Tancheng Ms 8 1/2 earthquake in 1668 AD occurred respectively in the Anqiu-Juxian fault segment(AJS)and Juxian-Tancheng fault segment(JTS)of the AJF.The JTS,as the seismic fault of Tancheng Ms 81/2 earthquake in 1668 AD,has been widely concerned about its activity in the Late Quaternary.Existing research shows that the activity of this segment follows the characteristic earthquake model,with a recurrence interval of 3000-3500a,a magnitude of about 8.5,and a co-seismic displacement of about 9m.In contrast,the AJS has not been well studied.At present,only the geometric distribution of the exposed segments has been determined,while the trace of the concealed parts is not clear.Although this segment is known to be an active Holocene fault,the surface rupture of the 70 BC Anqiu earthquake is not clearly understood.In this paper,the geometric distribution and earthquake rupture characteristics of the AJS were studied by means of micro offset-geomorphology analysis,geophysical exploration,joint drilling geologic section and trench exploration,and the following new understandings were obtained.(1)According to the results of the geological survey in the field,geophysical exploration and drilling,the geometric distribution of the AJS has been preliminarily identified.The segment starts from Juxian in the south,passes through the Anqiu City,and reaches the Changyi City in the north.The whole segment can be divided into four subsegments,among which the Changyi-Nanliu segment forms the left-step fault with the others,while the Anqiu-Mengtong segment,Qingfengling segment and Mengyan segment form right-stepping faults.The overall strike of the south three secondary faults is NNE,and each segment is separated by a stepover,whose width are both small,about 1km.The stepover between the Changyi-Nanliu segment and the Anqiu-Mengtong segment is bigger,about 5 km.(2)Based on the high-resolution orthophoto image and DEM data obtained by UAV aerial survey,87 displacement values were obtained.Statistical analysis shows that the cumulative displacement along the fault mainly concentrates in four intervals,with the peak value of 5.1,10.3,15.8 and 20.3 m respectively,all of which are multiples of 5 m.The results show that the AJS may also obey characteristic earthquake recurrence model,and the characteristic displacement is?5.0m,which is about half of that of the JTS,and the latest co-seismic displacement is?5.1m.(3)According to historical records,when the Anqiu earthquake occurred,the regional intensity about 960 km away from the epicenter was about ?-?.By comparing the intensity distribution of previous earthquakes with a magnitude greater than 7 in eastern China,the results show that the intensity distribution of the Anqiu earthquake is much greater than that of Ms 7 earthquake.Based on the empirical formula of the coseismic displacement and magnitude,and the coseismic displacement and surface rupture length,the coseismic displacement of 5m of the strike-slip fault is corresponding to a seismic event of about M7.5.Therefore,it is considered that the magnitude of the historical earthquake is more likely to be about 71/2 which may be the cascading rupture of the Anqiu-Mengtong,Qingfengling and Mengyan segments.(4)Compared with the JTS,the AJS shows different characteristics of seismic rupture.Besides,the width of the stepover between these two faults is?8km,which can prevent the rupture propagation of most strike-slip fault earthquakes theoretically.And the Tancheng Ms 8.5 earthquake in 1668 did not break through this stepover.Therefore,it is considered that the Juxian stepover results in the permanent rupture segmentation of these two fault segments. |
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