Study On The Borehole Fine Exploration Method Of The Buried Active Fault

Active faults developing in the city including buried active faults are called active faults buried directly beneath.Displacements along active faults buried directly beneath major cities create devastating earthquakes and surface dislocation that caused great damage to the city,seriously threaten to urban high-rise buildings,important engineering facilities,lifeline projects and people's lives and property safety.To detect active faults development within the city and areas nearby,know exactly fracture distribution and harmfulness of the activities,is extremely important foundational work of city development and construction of disaster prevention,and is of great significance to the social stability and economic sustainable development.Drilling detection is a common method to detect buried active faults,setting up drilling location scientifically and reasonably,with the use of drilling detection which is intenseve,we can determine the exact location of the fault and the depth of the upper breakpoint.This paper takes the concealed buried active faults in the Beijing plain area as the object,hole detection is applied to the Xiadian fault,Based on the formation of sediment grading curve to divide the stratum and the stratigraphic age to establish the equivalent time horizon,and eventually the paleoseismic events were analysised,and conclusions are as follows:1)On the basis of the comparison of the stratum from drilling joint profile and the stratum "face" by the stratigraphic age results,it is thought that the depth of sedimentary's bottom boundary of the fracture's hanging wall on late Pleistocene stratum is 57 m,In comprision,the depth of sedimentary's bottom boundary of the fracture's heading wall on late Pleistocene stratum is 28.5m and the bottom gap is about 28.5m.The average vertical slip rate of the fault is 0.24mm/a since late Pleistocene.2)The two sides of the fault's stratum's deposition rate were analyzed by the formation sedimentary age,the results showed that the sedimentary rate of the upper part of the rift can be divided into 7 stages,the deposition rate changed twice from fast to slow.The sedimentary rate of the footwall can be divided into 4 stages,the overall deposition rate is from slow to fast,it slowed down since Holocene.And on the whole,it shows a relatively uniform downward state.According to the analysis of the sedimentary rate of the two sets of faults,the average vertical slip rate at different stages of fracture is obtained which can be divided into 7 stages.Compared with the previous research,It is believed that Xiadian fault's slip rate greatly changed at about 100 ka,after that,the overall slip rate showed a decreasing trend,and the fault slip rate were different in timing and location.3)Through the analysis and identification of Xiadian fault's drilling undisturbed core paleoearthquake event,it is thought that the use of identification marks such as buried ground,colluvial wedge and soft sediment deformation,and the drill cores to study ancient earthquake is feasible.The 29 paleoearthquake events of Xiadian fault were identified since 120 ka,but it did not include Sanhe-Pinggu earthquake in 1679 whose magnitude was 8.With the M-D relation by Xiang Hongfa(1988)who used more than 30 dip slip of the seismic data in China,the 29 identified paleoearthquake events were estimated.The results showed that the maximum earthquake magnitude of Xiadian fault was 8.2 since 120 ka,and the earthquake's magnitude was more than 8,which occurred 2 times(including the M8 earthquake in 1679),21 times for the M7~M7.9 earthquake,and 6 times for the M6~M6.9 earthquake,and the average recurrence interval was 2.3ka since 30 ka.