Paleo-earthquake Study Of The Bedrock Normal Fault Of The Shanxi Rift

The Shanxi rift is located between the Ordos block and the North China plain,which consists of a series of the extensional graben and half-graben basins.It is not only anascent continental rift system,but also a strong seismic zone.The predecessors have carried out a lot of researches about the geometric distribution,segmentationcharacteristics,activity habits and seismic history of the faults in the Shanxi rift,which accumulated a wealth of fault activity and paleo-earthquake data.However,most of these previous studies only focused on faults in Quaternary strata coverage areas,and rarelyinvolved faults in bedrock areas.Currently,it is a significant problem that the lack ofresearch about the paleo-earthquakes history of the faults spreading in the bedrock area at the boundary of basin of the rift.The main reason for the above problems is the lack of Quaternary sediments in the bedrock area.Traditional evaluation methods and techniques are difficult to play a role in the bedrock area in the Shanxi rift,nor can they obtain aquantitative index of fault activity in the bedrock area.This is very likely to cause an insufficient understanding of the activity of faults in the Shanxi rift and incompleterecovery of paleo-earthquake history.Therefore,on the basis of existing researches,it is necessary to focus on the problem that the traditional method cannot have an effect on the fault in the bedrock area of the Shanxi rift.It is also urgent to build a key technicalsystem of"three-dimensional morphology of fault surfaces and the cosmogenic nuclide dating"to solve the core scientific problem of fault activity evaluation in the bedrock area of the Shanxi rift.The Yuguang basin southern marginal fault,the Jiaocheng fault and the Luoyunshan piedmont fault are located in the north,middle and south of the Shanxi rift,respectively,covering the Shanxi rift in space.All of these three faults have developed a large number of basin-mountain boundary bedrock faults and abundant bedrock fault scarps exposed in the bedrock area,which are ideal objects to carry out the paleo-earthquake study onbedrock fault surfaces.Therefore,in this paper,we selected these three faults as the target faults to carry out the paleo-earthquake research work on the bedrock fault surface.The study points of the bedrock fault surface were selected in the typical sections of the three target faults.The terrestrial Li DAR scanner was used to obtain high-precisionmorphology data of the bedrock fault surface.Based on the fractal analysis method,the weathering bands were divided to identify paleo-earthquake events and obtain co-seismic displacements.In the meantime,rock samples of cosmogenic nuclide ³⁶Cl exposuredating were collected to determine the age of paleo-earthquake events.Based on the research results above,the quantitative parameters such as the coseismic displacement,magnitude,slip rate,and latest activity time of the target fault were calculated andobtained.The main results obtained are as follows:1.The Luoyuncun segment of the northern Luoyunshan piedmont fault is an active fault since the late Pleistocene.The study of the bedrock fault surface revealed 2 paleo-seismic events occurred at 81.8-27.8ka and 34-10ka respectively,with the co-seismic dip slips of 2.0-2.9m and 2.8-3.1m,and the estimated magnitude of Ms7.5 to Ms7.7.The dip slip rate in this fault segment is 0.06-0.22mm/a.The Weicun segment is a Holoceneactive fault.The co-seismic dip slips of the latest two earthquake events,occurred at10.3-4.1ka and 4.6-1.4ka,are 1.6m and 1.2 to 1.3m,respectively,with the estimated magnitude of Ms7.4 to Ms7.3.Since the Holocene,the dip slip rate in this segment is estimated to be 0.27-0.71mm/a.2.The northern segment of Jiaocheng fault is an active fault since the Holocene.The research results of two bedrock fault surfaces at Shanglancun and Sixicun in the northern segment of the fault revealed 3 paleo-earthquake events with co-seismic dip-slipdisplacement of about 2m,and the estimated magnitude of Ms7.4-Ms7.6.The bedrock fault surface at Longwanggou identified the latest paleo-earthquake event with aseismogenic age of 6.6-2.5 ka and a co-seismic dip slips of 2.3-2.4 m.Since the end of the late Pleistocene,the dip slip rate in the north segment of the Jiaocheng fault isestimated to be not less than 0.20-0.55mm/a.3.The two paleo-earthquake events with a co-seismic dip slips of 3.1m and 3.2-3.3m were identified on the Tangshankou segment of the Weiguang basin southern marginal fault.,with estimated magnitudes of Ms7.6 and Ms7.8.The age of the latestearthquake event was 6.7-4.1ka.The Xizhuangtou bedrock fault surface of Tangshankou segment indicates a vertical slip rate of at least 0.7-0.85mm/a since 10.1-8.2ka.Based on the analysis of the above research results,the active tectonic significance of carrying out the paleo-seismic study of bedrock faults in the basin-mountain boundary zone of the Shanxi rift was discussed.The main understandings obtained are as follows:1.Carrying out"paleo-earthquake study on bedrock fault surfaces"is an effective method to study the history of fault activity and identify paleo-earthquakes.Based on the quantitative analysis of the high-precision morphology of the fault surface and thecosmogenic nuclide dating method,the two key technologies,we can better obtain theco-seismic displacement and the age of the paleo-earthquake event,and further estimate a series of quantitative parameters such as the slip rate and the magnitude.2.The normal fault in Shanxi rift often present a situation in which the fault has two branches:the one is close to the interior of the basin and cuts through Quaternary deposits,and the other is the bedrock fault near the mountain.Carrying out paleo-earthquake study on the bedrock fault near the mountain can effectively determine the activity of the bedrock fault,identify the potential source of seismic disaster risk,make the distribution of slip rate clear,which finally helps to conduct a more objective and reasonable seismic risk assessment.3.For the case where a fault is spatially spread in the bedrock area and the Quaternary deposit area in sequence,the paleo-earthquake study on bedrock faultsurfaces can extract more fault activity and paleo-earthquake information contained in bedrock area.The study can also help to effectively determine the time of the latest activity of the fault?segment?and restore a complete seismic activity history of a fault.It should be noted that the exposure age determination of the bedrock faults in this paper is only a preliminary exploration.The density of the ³⁶Cl samples collected from the bedrock fault plane is relatively sparse,which may have a certain impact on theoccurrence time of strong earthquakes and the slip rate of the fault.In particular,for the Weiguang basin southern marginal fault and the Jiaocheng fault,the composition of the overlying colluvium wedge on the production rate is not considered.Therefore,adding this factor into the future research may have a certain impact on the exposure age.In this paper,the exposure age calculation model based on classic formulas and simplifiedmodels is relatively simple and traditional.In future research work,we plan to consider more parameters to simulate the exposure history of bedrock fault planes,and the results may differ from those obtained in this thesis.