Detection Of Hidden Active Faults In The Tangyin Graben And Its Three-dimensional Model

Active faults are the main source of continental strong earthquakes and their associated disasters.The research on the planar distribution and spatial structure of active faults is of great significance for earthquake prevention and disaster reduction,as well as for the study of earthquake mechanisms.However,in the vast North China Plain,most active faults are covered by Neogene or Quaternary sediments and are hidden underground,which poses varying degrees of difficulty for research.The Tangyin graben,located on the southeastern edge of the Taihang Mountains,has multiple buried faults.Previous studies have shown that the Tangdong buried fault is an active fault,but the level of research on its structural characteristics,threedimensional spatial distribution,fault activity,and seismic hazard assessment is relatively low.In this study,we used the shallow seismic exploration method and ground penetration radar with small line spacing to obtain high-resolution shallow seismic reflection profiles and ground penetration radar images,which constrained the planar distribution and morphology of the Tangdong fault and revealed the existence of three main faults in the Tangyin graben: the Tangdong fault,Tangzhong fault,and Tangxi fault.The Tangdong fault is not a simple fault,but a combination of two active faults at the shallow level.The shallow seismic reflection profiles effectively constrained the specific positions of the two branches of the Tangdong fault.Three-dimensional structural modeling can finely depict and more comprehensively reveal the spatial characteristics of geological structures,and is one of the important means for studying the geometry and kinematics of active faults.Based on 19 high-resolution shallow seismic reflection profiles,1:200,000 geological maps,30 m resolution DEM data,and SKUA-GOCAD software platform for threedimensional modeling,we established a 3D structural model of the Tangyin graben and further revealed the 3D spatial distribution and geometric structure of the major faults in the Tangyin graben.The interpretation of the shallow seismic reflection profiles showed that the Tangdong fault is a high-angle normal fault with an NNE orientation,and the upper breakpoint cuts through the Quaternary strata,indicating that it has been active since the Quaternary period.Moreover,the Tangdong fault is not a simple single fault,but a composite fault formed by two branching faults in the northern section,which converge into a single fault in the southern section.Therefore,it is divided into east and west branches according to its combination style.The geological radar profile results showed a clear discontinuity of the same-phase axis of the reflection wave on the Fengtun Village geological radar profile,and combined with the shallow exploration profile HBL1,the fault trace on the geological radar profile was determined to be the extension of the west branch of the Tangdong fault at the shallow depth,with an upper breakpoint buried at a depth of about 2.5m.The three-dimensional structural model showed that the Tangdong fault zone has two branching faults at the shallow level,both of which are high-angle normal faults,and merge into a single fault in the south.Drilling data showed that the Tangdong fault is an active fault since the late Quaternary period.The Tangzhong fault trends eastward and has an inclination angle ranging from 51° to 67°,presenting a steep-dipping normal fault in general.Vertically,the steep-dipping angle zone of the fault is mainly located in the upper half of the fault,while the gentler zone is mostly in the lower half,presenting a shovel-shaped structure with a steep upper part and a gentle lower part.Drilling data shows that the Tangzhong Fault is a Early to Middle Pleistocene fault.The Tangxi fault trends eastward and does not have continuity on the plane,divided into northern and southern sections.In general,the upper part of the Tangxi fault has a steep inclination angle that gradually becomes gentler downwards,with an inclination angle ranging from 55° to 60° in the lower part.Field outcrops and profiles indicate that the Tangxi Fault is an Early to Middle Pleistocene fault.The petroleum exploration data and deep seismic reflection data show that the Tangdong fault zone has a shovel-shaped structure with a steep upper part and a gentle lower part,and the two branching faults converge and merge at a depth of about 2km,forming a typical listric normal fault.Deep seismic reflection data revealed that the Tangdong fault has a steep upper part and a gentle lower part,and the dip angle gradually becomes gentler with increasing depth.The Tangxi and Tangzhong faults intersect with the Tangdong fault at depth.In addition,there is two blind faults with a steep dip angle in the deep part of the Tangyin graben,which may extend to the Moho.Based on the spatial distribution of the major faults in the Tangyin graben and the earthquake catalog around the area,this study analyzed and discussed the relationship between the internal faults in the Tangyin graben and historical strong earthquakes,and proposed a possible earthquake structural model.The research results can provide a scientific basis for accurate avoidance of active faults,urban earthquake prevention and disaster reduction,and earthquake risk assessment in the region.