Study On The Quaternary Activity Of The Yellow River 's Buried Fault In Wuhai Basin, Inner

Buried active fault refers to a class of fault that with no obvious signs on the surface, which is concealed in Quaternary loose layers. It’s difficult to identify for the reason that there is no teaces on the ground. The activity of hidden fault may not only lead to the occurrence of earthquakes, but also cause serious casualties and property damage along the fault. To detect buried fault, shallow seismic exploration combine with drilling seems to be an effective method. The use of sequence stratigraphy to correlate stratums from large to small sequence can effectively avoid the blindness of stratigraphic correlation and reduce the uncertainty.The Yellow River buried fault in Wuhai rift basin of was previously speculated by the aeromagnetic data, and it may pass though Wuhai City. Thus, in the project "active fault detection in Wuhai City", the Yellow buried fault detection has been regarded as a top priority. This paper describes the use of seismic exploration and composite drilling sections to study the depth of the upper breakpoint and the Late Quaternary activity of the Yellow buried fault.Seismic reflection profiles laid all around the Wuhai basin reveals that the Yellow buried fault is not a single fault, but composed by three subparallel near north-south fault (F3, F4, F5). Among them, F3 is an east-dipping normal fault and terminates on the west-dipping normal faults F5 at about 8～10km. F4 is a west-dipping normal faults, which terminats on the east-dipping fault F3 at about 1.2-1.5km. Both form a "Y" on the section and composed of a center of deposition.Based on the shallow seismic exploration, four joint drill cross sections (Ulan village, Tongda street, Gang del River and Xinyuan village) have been dong along the X07, K2-2, K3, X09 four shallow seismic lines, respectively. Among them, Tongda street drilling is designed for detecting faults F3, Ulan village drilling and Xinyuan village drilling are designed for detecting faults F4, Gang del River drilling is for detecting faults F5.We use "folding" combine with "horizon gradient method" to dynamicly analyze and decide the next hole during the drilling of rach row. During the drilling process, we record the formation like depth, lithology, structure, construction for each laywer in time. Meanwhile, we timely collect the samples for OSL and 14C dating. After that, composite dilling sections have been established by sequence stratigraphy, lithology and facies analysis and Quaternary dating methods.Ulan village borehole surveys two across breakpoints FPx06 and FPx07 of lineX07. Four 100 meters drillings, numbered from west to east 2�'3�'4�'1, have been made with space 83m,30m,53m, respectively. Six signal layer (①～⑥) have been found on both sides of fault, with displacement 2.2m, 2.0m, 2.2m, 3.6m, 3.2m and 6.1m. Those displacements maybe result from three seismic events. Each event has a discement by 2.2m, 1.4m and 2.5m.According to the lithology and phase change of the sediments, we use sequence stratigraphy theory, we divide the strata into five parasequences (Sq1-Sq5) and 19 system. Though carefully comparetion, the Ulan village section reveals that the upper breakpoint of fault F4 is on the depth of about 26 meters, and the latest active time of F4 is about 25KaBP. Meanwhile, the section also reveals three seismic events which happen at about (25.6±0.11) Ka BP、(35.6±0.29) ～(41.7±0.57)Ka BP、(58.25±7.13)Ka BP with vertical break distance by 2.2 m,1.4 m and 3.6 m, respectively.Laid along the streets, the cross-section of the Tongda street joint drills also revealed the multi-phase activities of another branch of the Yellow River buried fault, rupture F3. The upper breakpoint of fault F3 is on a depth of 50.5 meters. Relatedly, the latest active time of the fault is about 40KaBP. What’s more, the section reveals two seismic events at all, which can be dated back to (39.5±0.45) Ka BP and (11.1±1.21) Ka BP, with vertical displacement about 2.8m and 2.0m, respectively. Fortunately, ZK4 pass though the fault surface at 67.2m.Gang del River Drilling was located so close to the east side of the Mount Gang del (less than 1.5 km). Quaternary sediments are mostly alluvial alluvial gravel. Three drills here all show that sediments are all gravels below 50m. Unfutunately, it is difficult to identify faults F5 effectively. Xinyuan villiage revealed that the strata upon 100m are free from the disturbance of F4, but the strata between 100 and 120m show some displacement.