Abstract:

In this paper, the presence of the Tianzhuang fault was confirmed using a combination of petroleum geophysical exploration, geology, remote sensing, and other data. The study concluded that the fault originated from the west of Tiancun, Jinyuan District, Taiyuan City with a total length of about 35 km from Houjiazhai to Tianzhuang. The fault trends from west to east with the pattern EW-NEE-NE, and tends to the SE as a high-angle normal fault. The Tianzhuang fault is a concealed fault associated with the piedmont fault of the East and West Mountains of the Taiyuan Basin. Through the joint drilling exploration across the Tianzhuang fault, near the Ma Lianying Road, there were three distinct sedimentary cycles of river lake swamp facies found in the strata: in the 80 ~ 60 m section, the sedimentary environment tends to frequent gradually, and the sedimentary facies is lake→swamp; in the 60 ~ 30 m section, the sedimentary environment tends to be stable and frequent twice, and the sedimentary facies is river→swamp→river→lake→swamp→river; in the 30 ~ 0 m section, the sedimentary environment tends to be stable gradually, and the sedimentary facies is swamp→lake→swamp. The Quaternary strata in the site gradually thickens from north to south in the horizontal direction, and the coarse-grained deposits become thinner. There is a magnitude change in the borehole, ZK3←→ZK4←→ZK7, and the first layer is thick in the vertical direction. Particle deposition occurs at 20 ~ 30 m, and the floating is not large; the sedimentation cycle number is roughly “M” from deep to shallow, and the sedimentation number reaches a peak at 30 ~ 40 m and 50 ~ 60 m. From the perspective of detecting the strata, all the boreholes in the silty layer of the Holocene boundary were exposed, and the depth was relatively small. It is believed that the sampling rate of the bored sand layer is not the same and hence it is expected that the fault of the Tianzhuang fault is not broken. There are three primary sets of fault-breaking strata in the Tianzhuang fault, all of which are from the Late Pleistocene strata; these did not penetrate the Upper Pleistocene, and thus the target fault was determined to be the late Pleistocene active fault. From top to bottom, the offset of faulted strata increases gradually: 0.4 m, 3.5 m, and 7.2 m, in turn. There are two coseismic displacements of about 3 meters in the exposed depth of the borehole, which can be used to judge the occurrence of two main dislocation events in the identified layer. This provides reliable geological evidence for analyzing the seismic risk of the Tianzhuang fault.

Key words: Tianzhuang fault, sedimentary facies, fault stratum