关键词: 汾河断裂, 浅层高分辨率地震探测, 太原

Abstract: This study focuses on a NNE-striking, normal-right-lateral-strike-slip fault, the Fenhe Fault in Taiyuan City. It is one of the faults from the NNE-trending fault system developed within the Shanxi faulted basin zone, buried beneath the ground surface. In this paper, we demonstrate the high-resolution seismic reflection data for a depth range of several hundred meters across the Fenhe Fault. In combination with the relevant borehole logs, these data provide useful constraints on the accurate position, geometry and deformation rate of the fault, as well as the kinematics of recent fault motion. Two high-resolution seismic reflection profiles across the Fenhe Fault in Taiyuan City clearly demonstrate two continuous, strong-reflection interfaces(at two-way time of 85～110ms for the upper one and of 180～200ms for the lower one), and three reflection layers separated by these two interfaces. In addition, they reveal the near-surface location, geometry and activities of this oblique-normal right-lateral fault. Although the westward dipping fault at shallow level cannot be ruled out, we consider the eastward dipping high angle fault, which is located between the canal and the western embankment of the Fenhe River, to be the west branch of the Fenhe Fault. Based on the geological and drilling data, three seismic reflection layers are inferred to represent the Holocene-upper Pleistocene, middle-lower Pleistocene and Neogene strata, respectively. A total vertical separation of ～10m has occurred on the fault since the beginning of Quaternary, terminating at ～70m below the ground surface. In the upper ～190m section of the borehole log near the two seismic profiles, two closely spaced, subvertical strike-slip faults can be recognized. The eastern branch fault is more active than the western one. It lies beneath the eastern embankment of the Fenhe River, dipping to the west and cutting into the upper layer of Holocene-late Pleistocene strata with a maximum vertical offset of ～8m. Another borehole log across the northern segment of the Fenhe Fault reveals an eastward dipping subvertical, right-lateral strike-slip oblique normal fault, which has cut into the upper layer of Holocene-late Pleistocene strata with a maximum vertical offset of ～6m. The afore-mentioned data provide a minimum average Pleistocene-Holocene vertical slip rate of ～0.06～0.08mm/a and a maximum average recurrence interval of 5.0～6.7ka for the Fenhe Fault, providing that the vertical offset caused by large earthquake is 0.4m per event(similar to the offset caused by the Tangshan earthquake). This is a long-recurrence interval and a low vertical-slip rate probably similar to or more than those of the Tangshan No.5 Fault. If the 0.08mm/a vertical-slip rate characterized the history of faulting at seismic profiling site, then the currently active strand of the east branch of the Fenhe Fault had been active since middle Pleistocene(～150ka), and before that time the west branch of the Fenhe Fault was active but probably had stopped moving at that time. Epicentral distribution of earthquakes clearly demonstrates the seismogenic structures of the Shanxi faulted basin zone. Within these NNE-NS-trending basins, some large historical earthquakes have occurred on the NNE-NS-trending faults. Although within the Taiyuan basin and on the Fenhe Fault occurred only some moderate earthquakes, their structures and activities are similar to those of the large earthquakes. It can be concluded, therefore, that the seismic risk of the Fenhe Fault is an objective reality. As the fault just lies beneath the urban population center of the Taiyuan City, a large earthquake(M_S 7.0～7.5)on it may cause enormous damages. An insight into the activity of the fault is needed for the earthquake prediction and hazard reduction of the Taiyuan City.

Key words: Fenhe Fault, high-resolution seismic reflection profiling, Taiyuan City