关键词: 郯庐断裂带, F₅断裂, 垂直滑动速率, 全新世, 古地震

Abstract:

The Anqiu-Juxian Fault(F₅)in the middle part of Tanlu fault zone is the most important seismically active fault in eastern China. The Fault F₅ is divided into the Anqiu-Juxian section, the Juxian-Tancheng section and the Xinyi-Sihong section, each of which is an independent rupture unit. There are no historical records about earthquakes with magnitude above 5 in the Xinyi-Sihong section, but it is revealed that there are Holocene paleoseismic events, so this section is a significant gap segment of surface rupture of historical earthquakes. In recent years, an important progress in the study of neotectonic activity of Xinyi-Sihong section of F₅ is to find that it extends southward to the region between Huai River and Nüshan Lake in Anhui Province, with a length of about 20km. The fault spreads on the gentle slope on the edge of Cretaceous red sandstone uplift(hillock)along the line from Fushan to Ziyangshan, and the latest activity can date back to the early Holocene. At present, there is a clear understanding of the geometric distribution, structural characteristics and activity nature of the Huai River-Nüshan Lake section of F₅(F₅-HRNL), but the paleoseismic research is relatively weak, the revealed paleoseismic events are relatively sporadic, and the research results are from single trench, so there is a lack of comprehensive and comparative analysis from multiple trenches. In addition, the study on slip rate has not been carried out in this section, which affects the understanding of the overall activity level of the fault. Therefore, based on the previous work, paleoseismic research is carried out by excavating trenches in key locations, and more reliable paleoseismic events are determined through comprehensive comparative analysis of multiple trenches. The vertical slip rate of the fault is calculated by measuring the height of the fault scarp near the trench and combining with the dating data of relevant strata. Based on the paleoseismic research results of the F₅-HRNL and combined with the data of other disciplines, the seismic risk of this fault section is analyzed. The results of this study enrich the understanding of the overall activity characteristics of F₅ in the Tanlu fault zone in the Late Quaternary, and provide new data for medium- and long-term earthquake prediction in the border area of Jiangsu and Anhui Provinces.
In this study, a new trench was excavated at the foot of Fushan Mountain on the south bank of the Huai River, named Santangnan trench, for the special research on ancient earthquake events. The trench reveals that four paleoseismic events have occurred on F₅, and the latest event occurred since the late Late Pleistocene, that is, since(15.7±2.0)ka BP, but the trench failed to constrain the age of each event. Based on the trenching work and combined with the previously published trench research data, the paleoseismic events in the F5-HRNL are further constrained by using the progressive constraining method. The results show that at least five paleoseismic events have occurred in the F₅-HRNL since the late Middle Pleistocene. The first three events occurred in the late Middle Pleistocene to the late Late Pleistocene, all of which were thrust in nature and manifested as gently dipping thrust faults, reverse faulting colluvial wedges and structural wedges in the trench; the latest two events occurred since the late Late Pleistocene, both of which were extensional in nature and manifested as splitting wedges in the trench; the age of the latest two events are constrained at 20.36~(18.7±0.3)ka BP and 10.92~7.83ka BP respectively.
At present, the research on the slip rate of F₅ mainly focuses on the horizontal slip rate on the Shandong Province section, where the water systems are relatively developed and the deformation is obvious. The vertical slip rate of the fault is rarely reported. Stable and continuous fault scarps are developed in local segments of the F₅-HRNL, and trenches are excavated across the scarps, which provides support for the calculation of vertical slip rate of this section. Through UAV topographic mapping, a high-precision digital elevation model near the scarp is constructed, the topographic profile across the scarp is extracted, and the vertical displacement of the fault is discussed. Based on the results of Quaternary stratum dating and paleoseismic event analysis in the trench near the scarp, the starting time of vertical displacement of the scarp is determined. The calculation shows that the vertical slip rate of the F₅-HRNL is about 0.05mm/a in the Ziyangshan area and about 0.07mm/a in the Doushan area, indicating that this fault section is weakly active as a whole.
The Sihong-Mingguang section of F₅ is from the south of Chonggang Mountain in Sihong County, Jiangsu Province to the north of Nüshan Lake in Mingguang City, Anhui Province, with a total length of about 65km. The latest paleoseismic event revealed in this section is about 8 000 years ago. Based on the research results of paleoearthquakes and combined with the research data of other disciplines, it is considered that the F₅ Sihong-Mingguang section is the surface rupture gap section of historical earthquakes, a long time has elapsed since the latest ancient earthquake, and the current small earthquakes are not active, the locking degree is high, and it is likely to accumulate stress, and there is a risk of strong earthquakes of magnitude 7 or above.

Key words: Tanlu fault zone, Fault F₅, vertical slip rate, Holocene, paleoearthquake