| As an important technology to paleoseismologic research, trenching has been used to identify paleo-earthquakes and the recurrence interval combined with dating methods. However, there have been some bigger uncertainties and limitations in the time and integrity of paleoearthquake sequences. This problem is especially serious for the strata that are relatively thick with unclear bedding, such as loess. Unclear bedding makes identifying the fault surface difficult, while subtle strata in loess sediment cannot be interpreted only by naked-eye, which seriously affect determination of paleoearthquake horizons and times. Therefore, how to reduce the uncertainty and improve the accuracy of paleoearthquake identification is an important issue to be addressed. The Dongyugou loess section, located in the northeastern corner of the Linfen basin, Shanxi Province, crosses the Huoshan piedmont fault. This section exposes not only well-developed loess sequence, but also several obvious faulting events. Thus, this loess section is an ideal site for paleoseismo logy. Using this site, this work attempted to make a high-resolution analysis of grain size and susceptibility, which facilitate subdivision of loess strata and paleoearthquake identification.Based on visual interpretation, previous work made a stratification of the Dongyugou loess section for ten and twelve layers between two fault walls. Four and five paleosols were identified on the hanging wall and the footwall, respectively. Marked by the corresponding paleosols of the two walls, five paleoearthquakes were recognized. The two early events occurred before d8 and d6, and the latest three events happened after formation of u2, u4 and u6. Combining with the OSL results, this work calculated the coseismic vertical displacements of the three events to be 0.7m,0.9m, and 1.1m, which occurred near 23.3ka,32.8ka, and 47.7ka, respectively. The resultant average recurrence interval of about 11ka is not consistent with that of 2ka in previous studies, which may be attributed to missing some events in the records by visual interpretation. Aiming at this problem, some scientists carried out the research on lacustrine sediments by the means of grain size and magnetic susceptibility analysis and identified the paleo-earthquake sequence effectively, extending the ancient history of earthquakes. Here, this work also used such an approach to research the paleoearthquakes in continuous deposition of the aeolian loess area to extend the records and make up the shortage of sequence.Firstly, the curves of high-resolution grain size and magnetic susceptibility show strong fluctuation on the section. In combination with dating results, comparison with other areas indicates that these curves are well consistent with the last glacial climate curves, which correspond to the six Heinrich events. In other words, almost all the curve fluctuations of the section are associated with climate change, while there are no abnormal fluctuations caused by tectonics. So the analysis of grain size and magnetic susceptibility cannot identify the trace of seismic accumulation.Secondly, identifying paleoearthquakes and accuracy of estimating the time based on the accuracy of the making a stratification of loess. Usually, the interpretation by naked-eye can only identify the loess layers and paleosol layers. Based on the high-resolution grain size and magnetic susceptibility analysis, this work made a high-resolution stratification of Dongyugou loess section, including high-resolution thickness of each stratum and its upper and bottom boundaries. Based on the high-resolution stratification and their comparison between two fault walls, this study recognizes three seismic events, which occurred after formation of u5-7, u4 and u2 corresponding to their stratification depth of 7.1 m,4.7 m and 2.9 m in the hanging wall, respectively. Based on results of OSL dating and average sedimentation rate of the hanging wall, this work estimated that the three events occurred near 45.8ka (between 48.1±1.5～43.2±2.5ka),32.8ka(between35.0±2.4～30.6±1.3ka) and 23.3ka (between 26.4±0.8～20.9±0.7ka), respectively. According to the thickness difference of three loess-paleosol sedimentary cycles between two fault walls, this work calculated the coseismic vertical displacements of the three events to be 0.5 m,0.4 and 1.3 m, respectively. Compared with other segments of the Huoshan piedmont fault zone, this work found the southernmost segment is the weakest, with longer recurrence interval of about 11ka and lower vertical slip rate of 0.05mm/a, and no earthquake occurred in the Holocene. Therefore, it is concluded that neither the southern end of the Huoshan piedmont fault zone nor the southern end of the rupture of the Hongtong M8.0 earthquake of 1303 has reached at the Subao village, instead nearby the Guangshengsi village.Finally, the analysis of grain size and susceptibility found the obvious H5 climate event of the last glacial period. With the divide 47.3ka, this period has two epochs. In the early time (67.7-47.3 ka), the content of coarse silt (20-63 μm) and sand fraction (> 63 μm) was overall low, but magnetic susceptibility was relatively high. On the contrary, in the later time (47.3-13.0 ka, coarse silt and sand fraction had high content, while magnetic susceptibility was law. This implies that Asian winter monsoon was weak and summer monsoon was strong in the early time, while Asian winter monsoon enhanced and summer monsoon weakened significantly in the later period. The evolution models of last glacial climate demarcated by H5 (-47.0 ka) are comparable in East Asia, Eastern Mediterranean and even North America, which may be the result of ice amount developing significantly in the Arctic in this period, leading to climate changes of East Asia and the Northern Hemisphere in the direction of the cold significantly. This cognition plays an important role in deep understanding of the evolution trend of MIS3 climate in East Asia. Of course, this initial cognition needs to be further tested in the future through high resolution studies. |
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