黄土地区粒度与磁化率分层对古地震研究的意义--以山西洞峪沟黄土剖面为例

SIGNIFICANCE OF HIGH-RESOLUTION LOESS STRATIFICATION BASED ON GRAIN SIZE AND MAGNETIC SUSCEPTIBILITY ANALYSIS TO PALEO-EARTHQUAKE STUDY: A CASE STUDY OF DONGYUGOU LOESS SECTION,AT HONGTONG,SHANXI PROVINCE

探槽技术虽然已经发展成为古地震研究的重要手段,结合定年技术可以识别出古地震事件与地震重复间隔,但是仍然存在较大的不确定性和局限性。例如,目视解译难以区分黄土沉积内部的细层理,严重影响了古地震事件发生位置或时间的判定。如何提高古地震研究的精度和准确性,降低古地震事件判定的不确定性,是目前面临的一个迫切问题。山西洞峪沟剖面位于山西临汾盆地东北角,横跨霍山山前断裂带,不仅揭示了较好的黄土沉积序列,还揭露了明显的地层错断事件。因此,该剖面是一个开展高精度探槽古地震研究,降低判定古地震事件不确定性的理想场所。根据高精度的粒度与磁化率变化曲线,结合目视分层解译结果,对洞峪沟黄土剖面进行了精细分层,界定了各层的厚度和边界。依据细分层和断层两盘地层的对应关系,将u6层沉积以来的3次断错事件的发生位置和时间作了再限定,它们分别发生在u5-7、u4以及u2的顶部,对应断层上盘埋深7.1m、4.7m与2.9m。根据释光测年结果以及断层上盘地层的平均沉积速率,推测3次断错事件发生的时间分别在(48.1±1.5)^(43.2±2.5)ka BP接近45.8ka BP、(35.0±2.4)^(30.6±1.3)ka BP接近32.8ka BP、(26.4±0.8)^(20.9±0.7)ka BP接近23.3ka BP。根据3次黄土-古土壤沉积旋回的厚度差,判定3次地震的同震垂直位移分别为0.5m、0.4m和1.3m,累计位移2.2m。依据高精度粒度与磁化率变化曲线的地层分层方法,为有效降低黄土地区古地震研究的不确定性提供了一种很好的参考。

As an important technology to paleoseismologic research, trenching has been used to identify paleo-earthquakes recorded in strata, combined with dating technology. However, there have been some bigger uncertainties and limitations. For instance, subtle strata in loess sediment cannot be interpreted only by naked-eye, which seriously affects identifying paleo-earthquake horizon and time. Therefore, how to improve the accuracy and reduce the uncertainty of paleo-earthquake identification is the important problem we are currently facing. Dongyugou loess section, located in the northeastern corner of Linfen Basin, Shanxi Province, cuts across the Huoshan piedmont fault. The section exposes not only the well-developed loess sequence, but also several obvious faulting events. Thus, this loess section is a better site to make a high resolution study to improve the accuracy and reduce the uncertainty of paleo-earthquake identification. Based on the high-resolution grain size and magnetic susceptibility analysis, and associated with visual interpretation by naked-eye, we 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, we identified three earthquake events, which occurred after formation of u5-7, u4 and u2, corresponding to their stratification depth of 7. lm, 4.7m and 2.9m in hanging wall. Based on results of OSL dating and average sedimentation rate of hanging wall, we estimated that the three events occurred around 45.8ka （ between （48.1 ± 1.5 ） ~ （ 43.2±2.5 ） ka） , 32.8ka （ between （35.0±2.4） ~ （30.6±1.3）ka） and 23.3ka（between （26.4±0.8） - （20.9±0.7）ka）. According to the thickness difference of three loess-paleosol sedimentary cycles between two fault walls, we calculated the coseismic vertical displacements of the three events as 0.5m, 0.4 and 1.3m, respectively. Compared with other segments of the Huoshan piedmont fault zone, we found the southernmost segment is the weakest, with longer recurrence interval of about 1 lka and lower vertical slip rate of 0. 048mm/a. The high-accuracy grain size and magnetic susceptibility analysis offers an effective method for reducing the uncertainties of the paleo-earthquake research in loess area.