基于探槽剖面证据探讨1668年郯城地震地表破裂南延问题

1668年山东郯城发生8.5级大地震,造成人民生命财产的极大损失。该地震产生上百公里的地表破裂,然而由于历史记载的缺失,至今对于该破裂的延伸问题一直存在异议,问题争论的焦点集中在该破裂是否跨过郯城窑上阶区。本研究主要基于航片及卫星影像解译、野外考察及探槽等方法重点研究了窑上阶区南北两侧断层的活动特征,为解决这一争议提供更多的地质资料约束。具体的,北侧的华侨探槽剖面揭露出新鲜断层泥发育有明显擦痕,显示断层活动具有明显的逆冲分量,南侧的麦坡探槽剖面揭示了U3地层被正断断错,位错量约2m,断层被全新世地层U1覆盖。另外,郯城地震地表破裂向北传播为右阶阶区,而窑上地区为3km宽的左阶阶区。整体上,本研究认为1668年郯城地震地表破裂向南并未穿越窑上阶区,这进一步表明郯庐断裂活动断层具有分段性,不同段落的活动性质具有差异性,因此对郯庐断裂不同段落开展活动特征研究和对比分析,并对不同段落分别进行地震危险性评价尤为重要。

Evidence of Southward Extend of the 1668 Tancheng Earthquake Surface Rupture from Trench Profiles

The M8.5 Shandong Tancheng earthquake in 1668, had caused tragic loss of life and disastrous damage to infrastructure along the densely populated Eastern China. The earthquake produced more than a hundred kilometers of surface rupture. Due to the lack of historical documentary records, so far, there have been disagreements for extension southward of the surface rupture. The debate focused on whether it across the Yaoshang sinistral restraining step-over. In this paper, through aerial photo and satellite image interpretation, field investigation and trenching, we study the fault activity characterized on both sides of the area in order to solve the controversy to provide more geological data constraints. The profile of Huaqiao village in the north expose fresh gouge and fault striation, which show that the earthquake faulting is main of thrust with strike slip; the Maipo profile in the south show that the strata of U3 is normal faulting and offset up to 2m, and the fault is covered by the Holocene stratum U1. In addition, the Tancheng earthquake surface rupture is characterized by dextral step-over northward, however, the Yaoshang area is 3km wide sinistral restraining step-over. Overall, we suggest that the surface rupture southward is ending at Yaoshang village. Our results show that activity characteristics of different segments of the fault is different. Thus, segmentation analysis, comparison research and segmented risk assessment of the Tanlu active fault is particularly important.