Abstract: The Langshan range-front fault (LRF)is a Holocene active normal fault that bounds the Langshan Mountain and Hetao Basin at the northwest corner of the Ordos Plateau. Paleoseismic trenching research at three sites, Dongshen Village trench (TC1), Qingshan trench (TC2)and Wulanhashao trench (TC3)from north to south was performed in this study to reveal the seismic hazard risk in Hetao Basin. The paleoevents ED1, ED2, ED3 from TC1 can be constrained to have occurred (6±1.3)ka, (9.6±2)ka and (19.7±4.2)ka respectively, while the paleoevent EQ1 from TC2 occurred about (6.7±0.1)ka and the paleoevents EW1, EW2, EW3 at TC3 took place about (2.3±0.4)ka, (6±1)ka and before 7ka respectively. In combination with paleoseismic results of previous researchers, the Holocene earthquake sequence of the LRF could be established as 2.3~2.43ka BP (E1), 4.41~3.06ka BP (E2), 6.71~6.8ka BP (E3), 7.6~9.81ka BP (E4), and (19.7±4.2)ka BP (E5). Although the possibility of missing events cannot totally be ruled out, based on the analysis on faulted geomorphology at Wulanhashao site, we argue the paleoearthquake history of the LRF during Holocene may be complete with an average recurrent interval about 2500 yrs. The apparent displacements associated with events E1, E3 and E4 are significantly larger than that of event, E2, that suggests that they might be great events with magnitudes 7.5 to even over 8 that ruptured the entire LRF, while the event E2 may be a smaller event that only ruptured a segment of the fault. The magnitude of event E2 might be about M7. This poses a significant seismic hazard to the area of the Linhe depression in the western Hetao graben region. With the further limitation of previous radiocarbon dating result near our trench site at Wulanhashao, the slip rate at Wulanhashao should be not smaller than, but close to 0.66mm/a since 15ka BP. And the slip rate at Qingshan site is supposed to be about 1.4~1.6mm/a since 6.8ka BP. Both our combined most recent paleoseismic cognition and current tectonic geomorphologic research results supports to reveal that the Langshan range-front fault now is an unsegmented fault, preferring to rupture the whole fault in a surface-rupture event. Considering the most recent event E1 and fault slip rate obtained above, the accumulated strain on the LRF could be estimated as about 1.52~3.94m. Given the ~2500a recurrent interval, we argue that the elapsed time since last major quake, E1, is approaching or even over the recurrence, and the seismic risk for another major quake is imminent, at least cannot be ignored.

Key words: Hetao Basin, Langshan range-front fault, paleoseismology, seismic hazard, fault slip rate

摘要： 位于鄂尔多斯西北缘的、狼山和河套盆地之间的狼山山前断裂是1条全新世活动断裂。沿狼山山前断裂，从北向南开挖了3个古地震探槽，分别为东升村探槽（TC1）、青山镇探槽（TC2）和乌兰哈少探槽（TC3）。由TC1限定的3次古地震事件（ED1、ED2、ED3）的发震时间可相应地限定为（6±1.3）ka BP、（9.6±2）ka BP和（19.7±4.2）ka BP；由TC2揭露的古地震事件EQ1的发震时间可以限定为（6.7±0.1）ka BP；由TC3限定的3次古地震事件（EW1、EW2、EW3）的发震时间可相应地限定为（2.3±0.4）ka BP、（6±1）ka BP和7ka之前。结合前人的研究，可以确定狼山山前断裂晚更新世以来的古地震序列为2.3～2.43ka BP（E1）、4.41～3.06ka BP（E2）、6.71～6.8ka BP（E3）、7.6～9.81ka BP（E4）和（19.7±4.2）ka BP（E5）。虽然由于事件E5可能包含了多次古地震事件，导致晚更新世以来古地震漏记的可能性仍然无法剔除，但仍可认为狼山山前断裂在全新世期间的古地震历史应该是完整的，强震平均复发周期约为2 500a。与古地震事件E1、E3和E4相关的地震同震位移值明显较其他事件更大，表明这3次古地震事件可能是震级为7.5～8级甚至>8级的破裂断裂全段的古地震事件，而古地震事件E2可能是较小一点的、仅破裂部分断裂段的古地震事件。狼山山前断裂在乌兰哈少点自15ka BP以来的滑动速率应该大于但接近于0.66mm/a。狼山山前断裂的古地震研究结果显示，断裂现今是1条倾向于在地表破裂事件中全段破裂的不分段断裂。假设约2 500a的强震复发周期成立，则自最近1次强震事件以来的离逝时间接近或者已经超过了强震复发周期，狼山地区再发生强震事件的危险性是非常大的。

关键词: 河套盆地, 狼山山前断裂, 古地震, 地震危险性, 断裂滑动速率