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

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