北京平原主要活动断裂全新世活动定量研究及未来地震危险性预测

Quantitative Study of Holocene Activity of Main Active Faults in the Beijing Plain and Prediction of Future Seismic Danger

一、前言北京平原位于华北平原西北角,地处燕山南缘与太行山东北缘衔接地带。源自燕山山脉的潮白河及太行山山脉的永定河构成北京平原区两大水系。北京平原属于山前冲洪积平原,总体地势西北高东南低,海拔高程50~10m,坡降5‰。北京市坐落在北京冲洪积平原的后缘。北京平原的活动构造为隐伏活动构造,增加了该地区地震构造研究的难度。本文将展示近年对这一地区主要活动断裂开展调查取得的主要结果,并对该地区未来强震危险性进行探讨。

The Holocene activities of some major active faults in the Beijing Plain have been studied quantitatively in recent years. The main results are introduced in this paper. The major active faults include: 1) the NNE-trending Xiadian fault, which controls the western boundary of the Dachang depression, 2) the NW-trending Nankou-Sunhe fault, which controls the northeastern boundary of the Machikou depression, and 3) the Gaoliying fault which extends along the western side of the Huairou-Shuenyi depression. Trenching profiles across the Xiadian fault near Qixinzhuang show that there were 4 paleo-earthquake events occurred during Holocene. The 4 events were dated as in 10850～9710 a.B.P., 7390～6680 a.B.P., 5416～2233 a.B.P., and AD 1679(the Sanhe-Pinggu M=8 earthquake), respectively. The average recurrence interval of these events was about 3336±396 years. The average co-seismic vertical displacement was about 1.4 m. By analyzing the throw of faulted strata obtained from drilling data of Pangezhuang near the Xiadian fault, it is estimated that there have been 8 paleo-events during the past 30000 years. The average recurrence interval of these events were about 3947～4625 years, while the average co-seismic vertical displacement was about 1.69 m. As to the Nankou-Sunhe fault, studies on the trenching profiles near Jiuxian, Changping County revealed that there had been 3 paleo-events occurred along the fault during Holocene. The 3 events were dated, respectively, as in 12240 a.B.P., 7894～7640 a.B.P., and 3987～3670 a.B.P. The time interval between them were 3938±186 and 4673±209 years. The average coseismic vertical displacement was about 1.67 m. According to some empirical formula concerning displacement-magnitude relation established for the North China region, the relevant magnitude of the paleo-events revealed in Jiuxian trenching profiles should be about 7.8. For the Gaoliying fault, drilling data in Gaogezhuang, Huairou County revealed 2 paleo-events during Holocene, and they were dated as in 3510 a.B.P, and 7050 a.B.P., respectively. Besides, it is estimated that there have been 6 times of fault activities during the period of the past 28000 years. The average interval of these paleo-events was about 4600 years, while the average co-seismic vertical displacement was about 4.6 m. Based on the above-mentioned quantitative researches on Holocene active faults, we attempt to predict the potential strong earthquake places and their magnitude upper limits in the area concerned. By means of seismic potential probability assessment of characteristic earthquakes, we have assessed the seismic potential in the areas concerned for the coming 20, 50, and 100 years. It is obtained that, for the Gaoliying fault and the Nankou-Sunhe fault, the accumulative probabilities reached 0.54 and 0.40, respectively, for earthquakes with M=7.5～7.8 to occur in the coming 50 years; and that, for the Daxing-Tongxian fault, the accumulative probability reached 0.66 for earthquakes with M=6～6.9 to occur in the coming 50 years.