A PRELIMINARY STUDY ON PALEOEARTHQUAKES IN THE EASTERN KALPIN NAPPE SYSTEM,SOUTHWESTERN TIANSHAN,CHINA

doi:10.3969/j.issn.0253-4967.2011.04.002

Abstract

Abstract:

There are several thrust-fold belts developed in the Kalpin nappe system of the southwestern Tianshan Mountains.Not only deformation rates of these thrust-fold belts are inconsistent,but also the paleoearthquakes recurrence laws on these thrust-fold belts in the nappe system are different.The Beichuan-Yingxiu Fault and the Pengguan Fault ruptured simultaneously in the M_w 7.9 Wenchuan earthquake.Therefore,it is worth discussing the question of how to determine the cascade-rupturing of a paleoearthquake on two or three thrust faults.We measured the scraps of different heights on the geomorphic surfaces(alluvial-proluvial fans)of different stages in eastern Kalpintage and Saergantage and analyzed the paleoearthquake events revealed by trenches in Shanchakou and Saergantage.Using the ¹⁰Be exposure age,we obtained the ages of the geomorphic surfaces.Then we got the upper and lower limit time of each paleoearthquake from the age of adjacent geomorphic surfaces.Finally,we got the recurrence intervals of different paleoearthquakes,the vertical dislocation of a single event,and the time range of the respective events.The results show that since 20ka BP,the average recurrence interval of paleoearthquake in the piedmont of east Kalpintage is 6.7±0.84ka,the vertical dislocation of a single event is 1m; the average recurrence interval of paleoearthquake in the piedmont of Saergantage is 5.4±0.50ka,and the vertical dislocation of a single event is 0.8～1.2m.The intensity of paleoseismicity is basically identical and the recurrence interval in Saergantage is slightly shorter.In the end,we discussed the possibility of cascade-rupturing accompanying these paleoearthquake events and found that the second and the third paleoearthquake events revealed by the trenches overlap in their occurrence time ranges,indicating the possibility of cascade-rupturing during the earthquake.

Key words: paleoearthquake, ¹⁰Be exposure age, Tianshan Mountains, Kalpin

摘要：

西南天山柯坪推覆系发育多排逆断裂-褶皱带,各排逆断裂-褶皱带的变形速率并不一致,古地震发生规律也可能具有不同特点。自汶川地震造成龙门山中央和前山两条断裂同时活动之后,对于如何判断多条逆断裂级联破裂的逆断裂古地震事件,是一个值得讨论的问题。文中通过对东柯坪塔格和萨尔干塔格两排逆断裂-褶皱带山前多期地貌面(冲洪积扇面)上发育的不同高度的断层陡坎进行测量和探槽古地震事件分析,通过¹⁰Be暴露年龄获得各级地貌面形成时间,利用相邻两级记录了不同次数古地震的地貌面年龄对各次古地震事件的上下限时间进行限制,得到了两排逆断裂-褶皱带山前不同的古地震复发周期、单次古地震垂直位错量和各自的古地震事件发生时间范围。结果表明距今20ka以来,东柯坪塔格山前古地震平均复发周期为(6.7±0.84)ka,单次事件垂直位错1m左右; 萨尔干塔格山前古地震平均复发周期为(5.4±0.50)ka,单次事件的垂直位错0.8～1.2m,古地震活动强度基本一致,萨尔干塔格的周期略短。根据这些结果对是否发生级联破裂进行了初步探讨,认为探槽揭露出的第2次和第3次古地震事件发生时间范围重合,存在发生级联破裂的可能。

关键词: 古地震, ¹⁰Be暴露年龄, 天山, 柯坪

CLC Number:

P315.2

LI An, RAN Yong-kang, XU Liang-xin, LIU Hua-guo, LI Yan-bao. A PRELIMINARY STUDY ON PALEOEARTHQUAKES IN THE EASTERN KALPIN NAPPE SYSTEM,SOUTHWESTERN TIANSHAN,CHINA[J]. SEISMOLOGY AND GEOLOGY, 2011, 33(4): 752-764.

李安, 冉勇康, 徐良鑫, 刘华国, 李彦宝. 西南天山东柯坪推覆系古地震初步研究[J]. 地震地质, 2011, 33(4): 752-764.

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