Paleoseismology along the range-front fault of Helan Mountains, north central China
Paleoseismology along the range-front fault of Helan Mountains, north central China
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DOI：
10.1029/95jb01814
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发表时间：
1996-03
影响因子：
--
通讯作者：
Q. Deng;Yuhua Liao
中科院分区：
文献类型：
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作者：
Q. Deng;Yuhua Liao
贺兰山沿银川地堑西缘的北北东向山前断裂是一条长13公里的高角度正右旋走滑断裂。它切割了晚更新世至全新世的冲积扇，形成一个面向东南的陡坡，与明代长城（约公元前400年）横向偏移1.45 m，纵向偏移0.95 m。长城的偏移可能是1739年银川-平洛M = 8级地震的结果。1739年的地震沿着断裂带形成了一个长88公里的不连续地表破裂带。北红果子和南苏鱼口断崖的长度和高度均最大。编制了红果子断裂带和宿鱼口断裂带的大尺度地形图和地质图，调查了断裂带上的88条剖面，挖掘了断裂带上的14条沟槽，研究了该断裂带全新世古地震活动性和大地震的重现期。在断层的隆起块体上，沿断裂冲积扇的沟槽发育了四层梯田。T4、T3等较老梯田向西倾斜，较年轻梯田向西倾斜较少。这表明阶地可能在多次断裂事件的作用下发生了倾斜和旋转。两个相邻梯田之间的高差约为1-2.7 m。在64条地形剖面上测量了坡地下部原始地表与T3、T2和T1阶地的高程差。结果表明：T3高程最高，沿宿鱼口断裂带和红果子断裂带分别平均为8.4±1.9 m和5.5±1.1 m；T2海拔高度分别为5.8±1.4 m和3.4±0.4 m， T1海拔高度分别为3.1±1.2 m和1.2±0.3 m。采用陡坡发育扩散模型计算，T3 ~ T2和T2 ~ T1之间梯田上升的年龄分别为3600 ~ 4600年和2000 ~ 2800年。断崖的形态复杂。部分早期活动的断崖发育2 ~ 3个坡面，上、下坡面分别对应T3、T2冲积阶地。免费面只存在于T1平台上。宿鱼口陡坡上、下坡段坡度角分别为6 ~ 22°和17 ~ 38°。在相同的剖面中，两者之间的差异始终在9-21°范围内。在断裂带较高的地方，自由面可能是1739年M = 8地震造成的，其坡度角可达60°。我们推断另外两个斜面代表两次古地震事件。在宿鱼口断裂带和红果子断裂带上分别开挖了8条和6条海沟。所有的海沟都显露出正断层、多偏移、沿陡坡发育的崩积和充填楔形。在这些海沟中，我们发现了4个不同时期形成的崩陷楔，表明有多次断裂事件。在此基础上，确定了四个全新世地震事件。这些事件的时间是通过从战壕中收集的14C测年样本确定的。根据台地偏移、断崖形态和古地震沟的特征，我们认为沿断裂带发生的4次大地震分别发生在B.P. 8400、4600 ~ 6300（或5700）、2600和256年，最近一次是1739年银川-平洛M = 8级地震。这些地震的复发间隔为2300-3000年。我们推断这些事件的震级可能为~ 8级，因为这些事件所产生的每个崩落楔的厚度和阶地隆起的高度与1739年地震所产生的相似。
The NNE trending range-front fault of the Helan Mountains along the western margin of Yinchuan graben, northcentral China, is a 13-km-long high-angle normal and right-lateral strike-slip fault. It cuts late Pleistocene to Holocene alluvial fans, forming a SE facing scarp, and offsets the Great Wall of the Ming Dynasty (about 400 years B.P.) right laterally for 1.45 m and vertically for 0.95 m. The offset of the Great Wall is probably a result of the M = 8 Yinchuan-Pingluo earthquake of 1739. The 1739 earthquake formed an 88-km-long discontinuous surface rupture zone along the fault scarps. Both the length and height of the fault scarps are largest at Hongguozi in the north and Suyukou in the south. We have prepared large-scale topographic and geologic maps along the Hongguozi and Suyukou scarps, surveyed 88 profiles across the scarps, and excavated 14 trenches across the fault, in order to study Holocene paleoseismicity and recurrence intervals of large earthquakes along the fault. Four levels of terraces were developed on the upthrown block of the fault along some gullies that dissected the alluvial fans. The older terraces, such as T4 and T3, are tilted to the west, whereas the younger terraces are less tilted. This indicates that the terraces may have been tilted and rotated by multiple faulting events. The height difference between the two adjacent terraces is about 1–2.7 m. The elevation differences between the lower original surface and the T3, T2, and T1 terraces were measured in 64 topographic profiles across the scarps. The results show that the elevation of T3 is highest, averaging 8.4 ± 1.9 m and 5.5 ± 1.1 m along the Suyukou and Hongguozi fault scarps, respectively. The elevations of T2 at the two sites are 5.8 ± 1.4 m and 3.4 ± 0.4 m, respectively, whereas elevations of T1 are 3.1 ± 1.2 m and 1.2 ± 0.3 m, respectively. Ages of terrace risers between T3 and T2, as well as between T2 and T1, are 3600–4600 years and 2000–2800 years, respectively, as calculated using a diffusion model of scarp development. The morphology of the fault scarps is complicated. Two or three bevels were developed on some fault scarps that were active in an earlier period, and the upper and the lower slope sections correspond to T3 and T2 alluvial terraces, respectively. A free face exists only on T1 terrace. The slope angles of the upper and lower slope sections of the scarp at Suyukou are 6–22° and 17–38°, respectively. The difference between the two in the same profiles is consistently in the range of 9–21°. In the place where the fault scarp is high, the free face, with a slope angle up to 60°, may be a result of the M = 8 earthquake of 1739. We infer that the other two bevels represent two paleoearthquake events. Eight and six trenches have been excavated across the fault scarps at Suyukou and Hongguozi, respectively. All the trenches reveal normal faults, multiple offsets, and colluvial and filled wedges along the scarps. In these trenches, we found colluvial wedges formed in four different periods indicating multiple faulting events. On the basis of this evidence, four Holocene seismic events have been identified. The timing of the events was determined through 14C dating samples collected from the trenches. On the basis of terrace offset, scarp morphology, and paleoseismic trenching, we conclude that the four large earthquakes along the fault occurred 8400, 4600–6300 (or 5700), 2600, and 256 years B.P. The latest event was the M = 8 Yinchuan-Pingluo earthquake of 1739. The recurrence interval of these earthquakes is 2300–3000 years. We infer that the magnitudes of these events were probably ∼8, because the thickness of each colluvial wedge and the height of terrace risers produced by these events are similar to those produced by the 1739 earthquake.