隐伏活动断层的多层次综合探测——以银川隐伏活动断层为例

COMPREHENSIVE MULTI-LEVEL EXPLORATION OF BURIED ACTIVE FAULT:AN EXAMPLE OF YINCHUAN BURIED ACTIVE FAULT

以银川活动断层试验探测为例,介绍了第四纪巨厚沉积区隐伏断层多手段、多层次探测的步骤与方法。在对前人资料综合分析的基础上,选择银川市兴庆区新渠梢村为综合探测试验场。首先,分层次布施道间距10m、5m和1m的浅层人工地震勘探,由深至浅将银川隐伏断层主断层逐步控制在可以布施浅钻勘探的程度。然后,实施钻孔联合剖面探测,确定了断层的准确位置和倾角,获得了由钻探资料可分辨的断层上断点埋深8.3m的信息。最后,通过大型探槽开挖,查明了断层实际上断点埋深1.5m和5期古地震活动事件。结合地层年龄的初步测定,得出了银川隐伏断层主断层中—晚全新世活动的结论。

Yinchuan Basin is a graben-like downfaulted Cenozoic era basin located on the west edge of Ordos Massif. Its activity is violent and deposition is very thick. Yinchuan City is located in the middle of Yinchuan Basin. The seismic petroleum exploration shows that a buried active fault lies in the east of Yinchuan City, named as the Yinchuan buried fault, which strikes NNE and dips west, with a total length of more than 80km. Because the seismic petroleum exploration did not gain any explained signals at the depth ranging from 0 to 400m, so whether the Yinchuan buried fault is active or not in the late Quaternary and its exact surface projective location hasn＇t been known yet. It has been a ＂worry＂ in the urban planning and development of Yinchuan for a long time. Under the financial support of the national and local governments, we launched the project entitled ＂ The prospecting of active fault and earthquake risk assessment in Yinchuan City＂. In order to facilitate the exploration, we selected Xinqushao village in the southeast suburb of Yinchuan City to be the site for the integrated test exploration of the Yinchuan buried fault before the exploration, based on the information obtained from the seismic petroleum exploration. Considering that the thick Quaternary sediment in Yinchuan reaches to 1609m, and that the depositional environment is the Yellow River flood plain and the lateral change of lithology is complex, we adopted in the test exploration the train of thoughts of ＂inferring an unknown fact from a known fact, and from deep to shallow and directly to the top＂. The experimentation has been developed step by step according the working order of multilevel seismic exploration→composite geological profile drilling→trenching. Along the same measuring line at Xinqushao, first, we adopted the seismic reflection exploration of primary wave in three levels with the group interval of 10m→5m→1m to catch the master fault of the Yinchuan buried fault, and by tracing upward layer by layer in the order of the three exploration ranges, i.e. 1400 ～ 400m→600 ～ 80m→150 ～ 20m, the position of the master fault at ±20m depth under the ground and its offset trace were primarily identified. And then, along the master fault and within the range of 100m at its both sides, 9 boreholes of 20. 5 ～ 100m were arranged for the composite geological profile drilling. The resulting information about the throws of the master fault was obtained, they are 20.34m, 9.66m and 2.25m respectively at the depth of 43.75m, 20. 33m and 13.04m from the ground, and the buried depth of the upper offset point ≤8.34m. At the same time, using the intact core specimen from thg fault plane of the borehole No. 7, we calculated the dip angle of the fault as 71°at the depth of 55.27m and figured out the exact position of its extension to the earth＇s surface. Finally, a large-scale trial trench, which is 40 meters long, 8 ～ 12 meters wide and 6 meters deep, was arranged across the master fault. The trenching revealed that the actual buried depth of the upper offset point of the master fault is 1.5 m and there are seismic remains, such as offsets of 5 stages, sand liquefaction and surface rupture, etc. Among the 5 stages offsets, 4 events occurred prior to 3170 ±80 a BP, belonging to the mid to late Holocene paleo-earthquakes. The age of the last event cannot be determined and it is inferred to be the result of the MS. 0 Yinchuan-Pingluo earthquake in 1737. In a word, through the comprehensive test exploration, we find that the Yinchuan buried fault is a Holocene active fault, which lays solid base for the next exploration.