龙门山断裂带映秀-北川断裂白沙河段滑动速率

doi:10.3969/j.issn.0253-4967.2024.06.005

摘要/Abstract

摘要：

龙门山断裂带是青藏高原块体和华南块体的重要边界断裂带, 是一条重要的地震活动构造带。几何上自西向东可分为后山断裂、中央断裂、前山断裂和前陆盆地(成都平原)变形带。中央断裂带是龙门山断裂带的主干断裂, 映秀-北川断裂是中央断裂带中段活动最强烈的一支断裂。前人对龙门山断裂带映秀-北川断裂滑动速率进行了部分研究, 但结果差异很大, 范围从0.07mm/a到1.1mm/a不等。断层滑动速率是分析断层活动特征、研究区域运动学和动力学机制的重要依据。文中依据遥感精细解译和野外调查资料, 通过10个阶地横剖面及阶地年龄对龙门山断裂带映秀-北川断裂白沙河段的滑动速率进行约束, 得到映秀-北川断裂白沙河段晚更新世以来的垂直滑动速率为(0.10±0.02)~(0.30±0.05)mm/a, 平均滑动速率为0.19mm/a。该结果可为理解汶川地震的发震构造、龙门山断裂带的整体活动特征及评价龙门山断裂带的长期地震危险性提供基础资料。

关键词: 龙门山断裂带, 映秀-北川断裂, 白沙河, 滑动速率

Abstract:

The tectonic belt stretches approximately 400km from Lushan County to Wenchuan County in an east-west direction. The Longmenshan fault zone can be geometrically divided into several sections, including the Houshan Fault, the Central Fault, the Qianshan Fault, and the Foreland Basin(Chengdu Plain)Deformation Zone. The Central Fault is the main segment of the active tectonic belt in the Longmenshan region, and the Yingxiu-Beichuan Fault is one of the most active segments within this central section. The Yingxiu-Beichuan Fault has experienced numerous moderate and strong earthquakes throughout its history, including the Wenchuan earthquake 2008. The 2008 Wenchuan earthquake was ahigh destructive natural disaster that profoundly impacted the Chinese mainland, leading to significant economic losses and casualties. This earthquake caused extensive building collapses, leading to the loss of tens of thousands of lives, and triggered severe secondary geological disasters such as landslides, rockfalls, and mudflows, severely affecting the normal operation of transportation and communication infrastructure. The Yingxiu-Beichuan Fault was one of the key surface rupture zones during this earthquake. However, there is still some uncertainty about the slip rate of this fault.

The Baisha River segment examined in this paper is located in the southern part of the Yingxiu-Beichuan Fault, measuring approximately 14km long. This area contains 14 fractures of varying lengths and complex geometric structures, forming a fracture zone that reaches a maximum width of nearly 300m. The overall orientation of the rupture zone is about 50 degrees; however, the orientation of each small secondary rupture varies, with differences ranging from 0 to 90 degrees. The coseismic displacement along the Baisha River section displays complexity and diversity. The thrust movement primarily occurs on the northern and western walls, with some local thrust faults. Additionally, the strike-slip motion is predominantly right-lateral, exhibiting a maximum horizontal displacement of approximately 4.8m, although some local areas show left-lateral displacement.

Previous studies have employed various techniques, such as geology and geodesy, resulting in a wide range of slip rate estimates from 0.07mm/a to 1.1mm/a. The slip rate of fault is a crucial factor for analyzing the characteristics of fault activity and for studying regional kinematics and dynamic mechanisms. According to river terrace longitudinal profiles estimates, the fault has a vertical slip rate of about 0.3mm/a to 0.6mm/a. Estimates based on displaced landforms indicate a vertical slip rate between 0.07mm/a and 1.1mm/a. According to GPS observations, the horizontal slip rate in the Longmenshan fault zone has a limit of 2mm/a, but the slip rate of individual faults is lower than 0.7mm/a.

In recent years, remote sensing techniques have been extensively utilized to study surface rupture zones, particularly during significant seismic events. This paper primarily employs aerial and QuickBird satellite images captured before and after the earthquake. The resolution of the aerial images is nearly 1m, while the QuickBird satellite images have a resolution of 0.6m, both of which allow for precise interpretation of tectonic landforms. River terraces consist of terraced units, including terraced surfaces, steep terraces, terrace fronts, and terrace backs. As geomorphic markers that are relatively easy to identify and measure, river terraces are among the most essential geomorphic units in the quantitative study of active tectonics. They also serve as crucial geological relics documenting Quaternary tectonic movements and climate changes. By examining river terraces and their deformations, researchers can discuss the timing and scale of tectonic activity, making this a long-term area of research.

This paper focuses on the Baisha River section, situated in the southern part of the Yingxiu-Beichuan Fault. We employed geological and geomorphological methods along with optically stimulated luminescence dating, remote sensing interpretation, field investigations, and data analysis to assess the slip rate of the Baisha River section of the Yingxiu-Beichuan Fault within the Longmenshan fault zone. Additionally, we analyze the spatio-temporal variation characteristics of this slip rate. This study constrains the slip rate of the Baishahe segment of the Yingxiu-Beichuan Fault in the Longmenshan fault zone using 10 terrace cross-sections and terrace ages. The results indicate that the Yingxiu-Beichuan Fault Baisha River segment has a vertical slip rate since the Late Pleistocene ranges from(0.10±0.02)mm/a to(0.30±0.05)mm/a. Considering that only one event, the 2008 Wenchuan earthquake, is associated with the T1 terrace, we believe the calculated rate based on the dislocation and age of the T1 terrace may significantly deviate from reality. If we exclude the sliding rate of the T1 terrace, the vertical slip rate since the late Quaternary ranges between(0.10±0.03)mm/a and(0.30±0.05)mm/a. The linear fitting results indicate that the average vertical sliding rate since the late Quaternary is approximately 0.19mm/a.

These findings provide fundamental data for understanding the seismogenic structure of the Wenchuan earthquake and the overall characteristics of the Longmenshan fault zone, as well as for assessing its long-term seismic hazard.

Key words: Longmenshan fault zone, Yingxiu-Beichuan Fault, Baisha River segment, slip rates

花春雨, 石峰, 魏占玉. 龙门山断裂带映秀-北川断裂白沙河段滑动速率[J]. 地震地质, 2024, 46(6): 1295-1313.

HUA Chun-yu, SHI Feng, WEI Zhan-yu. STUDY ON THE SLIP RATE OF THE BAISHA RIVER SEGMENT IN THE YINGXIU-BEICHUAN FAULT IN THE LONGMENSHAN FAULT ZONE[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(6): 1295-1313.

图/表 9

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样品编号	埋深 /m	颜色	采样位置	测年手段	环境剂量率 /Gy·ka^-1	等效剂量 /Gy	测年结果
LEDL09-71	0.3	灰黑色	虹口乡八角庙T1阶地	OSL	4.22±0.42	20.20±2.50	(4.80±0.60)ka BP
LEDL09-74	0.5	黄褐色	虹口乡八角庙T3阶地	OSL	5.54±0.55	354.40±28.20	(63.90±8.20)ka BP
LEDL09-75	1.6	黑色	虹口乡八角庙T3阶地	OSL	4.82±0.48	194.00±13.10	(40.30±4.80)ka BP
LEDL09-76	1.5	黄褐色	虹口乡八角庙T2阶地	OSL	6.09±0.61	129.80±10.90	(21.30±2.70)ka BP
LEDL09-77	1.1	灰色	虹口乡八角庙T3阶地	OSL	4.01±0.40	222.00±18.70	(55.40±7.20)ka BP
LEDL09-78	0.7	黄色	虹口乡八角庙T2阶地	OSL	3.73±0.37	66.90±6.30	(17.90±2.40)ka BP
LEDL09-178	2.0	灰黄色	虹口乡高原村T2阶地	OSL	4.44±0.44	75.00±5.50	(16.90±2.10)ka BP
LEDL09-180	5.0	灰黄色	虹口乡马家咀T2阶地	OSL	3.58±0.36	59.80±6.00	(16.70±2.40)ka BP

样品编号	埋深 /m	颜色	采样位置	测年手段	环境剂量率 /Gy·ka^-1	等效剂量 /Gy	测年结果
LEDL09-71	0.3	灰黑色	虹口乡八角庙T1阶地	OSL	4.22±0.42	20.20±2.50	(4.80±0.60)ka BP
LEDL09-74	0.5	黄褐色	虹口乡八角庙T3阶地	OSL	5.54±0.55	354.40±28.20	(63.90±8.20)ka BP
LEDL09-75	1.6	黑色	虹口乡八角庙T3阶地	OSL	4.82±0.48	194.00±13.10	(40.30±4.80)ka BP
LEDL09-76	1.5	黄褐色	虹口乡八角庙T2阶地	OSL	6.09±0.61	129.80±10.90	(21.30±2.70)ka BP
LEDL09-77	1.1	灰色	虹口乡八角庙T3阶地	OSL	4.01±0.40	222.00±18.70	(55.40±7.20)ka BP
LEDL09-78	0.7	黄色	虹口乡八角庙T2阶地	OSL	3.73±0.37	66.90±6.30	(17.90±2.40)ka BP
LEDL09-178	2.0	灰黄色	虹口乡高原村T2阶地	OSL	4.44±0.44	75.00±5.50	(16.90±2.10)ka BP
LEDL09-180	5.0	灰黄色	虹口乡马家咀T2阶地	OSL	3.58±0.36	59.80±6.00	(16.70±2.40)ka BP

剖面编号	T1		T2		T3
剖面编号	位错/m	速率/mm·a^-1	位错/m	速率/mm·a^-1	位错/m	速率/mm·a^-1
P1			5.15±0.50	(0.24±0.04)~ (0.30±0.05)
P2	3.80±0.50	(0.79±0.14)			7.53±0.50	(0.12±0.02)~(0.19±0.03)
P3	2.20±0.50	(0.46±0.12)			10.30±0.50	(0.16±0.02)~(0.26±0.03)
P4					8.40±0.50	(0.13±0.02)~(0.21±0.03)
P5					8.32±1.50	(0.13±0.02)~(0.21±0.03)
P6					9.71±0.50	(0.15±0.02)~(0.24±0.03)
P7					8.05±0.50	(0.13±0.02)~(0.20±0.03)
P8			2.20±0.50	(0.10±0.03)~ (0.13±0.03)
P9					6.90±0.50	(0.11±0.02)~(0.17±0.02)
P10

剖面编号	T1		T2		T3
剖面编号	位错/m	速率/mm·a^-1	位错/m	速率/mm·a^-1	位错/m	速率/mm·a^-1
P1			5.15±0.50	(0.24±0.04)~ (0.30±0.05)
P2	3.80±0.50	(0.79±0.14)			7.53±0.50	(0.12±0.02)~(0.19±0.03)
P3	2.20±0.50	(0.46±0.12)			10.30±0.50	(0.16±0.02)~(0.26±0.03)
P4					8.40±0.50	(0.13±0.02)~(0.21±0.03)
P5					8.32±1.50	(0.13±0.02)~(0.21±0.03)
P6					9.71±0.50	(0.15±0.02)~(0.24±0.03)
P7					8.05±0.50	(0.13±0.02)~(0.20±0.03)
P8			2.20±0.50	(0.10±0.03)~ (0.13±0.03)
P9					6.90±0.50	(0.11±0.02)~(0.17±0.02)
P10