InSAR数据约束的鲜水河断裂带北西段震间滑动速率及浅部蠕滑特征

doi:10.3969/j.issn.0253-4967.2023.05.003

摘要/Abstract

摘要：

作为中国大陆最为活跃的断裂带之一, 鲜水河断裂现今的滑动模式, 尤其是北西段的浅部蠕滑特征长期以来备受关注。文中首先利用Sentinel-1卫星降轨数据, 基于小基线集时序分析(SBAS)方法获取鲜水河断裂带北西段2014—2021年的地表视线向(LOS向)平均速度场; 再采用弹性螺旋位错模型拟合InSAR跨断层剖面速度, 估计断层的长期滑动速率和浅部蠕滑速率; 最后结合黏弹性地震周期模型分析炉霍段地震的复发周期、下地壳和上地幔流变对滑动速率估计的影响。InSAR处理结果显示, 断层两侧LOS向速度场呈现出明显的速度差异。使用弹性螺旋位错模型估计得到的鲜水河断裂北西段的断层滑动速率为7.2~11.0mm/a, 自西向东逐渐减小。断裂带浅部蠕滑速率为0.3~3.1mm/a, 蠕滑主要集中在虾拉沱和道孚—松林口之间。基于历史地震的复发周期及青藏高原东缘下地壳和上地幔黏滞系数的研究, 文中采用黏弹性地震周期模型反演得到炉霍段的滑动速率为(7.91±0.3)~(9.85±0.4)mm/a, 略低于纯弹性螺旋位错模型的结果((10.14±0.5)mm/a)。

关键词: 鲜水河断裂带, 震间滑动速率, 浅部蠕滑, 黏弹性地震周期模型

Abstract:

Located in the eastern boundary of the Qinghai-Tibetan plateau, the Xianshuihe fault zone is one of the most active left-lateral strike-slip faults in Chinese mainland. As the southern boundary of the Bayanhar block, the Xianshuihe Fault accommodates the southeastward transport of material toward southeastern Asia. Earthquakes have occurred frequently along this fault, especially in the northwestern segment. More than 20 earthquakes with M_W>6.0 have ruptured since 1700. The most recent M_W>7 earthquake was the Luhuo earthquake in 1973, and the most recent M_W>6 earthquake was the M_W6.6 Luding earthquake in 2022. As one of the most active faults in mainland China, the present slip pattern of the Xianshuihe Fault, especially the shallow creep characteristics along its northwestern segment, has attracted much attention.

The primary goal of determining slip rates of active faults using geodetic data is to quantify the seismic potential of the faults. Illuminating the long-term slip rate and shallow creep distribution of faults is the basis for calculating the seismic moment rate and evaluating the seismic potential. Due to the backwardness of early measurement methods, the seismic deformation along the Xianshuihe Fault was previously based on geologic, cross-fault short baseline and leveling surveys. With the application of GPS in tectonic geodesy, more and more GPS stations are installed near active faults, which provide accurate constraints on the long-term slip rates of the fault. Subsequently, the appearance of InSAR technology has brought a beneficial supplement to GPS, providing high spatial resolution surface velocity maps, which have been widely used to measure deep and shallow creep along active faults. It is the key to accurately characterize the fault slip behavior and evaluate the seismic potential.

In this study, 119 Sentinel-1 satellite descent data from December 2014 to December 2021 were processed to obtain the average line-of-sight(LOS)velocity field of the northwestern segment of the Xianshuihe Fault based on the small baseline InSAR method. Then the elastic screw dislocation model was used to fit the fault normal InSAR LOS velocity profiles to estimate the long-term slip rates and shallow creep rates. Combined with the viscoelastic earthquake cycle model, the effects of the earthquake recurrence period, and rheology of the lower crust and upper mantle on slip rate estimation in Luhuo segment are analyzed. The main results are as follows:

(1)The average InSAR LOS velocity field is in the northwestern segment of the Xianshuihe Fault during 2014—2021 has been obtained with a large range and high spatial resolution. The velocity field results show an obvious velocity gradient across the surface trace of the Xianshuihe Fault, which is consistent with the left-lateral strike-slip characteristics of the Xianshuihe Fault.

(2)To investigate the slip rate variation along the northwestern segment of the Xianshuihe Fault, we used the two-dimensional elastic screw dislocation model to fit the 14 fault-normal velocity profiles selected along the northwestern segment of the Xianshuihe Fault and estimated the long-term slip rates and shallow creep rates using the Markov Chain Monte Carlo(MCMC)method. The results show that the overall slip rates of the NW segment of the Xianshuihe Fault range from 7.2mm/a to 11.0mm/a, and gradually decrease from west to east. The shallow creep rate ranges from 0.3mm/a to 3.1mm/a. The high creep rate appears mainly at Xialatuo and the segment from Daowu to Songlinkou. The shallow creep rates in other places are close to zero, implying that the fault is completely locked.

(3)According to historical earthquake records, the recurrence interval of the Luhuo segment is set to be 150 years, 200 years, and 400 years, and the viscosity of the lower crust and upper mantle is set to be 5.0×10¹⁸Pa·s, 1.0×10¹⁹Pa·s, and 5.0×10¹⁹Pa·s. The slip rate of the Luhuo segment is estimated to be (7.91±0.3)~(9.85±0.4)mm/a using the MCMC method, which is slightly lower than the (10.14±0.5)mm/a obtained by the pure elastic model. In addition, when the earthquake recurrence interval is 150 years and the viscosity of the lower crust and upper mantle is 5.0×10¹⁹Pa·s, we simulate the fault-normal velocity at 5 years, 20 years, 75 years, and 125 years after the 1973 Luhuo earthquake, and find that in any period of the seismic cycle, the estimation of fault slip rate will be biased to some extent if the viscoelastic contribution of the lower crust and upper mantle is ignored.

Key words: Xianshuihe Fault, inter-seismic slip rate, shallow creep, viscoelastic earthquake cycle model

陈毅, 赵斌, 熊维, 王伟, 余鹏飞, 余建胜, 王东振, 陈威, 乔学军. InSAR数据约束的鲜水河断裂带北西段震间滑动速率及浅部蠕滑特征[J]. 地震地质, 2023, 45(5): 1074-1091.

CHEN Yi, ZHAO Bin, XIONG Wei, WANG Wei, YU Peng-fei, YU Jian-sheng, WANG Dong-zhen, CHEN Wei, QIAO Xue-jun. INTERSEISMIC SLIP RATES AND SHALLOW CREEP ALONG THE NORTHWESTERN SEGMENT OF THE XIANSHUIHE FAULT FROM INSAR DATA[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(5): 1074-1091.

图/表 11

图1 鲜水河断裂带构造背景黑色细实线表示断裂带, 黑色粗实线表示鲜水河断裂带, 红色实线为鲜水河断裂带北西段; 黑色矩形框表示Sentinel-1卫星D135轨道重组影像的覆盖范围; 蓝色箭头表示以红色圆圈内的GPS站为参考的GPS速度场, 速度场数据来自Wang等 (2020)。历史地震数据来源于USGS网站①(①htlps://www.usgs.gov/programs/earthquake-hazards/earthquakes。), 震源机制解参考Papadimitriou等(2004)的研究

Fig. 1 Tectonic setting of the Xianshuihe fault zone.

图2 降轨T135时空网络基线图

Fig. 2 Spatio-temporal baseline plots of descending track T135.

图3 a 鲜水河断裂带的LOS向速度场; b 标准差

Fig. 3 Line-of-sight velocities across the Xianshuihe fault zone(a), standard deviation(b).

图4 鲜水河断裂带平行于断层的速度黑色箭头表示GPS速度场(同图1), 黑色矩形框表示弹性模型选取的14个剖面的位置, 红色矩形框表示黏弹性地震周期模型中炉霍段跨断层的剖面位置

Fig. 4 Fault-parallel velocities along the Xianshuihe fault zone.

图5 螺旋位错模型示意图

Fig. 5 A schematic of the screw dislocation model.

图6 剖面上与断层平行方向的速率观测值与拟合值的对比粉色点表示InSAR获取的剖面上与断层平行的速度, 蓝色实线表示模型的最佳拟合结果, CC'、 FF'、 II'、 KK'中的蓝色点表示GPS在LOS向投影的速度

Fig. 6 Comparied observed fault-parallel velocities to modeled velocities.

表1 鲜水河断裂带炉霍段的滑动速率估计

Table 1 Slip rate of Luhuo segment, Xianshuihe Fault

地震复发周期/a	黏滞系数/Pa·s	滑动速率/mm·a^-1	闭锁深度/km	WRMS/mm·a^-1
150	5.0×10¹⁸	8.99±0.4	4.73±0.8	0.7886
150	1.0×10¹⁹	9.13±0.4	6.96±1.0	0.7883
150	5.0×10¹⁹	9.85±0.4	8.52±1.2	0.7881
200	5.0×10¹⁸	7.91±0.3	4.67±0.7	0.7889
200	1.0×10¹⁹	8.22±0.3	7.35±1.0	0.7920
200	5.0×10¹⁹	9.56±0.4	9.07±1.2	0.7906
400	5.0×10¹⁸	5.21±0.2	4.43±0.5	0.7984
400	1.0×10¹⁹	5.64±0.2	8.03±1.0	0.7997
400	5.0×10¹⁹	8.26±0.4	11.17±1.1	0.8022

图7 炉霍最佳拟合模型

Fig. 7 Best-fit profile of Luhuo.

图8 深部滑动速率和浅部蠕滑速率沿走向的变化

Fig. 8 Deep and shallow creep rates along the Xianshuihe Fault.

图9 炉霍段不同地震周期阶段的速度模拟

Fig. 9 Calculated velocity profiles across the Luhuo segment of the Xianshuihe Fault at different times after the last earthquake.

图10 基于不同Maxwell松弛时间计算的炉霍地震震后8a跨断层地表速度的模拟值与跨断层观测值的比较

Fig. 10 Comparison of observed surface velocities to calculated velocities across Luhuo segment of Xianshuihe Fault at 8 years after the Luhuo earthquake with different Maxwell relaxation times.

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