| 龙门山断裂带中上地壳速度结构的短周期环境噪声成像 |
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| 引用本文： | 赵盼盼,陈九辉,刘启元,郭飚,李顺成,李昱.龙门山断裂带中上地壳速度结构的短周期环境噪声成像[J].地球物理学报,2015,58(11):4018-4030. |
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| 作者姓名： | 赵盼盼 陈九辉 刘启元 郭飚 李顺成 李昱 |
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| 作者单位： | 中国地震局地质研究所地震动力学国家重点实验室, 北京 100029 |
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| 基金项目： | 国家自然科学基金项目(41274060)和地震动力学国家重点实验室(LED2010A01)联合资助. |
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| 摘 要： | 本文利用在龙门山断裂带周边布设的57个台站自2008年11月至2009年11月为期一年的垂直分量连续地震环境噪声数据,通过短周期地震环境噪声成像方法,获得了龙门山断裂带中北段地壳25km深度范围的S波精细速度结构.结果表明:(1)龙门山断裂带周边区域10km以上的速度结构与地表断裂的分布形态具有良好的一致性,速度结构控制了龙门山主要断层的深部延展特征;在15km及以下深度,S波速度结构呈现沿龙门山和沿岷山隆起走向的交叉构造格局,由此造成的速度结构差异可能影响了汶川地震的破裂过程;(2)速度结构随深度的分布特征为龙门山断裂带主要断层的深部延伸形态给出了良好的约束,结果进一步确认了龙门山断裂中段的高角度铲型断裂构造特征;(3)研究区的南端发现了龙门山断裂下方20km以下深度具有与松潘地块中地壳低速层相关的低速结构的迹象,这可能是汶川地震破裂带南段22km左右深度存在脆韧转换带的一个证据.研究结果显示出密集台阵和短周期环境噪声成像方法在地壳浅部精细结构和断层探测研究中具有巨大潜力.
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| 关 键 词： | 龙门山断裂带 速度结构 地震环境噪声 汶川地震 龙门山断裂带 速度结构 地震环境噪声 |
| 收稿时间： | 2015-06-10 |
Fine structure of middle and upper crust of the Longmenshan Fault zone from short period seismic ambient noise |
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| ZHAO Pan-Pan,CHEN Jiu-Hui,LIU Qi-Yuan,GUO Biao,LI Shun-Cheng,LI Yu.Fine structure of middle and upper crust of the Longmenshan Fault zone from short period seismic ambient noise[J].Chinese Journal of Geophysics,2015,58(11):4018-4030. |
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| Authors: | ZHAO Pan-Pan CHEN Jiu-Hui LIU Qi-Yuan GUO Biao LI Shun-Cheng LI Yu |
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| Institution: | State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China |
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| Abstract: | On 12 May 2008, the Mw7.9 earthquake occurred on the Longmenshan (LMS) fault zone in Sichuan province of China. Geological surveys, inversion of the seismic rupture process and aftershock relocation indicate that this temblor resulted from a thrust on a high-angle fault with right-slip component. The study of the fine velocity structure of this fault and earthquake-generating environment is of great importance to understanding the tectonic setting and dynamic process of the Wenchuan earthquake as well as the dynamics of the east edge of the Tibet Plateau.Seismic ambient noise tomography has been proved to be a new and efficient way to study the crust and upper mantle structure in recent years. This method extracts surface wave Green's Function between station pairs from continuous ambient noise data, uses the surface wave tomography and S wave inversion method to obtain the velocity structure. Since the spatial resolution for ambient noise tomography mainly depends on the distribution of the stations, short period surface waves from ambient noise have been increasingly used to study the shallow structure of faults with dense seismometer arrays.This work is based on the vertical component data of 57 stations of the array across the LMS fault for 1 year (2008-11-2009-11). It utilizes the short-period seismic ambient noise imaging to invert the S-wave velocity structure of the top 25 km of the central and northern LMS zone. The procedures include: ambient noise data preprocessing, computing cross-correlation functions, measuring group velocity dispersion curves, surface wave tomography, and S-wave velocity inversion. During the preprocessing, a method with short time windows is used instead of time domain normalization (e.g. one-bit) to improve the SNR of cross-correlation functions. This study implements a frequency time analysis to measure Rayleigh wave group velocity for periods of 1~25 s, and then uses a generalized inversion program to generate group velocity maps at 2~20 s. According to the checkboard tests, the lateral resolution is about 10 km. At last, this thesis inverts the group velocity at each grid node for S-wave velocity using a linearized inversion method.The results show that (1) Velocity structure above 10 km keeps good consistency with the surface fault system around Longmenshan, and controls the deep extension features of most major faults. Below the depth of 15 km, the velocity structure presents cross tectonic framework along both Longmenshan and Minshan. The complex structure may have affected the rupture process of the Wenchuan earthquake. (2) The depth velocity structure profiles give good constraint for the deep geometry of main faults. The characteristics of the high angle, listric, reverse structure of the Longmenshan faults is further confirmed by our results.(3) At southern part of the study area, low-velocity structure is found at about 20 km depth beneath the Pengguan massif, which is related to the low velocity layer in the middle crust of Songpan-Garzê block. This may be an evidence for the existence of brittle-ductile transition zone in southern part of the rupture zone of the Wenchuan earthquake at the depth around 22 km. Our results show the great potential of short period ambient noise tomography with data from dense passive seismic array in the study of fine velocity structure and fault zone imaging. |
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| Keywords: | Seismic ambient noise Wenchuan earthquake Longmenshan fault Fine velocity structure Seismic ambient noise |
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