Are there links between the localized deformation in the northeastern section of Wenchuan earthquake source zone and the May 25，2008 Qingchuan strong aftershock?
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摘要: 2008年汶川地震之后，通过InSAR观测到青川县木鱼镇附近存在一个长约为15 km、宽约为10 km、地表位移数十厘米的局部形变区。前人分析认为，该形变区是由MS6.4的青川强余震造成的，但拟合地表形变数据所采用的震源深度和震源机制解与地震学反演的结果具有较大差异。本文利用远震体波和瑞雷波振幅谱进一步测定了青川强余震的震源深度和震源机制解，计算了此次事件造成的地表位移场，认为青川强余震并非造成木鱼镇地区局部形变的直接原因，并讨论该局部形变区可能的成因。Abstract: A localized deformation zone near Muyu town of Qingchuan county was observed on the InSAR interferograms of the great 2008 Wenchuan earthquake. The Muyu deformation zone was about 15 km long and 10 km wide, with line of sight displacement up to dozens of centimeters. Some researchers proposed that the deformation zone is caused by a strong aftershock （MW6.1） in Qingchuan county on May 25, 2008, and they adopted seismic source parameters very different from seismic inversions. To verify reliability of the seismic source inversions, we use teleseismic body wave and local Rayleigh wave spectra to constrain focal depth and fault plane of the strong aftershock. Thereafter, we compute ground displacement and proposed that the aftershock was not the direct cause of the localized deformation zone. Then we discuss some candidate mechanisms which might explain the localized deformation.
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Keywords:
- focal depth /
- source mechanism /
- ground deformation /
- InSAR
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图 1 汶川地震震源区东北端InSAR位移场及余震分布(修改自Hashimoto et al，2009 )
Figure 1. Deformation observed by InSAR and after-shocks (circles) in the northeastern section of Wenchuan earthquake source zone（modified from Hashimoto et al, 2009 ）
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图 2 青川强余震及用于震源参数反演的远震台站分布（a）和波形拟合残差随深度分布（b）
Figure 2. Teleseismic stations and the Qingchuan strong aftershocks （a） as well as waveforms misfit versus depth （b）
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图 3 青川强余震震源机制解及CAPtel反演后的远震体波观测波形（黑色）与理论波形（红色）对比图
台网及台站代号标记在波形左侧上方，台站下方分别为震中距/方位角，波形下方的两行数字分别为波形对齐所需的时移 （上行）和相关系数百分值 （下行）
Figure 3. Comparison between observed (black) and synthetic (red) seismograms after the CAPtel inversion of the Qingchuan strong aftershock.
Seismic network and stations are labeled to the left of the traces，while epicentral distance and azimuth are displayed below the station code，time shift （above） and percentile of cross correlation coefficient （below） are displayed under the waveforms
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图 4 ABKT地震台的远震直达P波及其深度震相观测与理论波形对比
P波按照实际到时对齐，理论地震图所使用的深度标记于波形的下方
Figure 4. Observed and synthetic P，pP and sP for station ABKT
The waveforms are aligned on onset of P wave，and the focal depth used in synthetics are displayed below each trace
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图 5 恩施地震台的三分量波形对比
Figure 5. Comparison among radial，tangential and vertical waveform recorded by station ENH
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图 6 恩施台的振幅谱 （a），频散曲线测量图 （b） 以及时间域地震波形图 （c）
Figure 6. Spectral amplitude versus period （a），group velocity versus period （b） and seismograms （c） at station ENH
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图 7 恩施台的瑞雷波频谱极小点周期与震源深度的关系
Figure 7. The dependence of period at spectral null and focal depth
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图 8 不同质心深度时的InSAR位移场
断层位错上端点深度分别为 6.02 km （a）， 11.02 km （b）， 16.02 km （c）及21.02 km （d）
Figure 8. Prediction InSAR deformation with different depths
The depth of the upper edge of the ruptured fault is placed at 6.02 km （a），11.02 km （b）， 16.02 km （c） and 21.02 km （d），respectively
表 1 汶川强余震目录
Table 1 Catalog of strong aftershocks of the Wenchuan earthquake
序号 发震时刻 （UTC） 震中位置 震源
深度/kmMS MS7 mL mb mB MW 年-月-日 时：分：秒 东经/° 北纬/° 1 2008-05-12 06：43：14 103.82 31.27 14 6.3 5.9 5.7 6.2 6.3 2 2008-05-12 06：54：16 103.59 31.26 13 5.8 5.8 5.3 5.8 6.2 3 2008-05-12 11：11：01 103.67 31.26 14 6.3 6.2 5.8 5.8 6.2 6.1 4 2008-05-13 07：07：08 103.42 30.95 14 6.1 5.9 5.7 5.6 6.0 5 2008-05-17 17：08：24 105.08 32.20 13 6.1 5.9 5.9 5.5 6.1 6 2008-05-25 08：21：47 105.48 32.55 14 6.4 6.2 6.3 5.7 6.1 6.1 7 2008-07-24 07：09：27 105.61 32.76 10 6.0 5.6 5.7 5.7 5.7 8 2008-08-01 08：32：41 104.85 32.02 14 6.2 6.0 5.9 5.6 6.1 9 2008-08-05 09：49：15 105.61 32.72 13 6.5 6.3 6.0 5.8 6.3 6.0 注：表中序号为3，6，9的数据来自GCMT （2008），其余来自中国地震台网中心 （2008）。 
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表 2 2008年5月25日青川MW6.1余震震源参数对比
Table 2 The source parameters of the May 25，2008 MW6.1 Qinchuan aftershock from various authors or agencies
数据来源 震中位置 深度/km 节面Ⅰ 节面Ⅱ 东经/° 北纬/° 走向/° 倾角/° 滑动角/° 走向/° 倾角/° 滑动角/° GCMT （2008） 105.45 32.57 27 59 84 178 149 88 6 郑勇等 （2009） 105.39 32.62 18 63 64 −171 329 82 −26 郭祥云等 （2010） 251 74 170 344 80 16 吕坚等 （2008） 105.37 32.62 19* 中国地震台网中心 （2008） 105.48 32.55 14* 注：深度为质心深度，标*为破裂起始深度。
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表 3 理论地震图及地表形变使用的速度结构模型
Table 3 Crustal structure used in this study
底部深度/km vP/(km·s−1） vS/（km·s−1） 密度/（103 kg·m−3） 1 2.50 1.00 1.20 2 4.00 2.10 2.40 22 6.10 3.50 2.75 42 6.30 3.60 2.80 46 7.20 4.00 3.10 ∞ 8.00 4.47 3.35
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