基于近场强震资料的汶川地震破裂过程反演
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摘要
5.12汶川大地震发震断层复杂、破裂尺度大、强震动影响范围广，其震源过程的研究对于揭示大地震断层破裂机理具有重要意义。震后，一些研究小组基于远场长周期波形观测资料反演了断层破裂过程，结果表明汶川地震震源破裂持续时间长达100s，较大错动量的主要区域分布在北川、映秀附近。
迄今为止，几乎所有对于汶川地震震源特性的研究都是基于远场长周期波形数据进行的，由于选取的频率上限较低难以揭示更为精细的震源破裂过程。汶川地震中，国家强震动台网中心记录了大量的近场强震记录。同时，震后调查获得了大量地表破裂数据。本文基于近场的48个强震台记录并结合地表破裂条件，采用非负最小二乘法和多视窗技术，对震源过程进行了反演，就其静态和动态破裂过程进行了详细的分析。反演的断层模型中，考虑了中央断裂和前山断裂同时发震，同时利用两段不同倾角的断层模拟了中央断裂曲面断层的影响。
反演结果显示的主要结论如下：
（1）汶川地震的震源破裂过程异常复杂，震源总破裂持续时间长达90s，并且断层的北部区域的破裂速度普遍高于南部区域，其平均破裂速度在3.1km/s左右。
（2）静态破裂分析显示，主要错动发生在断层倾向深度15km以上的浅层区域，在中央断裂的中北部区域的映秀附近，汶川附近和北川附近形成3个明显的Asperity，最大的错动量为8.2m发生在映秀附近；前山断裂倾向深度10km以上的浅层区域存在两个小的Asperity，最大错动5.2m出现在汶川附近。
（3）动态破裂分析显示，破裂波前十分不规则，破裂从震源开始向东北和西南双向扩展，断层面上的南北两个部分呈现出截然不同的破裂状态。断层的中北部区域滑移速率普遍较大，总的持续时间相对较长，特别是映秀附近的Asperity区域的破裂持续时间长达30s以上，并伴随着断层的破裂停顿和重复破裂；而断层南部区域滑移速率普遍较短，总的持续时间相对较短。从动态破裂过程分析中可以看出，这些Asperity区域的破裂持续时间较长，大多超过了25s，子断层的最大滑移时间发生在中央断裂映秀附近，长达58s。
本文的研究结果提高了汶川地震破裂细节的认识，同时反演所得震源模型也可用于模拟汶川地震近场强地面运动。
As a big earthquake with complicated and large rupture fault, the5.12Wenchuan earthquake is important for realizing the large earthquake sourcemechanism. After the earthquake, several research groups have implementedthe source inversion using the far-field long-period teleseismic waveform dataand different fault model. Most results show that the earthquake rupturedmore than100s, and it formed2asperities in Yingxiu and Beichuan area.
By now, all studies on source inversion of Wenchuan earthquake arebased on far-field long-period teleseismic waveform data, which can hardlygive the detail of the rupture due to the use of low frequency wave.
During the earthquake, the strong-motion network of China EarthquakeAdministration recorded many three-component accelerograms and a lot ofsurface offset data are acquired through the field investigations. So in thispaper, based on3components velocity records of48near field strong motionstation and the field investigated surface rupture data, we inverse the sourcerupture process of the Wenchuan earthquake by the non-negative least-squaresmethod and the multiple-time widow technique. Two causative faults aretaking into inverse; one is the Beichuan-Yingxiu which is represented as acurved fault, the other is Jiangyou-Guanxian fault which is a single fault. Themain results include:
(1) The fault rupture process are very complex, which lasted for90s, andthe rupture front propagate rapidly in the north direction with average velocityof about3.1km/s than that in the southwest direction.
(2) In the middle and north part of the faults, whose surface near Yingxiu,Wenchuan, and Beichuan area, the fault persisted for a majority of slipsequence. The rupture result in3asperities in Beichuan fault with maximumslip of the8.2m and2small asperities in the Pengguan fault with maximumslip of5.2m.
(3) The dynamical rupture process show that the rupture fronts are veryirregular, and it propagated in two directions (northeast and southwest), withdifferent rupture phenomenon in the north and south parts. In northern area,slip velocity is generally high and the total rupture duration time is relativelylonger than that in the south part, especially for the Yingxiu asperity regionwith rupture duration up to more than30s, and accompanied by rupture pauseand re-rupture. In the southern region, slip velocity is generally low, and thetotal rupture duration time is relatively short. In these asperities, the slipduration time sustained relatively long with most of them over25s, and thelongest one is more than58s in the Yingxiu area.
迄今为止，几乎所有对于汶川地震震源特性的研究都是基于远场长周期波形数据进行的，由于选取的频率上限较低难以揭示更为精细的震源破裂过程。汶川地震中，国家强震动台网中心记录了大量的近场强震记录。同时，震后调查获得了大量地表破裂数据。本文基于近场的48个强震台记录并结合地表破裂条件，采用非负最小二乘法和多视窗技术，对震源过程进行了反演，就其静态和动态破裂过程进行了详细的分析。反演的断层模型中，考虑了中央断裂和前山断裂同时发震，同时利用两段不同倾角的断层模拟了中央断裂曲面断层的影响。
反演结果显示的主要结论如下：
（1）汶川地震的震源破裂过程异常复杂，震源总破裂持续时间长达90s，并且断层的北部区域的破裂速度普遍高于南部区域，其平均破裂速度在3.1km/s左右。
（2）静态破裂分析显示，主要错动发生在断层倾向深度15km以上的浅层区域，在中央断裂的中北部区域的映秀附近，汶川附近和北川附近形成3个明显的Asperity，最大的错动量为8.2m发生在映秀附近；前山断裂倾向深度10km以上的浅层区域存在两个小的Asperity，最大错动5.2m出现在汶川附近。
（3）动态破裂分析显示，破裂波前十分不规则，破裂从震源开始向东北和西南双向扩展，断层面上的南北两个部分呈现出截然不同的破裂状态。断层的中北部区域滑移速率普遍较大，总的持续时间相对较长，特别是映秀附近的Asperity区域的破裂持续时间长达30s以上，并伴随着断层的破裂停顿和重复破裂；而断层南部区域滑移速率普遍较短，总的持续时间相对较短。从动态破裂过程分析中可以看出，这些Asperity区域的破裂持续时间较长，大多超过了25s，子断层的最大滑移时间发生在中央断裂映秀附近，长达58s。
本文的研究结果提高了汶川地震破裂细节的认识，同时反演所得震源模型也可用于模拟汶川地震近场强地面运动。
As a big earthquake with complicated and large rupture fault, the5.12Wenchuan earthquake is important for realizing the large earthquake sourcemechanism. After the earthquake, several research groups have implementedthe source inversion using the far-field long-period teleseismic waveform dataand different fault model. Most results show that the earthquake rupturedmore than100s, and it formed2asperities in Yingxiu and Beichuan area.
By now, all studies on source inversion of Wenchuan earthquake arebased on far-field long-period teleseismic waveform data, which can hardlygive the detail of the rupture due to the use of low frequency wave.
During the earthquake, the strong-motion network of China EarthquakeAdministration recorded many three-component accelerograms and a lot ofsurface offset data are acquired through the field investigations. So in thispaper, based on3components velocity records of48near field strong motionstation and the field investigated surface rupture data, we inverse the sourcerupture process of the Wenchuan earthquake by the non-negative least-squaresmethod and the multiple-time widow technique. Two causative faults aretaking into inverse; one is the Beichuan-Yingxiu which is represented as acurved fault, the other is Jiangyou-Guanxian fault which is a single fault. Themain results include:
(1) The fault rupture process are very complex, which lasted for90s, andthe rupture front propagate rapidly in the north direction with average velocityof about3.1km/s than that in the southwest direction.
(2) In the middle and north part of the faults, whose surface near Yingxiu,Wenchuan, and Beichuan area, the fault persisted for a majority of slipsequence. The rupture result in3asperities in Beichuan fault with maximumslip of the8.2m and2small asperities in the Pengguan fault with maximumslip of5.2m.
(3) The dynamical rupture process show that the rupture fronts are veryirregular, and it propagated in two directions (northeast and southwest), withdifferent rupture phenomenon in the north and south parts. In northern area,slip velocity is generally high and the total rupture duration time is relativelylonger than that in the south part, especially for the Yingxiu asperity regionwith rupture duration up to more than30s, and accompanied by rupture pauseand re-rupture. In the southern region, slip velocity is generally low, and thetotal rupture duration time is relatively short. In these asperities, the slipduration time sustained relatively long with most of them over25s, and thelongest one is more than58s in the Yingxiu area.
引文
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[3]邓起东,陈社发,赵小麟，等.龙门山及其邻区的构造和地震活动及动力学[J].地震地质,1994,16(4):389-403.
[4]丁志峰,何正勤,孙为国,等.青藏高原东部及其边缘地区的地壳上地慢三维速度结构[J].地球物理学报,1999,2.
[5]黄媛,吴建平,张天中,等.汶川8.0级大地震及其余震序列重定位研究[J].中国科学,2008,38,1242–1249.
[6]李传友,宋方敏,冉勇康.龙门山断裂带北段晚第四纪活动性讨论[J].地震地质,2004,26(2):248-258.
[7]李廷栋,郑英龙.青藏高原地质调查研究历史[M].青藏高原的构造演化与隆升机制.广州:广东科技出版社,2000.297-311.
[8]刘启元,李昱,陈九辉,等.汶川8.0地震:地壳上地幔S波速度结构的初步研究[J].地球物理学报,2009,52(2)：309-319.
[9]马宏生,张国民,闻学泽,等.川滇地区三波速度结构反演与构造分析[J].地球科学—中国地质大学学报.2008,33(5):591-602.
[10]滕吉文,熊绍柏.喜马拉雅山北部地区的地壳结构模型和速度分布特征[J].地球物理学报,1983,26(6):525-540.
[11]王椿镛,D.Mooney,W.,川滇地区地壳上地幔三维速度结构研究[J].地震学报.2002.24,1-16.
[12]王椿镛,吴建平,楼海,等.川西藏东地区的地壳P波速度结构[J].中国科学:D辑.2003,33,181–189.
[13]王卫民,等.GPS测定的2008年汶川Ms8.0级地震的同震位移场[J].中国科学D辑.2008.38,1195-1206.
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