| 用接收函数建立区域模型的震源机制反演及其在芦山地震序列研究中的应用 |
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| 引用本文： | 杨宜海,梁春涛,苏金蓉.用接收函数建立区域模型的震源机制反演及其在芦山地震序列研究中的应用[J].地球物理学报,2015,58(10):3583-3600. |
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| 作者姓名： | 杨宜海 梁春涛 苏金蓉 |
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| 作者单位： | 1. 成都理工大学地质灾害防治与地质环境保护国家重点实验室, 成都 610059;
2. 成都理工大学地球探测与信息技术教育部重点实验室, 成都 610059;
3. 四川省地震局, 成都 610041 |
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| 基金项目： | 国家自然科学基金(41340009,41374058,U1262206),成都理工大学地质灾害防治与地质环境保护国家重点实验室自主课题基金,成都理工大学四川盆地及周边地震活动性研究创新团队联合资助. |
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| 摘 要： | 本文提出并试验了一种基于接收函数建立区域模型进行震源机制反演的方法.选取四川地震台网记录的M≥3且信噪比高的近震波形资料,反演得到了芦山地震序列中74个地震的震源机制.通过对震源深度和震源机制的综合分析,探讨了芦山地震的发震构造和区域应力场状态.采用接收函数方法反演获取了26个台站下方的S波速度结构,对不同区域的台站反演结果进行叠加平均,以此区域平均S波速度作为本文震源机制反演使用的区域模型的S波速度;区域模型的P波速度由经验公式给出.反演稳定性测试表明,使用不同模型或对原始波形记录加入随机噪声的反演结果与原始反演相比,震源深度最大误差为1km,断层面各参数误差水平也很低,且显示的发震类型是一致的,其中随机噪声带来的误差小于模型带来的误差.主震反演得到的震源机制解为:震源深度17km,矩震级6.47;节面Ⅰ走向213°,倾角51°,滑动角98°;节面Ⅱ走向20°,倾角40°,滑动角80°;显示芦山主震可视为纯逆冲型地震,发震构造可能是某个具有较大倾角的逆冲断层,而不是低缓的推覆构造的基底滑脱面.同时本文反演获取的73个M≥3余震的震源机制绝大多数也显示了类似的发震类型,逆冲型地震为67个,占92%,具有绝对优势;走滑型地震为5个,正断型地震为1个.其中5个走滑型地震中的4个均分布在震源区的东北端.整个芦山地震序列深度集中在12~20km,且沿震源区短轴的余震深度剖面有自西向东呈逐步变浅的趋势,呈现清晰的铲形断面结构,结合本地地质构造,可以推断芦山地震序列主要发生在龙门山前山断裂以东的逆冲推覆体内的一个隐伏断裂上.P轴方位角优势方位与区域应力场及汶川震源区南段的相一致,表明芦山序列地震活动主要受区域应力场控制,且汶川震后该区应该不存在应力场变化.P轴仰角随深度分布则显示了孕震层在浅部为脆性上地壳,而深部已经进入了中地壳低速层.断层面的几何形态简单,倾角均值在不同深度保持稳定在55°左右,与主震倾角接近,这与汶川震源区南段的研究结果明显不同,揭示了龙门山断裂带南段与此次芦山发震断裂在断层面几何形态上的明显差异.
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| 关 键 词： | 芦山地震 震源机制 接收函数 震源深度 全波形反演 发震构造 |
| 收稿时间： | 2014-09-22 |
Focal mechanism inversion based on regional model inverted from receiver function and its application to the Lushan earthquake sequence |
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| YANG Yi-Hai,LIANG Chun-Tao,SU Jin-Rong.Focal mechanism inversion based on regional model inverted from receiver function and its application to the Lushan earthquake sequence[J].Chinese Journal of Geophysics,2015,58(10):3583-3600. |
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| Authors: | YANG Yi-Hai LIANG Chun-Tao SU Jin-Rong |
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| Institution: | 1. State Key Lab. of Geohazard Prevention and Geoenviroment Protection, Chengdu University of Technology, Chengdu 610059, China;
2. Key Lab. of Earth Exploration and Information Technique of Education Ministry of China, Chengdu University of Technology, Chengdu 610059, China;
3. Earthquake Administration of Sichuan Province, Chengdu 610041, China |
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| Abstract: | In this paper, we propose and test a focal mechanism inversion algorithm that uses receiver function inversion to obtain S-wave velocity of a regional model. Totally 74 focal mechanisms of the Lushan earthquake sequence have been determined by selecting high signal-to-noise ratio near field waveforms from the Sichuan Seismic Network. We analyze the focal mechanism solutions and depths to discuss the seismogenic structure and stress field. We use receiver function inversion to obtain the S-wave velocity structures of the crust and upper mantle beneath 26 stations, and then stack S velocity models beneath stations in one area to get the average model for corresponding area. The P-wave velocity is derived using the equation Vp=1.732 Vs, here Vs is the average S-wave velocity model. Vp and Vs constitute the regional model used in moment tensor inversion. Comparing the results of different models or the original waveforms added with random noise with the original result, the inversion stability test shows a maximum depth error of 1km. Fault plane parameters are also at low level and reflect the same seismogenic type with the original result. The error from model is less than that from random noise.
The focal mechanism solution shows the mainshock to be a nearly pure thrust with depth of 17km, and MW of 6.47; the two nodal planes' parameters are: strike=213°, dip=51°, rake=98°; and strike=20°, dip=40°, rake=80°, respectively. The seismogenic structure is a thrust fault with a large dip angle, and may not be the basal slip plane of lower nappe structure. In statistics, 67 of 73 aftershocks with M≥3 that account for 92% of the total bear thrust fault types; 5 events are strike-slip, and one is normal faulting. Four of the five strike-slip type events are located in the northeast side of the source area. The depths of the Lushan earthquake sequence are between 12 and 20 kilometers, and the depths of aftershocks are gradually shallower from west to east along the profile of the short axis of the source area, showing a clear listric fault plane structure. We can infer that the Lushan earthquake sequence occurs mainly along a buried fault in the thrust nappe structure in the east of the Longmenshan piedmont fault. The dominant orientations of P-axis are consistent with the regional stress field and the southern segment of the Wenchuan earthquake source area, showing that the Lushan earthquake sequence is mainly controlled by the regional stress field, and the stress field does not change after the Wenchuan earthquake. P-axis plunges with depth distribution reflect a brittle upper crust and a low velocity layer in middle crust. The stable mean dip with depth distribution is about 55°. This is significantly different from the result of the southern segment of the Wenchuan earthquake source area, indicating the variation of the fault plane geometry between the two segments. |
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| Keywords: | Lushan earthquake Focal mechanism Receiver function Source depth Full waveform inversion Seismogenic structure |
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