| 芦山震区地壳三维P波速度精细结构及地震重定位研究 |
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| 引用本文： | 王小娜,于湘伟,章文波.芦山震区地壳三维P波速度精细结构及地震重定位研究[J].地球物理学报,2015,58(4):1179-1193. |
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| 作者姓名： | 王小娜 于湘伟 章文波 |
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| 作者单位： | 1. 中国科学院大学地球科学学院, 北京 100049;
2. 中国科学院计算地球动力学重点实验室, 北京 100049 |
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| 基金项目： | 国家自然科学基金(41374105,41274068),中国科学院、国家外国专家局创新团队国际合作伙伴计划(KZZD-EW-TZ-19)和中国科学院知识创新工程重要方向项目(KZCX2-EW-QN603)联合资助. |
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| 摘 要： | 联合芦山地震序列5285个地震的50711条P波初至绝对到时数据及7294691条高质量的相对到时数据,利用双差地震层析成像方法联合反演了芦山震源区高分辨率的三维P波速度精细结构及5115个地震震源参数.反演结果表明,芦山主震震中为30.28°N,103.98°E,震源深度为16.38km,主震南西段余震扩展长度约23km,余震前缘倾角较和缓,主震北东段余震扩展长度约12km,余震前缘呈铲形,倾角较陡.芦山震源区P波三维速度结构表现出明显的横向不均匀性,近地表处的P波速度异常与地形起伏及地质构造密切相关:宝兴杂岩对应明显的高速异常,此异常由地表延伸到地下15km深度附近,而中新生代岩石表现为低速异常;大兴附近区域亦显示出小范围的大幅度高速异常,宝兴高速异常与大兴高速异常在10km深度附近相连,进而增加了芦山震源区的高低速异常对比幅度.在芦山主震的南西、北东两段速度结构存在着较大差异,芦山主震在水平向位于宝兴及大兴高速异常所包围的低速异常的前缘.主震南西段余震主要发生在倾向北西的高低速异常转换带上并靠近低速一侧,其下盘为低速异常,上盘为高速异常.而芦山主震北东段的余震主要分布在宝兴高速体与大兴高速体之间,主发震层向北西倾斜,主发震层上方的宝兴高速异常下边界出现一条南东倾向的反冲地震带,两地震带呈"y"型分布.
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| 关 键 词： | 芦山地震 双差地震层析成像 三维P波速度结构 震源参数 芦山震源区 |
| 收稿时间： | 2014-03-19 |
3D P-wave velocity structure of the crust and relocation of earthquakes in the Lushan source area |
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| WANG Xiao-Na,YU Xiang-Wei,ZHANG Wen-Bo.3D P-wave velocity structure of the crust and relocation of earthquakes in the Lushan source area[J].Chinese Journal of Geophysics,2015,58(4):1179-1193. |
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| Authors: | WANG Xiao-Na YU Xiang-Wei ZHANG Wen-Bo |
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| Institution: | 1. College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China;
2. Key Laboratory of Computation Geodynamics, CAS, Beijing 100049, China |
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| Abstract: | Many Chinese researchers have investigated the Lushan earthquake source area with geological and geophysical approaches since Lushan earthquake happened. Compared with the previous tomographic studies, we have used a much large data set and an updated tomographic method to determine a small scale three-dimensional P wave velocity structure with spatial resolution less than 5km, which plays the important role for understanding the deep structure and the genetic mechanism beneath Lushan area. Furthermore, our studies focus on the relation between relocated focal distribution and 3D P wave velocity anomaly.The double difference seismic tomography method is applied to 50711 absolute first arrival P wave arrival times and 7294691 high quality relative P arrival times of 5285 events of Lushan seismic sequence to simultaneously determine the detailed crustal 3D P wave velocity structure and the hypocenter parameters in the Lushan seismic area. This method takes account of the path anomaly biases explicitly by making full use of valuable information of seismic wave propagation jointly with absolute and relative arrival time data. In this study, we use an evolving weighting scheme for the absolute and differential arrival times in order to resolve the entire area covered by the ray and the detailed structures around the hypocenters outside the source region.The results show that Lushan mainshock locates at 30.28°N, 103.98°E, with the depth of 16.38 km. The front edge of aftershocks in the northeast of mainshock is listric with a steep dip angle, the aftershocks extended about 12 km in this direction. In the southwest of Lushan mainshock, the front edge of aftershocks is in a low velocity zone with a gentle dip angle, the aftershocks extended about 23 km on this side. Our high-resolution tomographic model not only displays the general features contained in the previous models, but also reveals some new features. The Tianquan, Shuangshi and Daguan line lies in the transition zone between high velocity anomalies to the southeast and low velocity anomalies to the northwest at the ground surface. An obvious high-velocity anomaly is visible in Daxing area. With the depth increasing, Baoxing high velocity anomaly extends to Lingguan, while the southeast of the Tianquan, Shuangshi and Daguan line still shows low velocity. The high-velocity anomalies beneath Baoxing and Daxing connect to each other in 10 km depth, which makes the contrast between high and low velocity anomalies sharper. Above 20 km depth, the velocity structure in southwest and northeast segment of mainshock shows a big difference:low-velocity anomalies are dominant in the southwest segment, while high-velocity anomalies dominate the northeast segment. Lushan aftershocks in southwest are distributed in low-velocity anomalies or the transition belt:the footwall represents low-velocity anomalies, while the hanging wall shows high-velocity anomalies. The northeastern aftershocks are distributed at the boundary between high-velocity anomalies in Baoxing and Daxing area.The P wave velocity structure of Lushan seismic area shows obviously lateral heterogeneity. The P wave velocity anomalies have a close relationship with topographic relief and geological structure. In Baoxing area the complex rocks correspond to obvious high-velocity anomalies extending down to 15 km depth,while the Cenozoic rocks are correlated with low-velocity anomalies. Lushan mainshock locates at the leading edge of a low-velocity anomaly surrounded by the Baoxing and Daxing high-velocity anomalies. The main seismogenic layer dips to northwest. Meanwhile, a recoil seismic belt dipping to southeast above the main seismogenic layer exists at the lower boundary of Baoxing high-velocity anomaly. A "y" distribution pattern is shown between two seismic belts. |
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| Keywords: | Lushan earthquake The double difference seismic tomography method Crustal 3D P wave velocity structure Hypocenter parameters Lushan seismic area |
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