Evaluation of investigating magnitude, source mechanism and rupture process based on restored seismic data——Taking the 2013 Lushan MW6.6 earthquake in Sichuan as an example
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摘要:
区域地震波形对于震源研究非常重要，但限幅问题限制了区域地震台网数据的运用，并影响到震源参数测定的准确度.本文利用恢复后的芦山地震区域地震波形，研究了芦山地震的震级、点源机制解以及破裂过程.基于震中距99~300 km恢复前与恢复后地震数据获取的面波震级分别为7.01与7.06级.分别利用7个震中距150~250 km宽频带台站的恢复前和恢复后的数据反演点源机制解，与参考机制解相比，滑动角偏差自13°减小到了4°.基于7个震中距81~134 km的区域地震波形联合远场数据获得的震源破裂过程结果，其主要参数（如滑动分布、破裂速度等）与强地面运动波形联合远场数据得到的结果具有很好的一致性.研究结果表明，本文所采用的数据恢复方法具有较高的可靠性，有效提高了震源参数测定的准确度.
Abstract:Regional seismic waveforms are very useful for investigating earthquake source. However, partial near-field seismic data of Regional Seismic Network are clipped. Therefore, it could not be applied to determine the accurate source parameters directly. In this paper, the magnitude, source mechanism and rupture process of MW6.6 Lushan earthquake were studied based on the restored regional seismic waveforms. The magnitude changed from 7.01 to 7.06 based on clipped and restored surface waves of stations with epicentral distances from 99km to 300 km. Clipped and restored regional waveforms of 7 stations with epicentral distance 150 km to 250 km are utilized to get the source mechanism respectively. Compared to the reference mechanism solution, the misfit of rake angle is reduced from 13° to 4°. Based on 7 regional seismic waveforms with epicentral distance from 81 km to 134 km combined with teleseismic data, the main parameters (such as slip distribution, rupture velocity, etc.) of the rupture process are similar to that of the result constrained by local strong motion data and teleseismic waveforms. These studies verify that this method is reliable and can play an important role in the earthquake source research.
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图 1
用于计算震级的台站分布以及震级差异.(a)台站分布图，五角星和三角形分别表示震中位置以及台站分布; (b)由恢复前和恢复后数据所获取的震级之差随震中距的变化图
Figure 1.
The distribution of stations used to investigate the magnitude and the difference of magnitudes.(a)The distribution of stations. The star and triangles show the location of epicenter and the stations, respectively; (b) The difference between the magnitudes based on clipped data and that based on restored data with epicentral distances
图 2
台站分布与点源机制解比较.(a)台站分布图；红色五角星表示了震中位置，黑色三角形以及黑色台站名表示用来获取参考机制解的台站分布，红色三角形以及红色台站名表示使用到的含有限幅数据的台站分布.(b)基于恢复前和恢复后数据，震源机制与误差函数随深度的变化；红色的沙滩球表示恢复后数据的结果，蓝色的沙滩球表示恢复前数据的结果；未填充颜色的大沙滩球中的黑线、蓝线和红线分别表示参考机制解、恢复前机制解和恢复后机制解的一个节面
Figure 2.
The distribution of stations and comparison of focal mechanisms. (a) The distribution of stations. The red star and black triangles show the location of epicenter and the stations used to get the reference mechanism, respectively. The red triangles indicate the used stations with clipped data. (b) The focal mechanisms and misfits in different depths based on the clipped and restored data. The beach balls connected by the red and blue line represent the results of restored data and clipped data, respectively. The black, blue and red line in the large beach ball represent a nodal plane of the reference mechanism, clipped mechanism and restored mechanism, respectively
图 3
参考机制解的波形比较.细线和粗虚线分别表示实际记录和理论波形，每道数据左边为台站名，N、E和Z分别表示北向、东向和垂向分量，左边数字上下分别表示方位角和震中距(单位km)，右边的数字表示实际记录的峰值(单位cm·s-1)，波形使用实际记录的峰值进行了归一化
Figure 3.
Comparison of waveforms predicted using reference mechanism. Comparison of three-component velocity records (thin solid line) and synthetic seismograms (thick dashed line) predicted using reference mechanism. The station name is indicated at the left side of each seismogram. N, E and Z indicate the North-south component, East-west component and Vertical component, respectively. The value above the beginning of each trace is the source azimuth in degrees, and below is the epicentral distance in kilometers. The peak velocity of the observation in cm/s is indicated above the end of each trace, which is used to normalize both data and the corresponding synthetic seismogram
图 4
恢复后数据在最佳机制解下的波形比较.细线和粗虚线分别表示实际记录和理论波形，每道数据左边为台站名，N、E和Z分别表示北向、东向和垂向分量，左边数字上下分别表示方位角和震中距(单位km)，右边的数字表示实际记录的峰值(单位cm·s-1)，波形使用实际记录的峰值进行了归一化
Figure 4.
Comparison of waveforms predicted by preferred mechanism based on restored data. Comparison of three-component velocity records (thin solid line) and synthetic seismograms (thick dashed line) predicted using reference mechanism. The station name is indicated at the left side of each seismogram. N, E and Z indicate the North-south component, East-west component and Vertical component, respectively. The value above the beginning of each trace is the source azimuth in degrees, and below is the epicentral distance in kilometers. The peak velocity of the observation in cm·s-1 is indicated above the end of each trace, which is used to normalize both data and the corresponding synthetic seismogram
图 5
恢复前数据在最佳机制解下的波形比较.细线和粗虚线分别表示实际记录和理论波形，每道数据左边为台站名，N、E和Z分别表示北向、东向和垂向分量，左边数字上下分别表示方位角和震中距(单位km)，右边的数字表示实际记录的峰值(单位cm·s-1)，波形使用实际记录的峰值进行了归一化
Figure 5.
Comparison of waveforms predicted by preferred mechanism based on clipped data. Comparison of three-component velocity records (thin solid line) and synthetic seismograms (thick dashed line) predicted using reference mechanism. The station name is indicated at the left side of each seismogram. N, E and Z indicate the North-south component, East-west component and Vertical component, respectively. The value above the beginning of each trace is the source azimuth in degrees, and below is the epicentral distance in kilometers. The peak velocity of the observation in cm·s-1 is indicated above the end of each trace, which is used to normalize both data and the corresponding synthetic seismogram
图 6
破裂过程、台站分布与地震矩释放率.(a)强地面运动加远场波形数据获取的破裂过程(Hao et al., 2013), 颜色、箭头和等值线分别表示滑动量的大小、滑动方向以及破裂传播时间.(b)恢复后的区域宽频带数据加远场数据获取的破裂过程.(c)恢复前的区域宽频带数据加远场数据获取的破裂过程.(d)台站分布图.红色五角星表示了震中位置，黑色三角形表示台站分布，黑色矩形框为断层在地表的投影.(e)地震矩释放率随时间变化的比较
Figure 6.
Rupture process, distribution of stations and moment rate. (a) Cross sections of slip distributions of the model constrained by the strong motion and teleseismic data. The red star indicates the hypocenter. The color and the white arrows denote the slip amplitude and direction, respectively. The contours show the rupture initiation time. (b) Cross sections of slip distributions of the model constrained by the restored regional broadband waveforms and teleseismic data. (c) Cross sections of slip distributions of the model constrained by the clipped regional broadband waveforms and teleseismic data. (d) The distribution of stations. The red star and black triangles show the location of epicenter and the used stations, respectively. The black box shows the surface projection of the fault plane used in this study. (e) Comparison of moment rate functions
图 7
远场体波波形比较.细线和粗虚线分别表示实际记录和理论波形，每道数据左边为台站名，P和SH分别表示垂向P波速度记录和切向S波位移记录，左边数字上下分别表示方位角和震中距(单位°)，右边数字表示实际记录的峰值(单位为cm·s-1或cm)，波形使用实际记录的峰值进行了归一化
Figure 7.
Comparison of teleseismic body waveforms. Comparison of teleseimic body waveforms (thin solid line) and synthetic seismograms (thick dashed line). The station name is indicated at the left side of each seismogram. P and SH indicate the Vertical P wave component and Tangential SH component, respectively. The value above the beginning of each trace is the source azimuth in degrees, and below is the epicentral distance in degrees. The peak velocity of the observation in cm·s-1 is indicated above the end of each trace, which is used to normalize both data and the corresponding synthetic seismogram
图 8
远场面波波形比较.细线和粗虚线分别表示实际记录和理论波形，每道数据左边为台站名，UD和SH分别表示垂向记录和切向记录，左边数字上下分别表示方位角和震中距(单位°)，右边的数字表示实际记录的峰值(单位mm)，波形使用实际记录的峰值进行了归一化
Figure 8.
Comparison of teleseismic surface waveforms. Comparison of teleseimic surface waveforms (thin solid line) and synthetic seismograms (thick dashed line). The station name is indicated at the left side of each seismogram. UD and SH indicate the Vertical component and Tangential component, respectively. The value above the beginning of each trace is the source azimuth in degrees, and below is the epicentral distance in degrees. The peak velocity of the observation in mm is indicated above the end of each trace, which is used to normalize both data and the corresponding synthetic seismogram
图 9
区域宽频带波形记录比较.细线和粗虚线分别表示实际记录和理论波形，每一道数据左边为台站名，N、E和Z分别表示北向、东向和垂向分量，左边数字上下分别表示方位角和震中距(单位°)，右边的数字表示实际记录的峰值(单位为cm·s-1)，波形使用实际记录的峰值进行了归一化
Figure 9.
Comparison of regional broadband waveforms. Comparison of three-component velocity records (thin solid line) and synthetic seismograms (thick dashed line). The station name is indicated at the left side of each seismogram. N, E and Z indicate the North-south component, East-west component and Vertical component, respectively. The value above the beginning of each trace is the source azimuth in degrees, and below is the epicentral distance in kilometers. The peak velocity of the observation in cm·s-1 is indicated above the end of each trace, which is used to normalize both data and the corresponding synthetic seismogram
表 1
基于恢复前和恢复后数据的震级
Table 1.
The magnitudes based on clipped data and restored data
台站名 震中距
(km)最大振幅周期(s) 恢复后震级
MS_restored恢复前震级
MS_clippedΔMS XJI 99 25.0 7.5582 7.2694 0.2888 CD2 99 25.0 7.3969 6.9481 0.4488 JYA 106 25.0 7.3994 7.2707 0.1287 YGD 107 25.0 7.5845 7.4078 0.1767 SMI 135 23.0 7.1512 7.1392 0.0120 JJS 167 13.0 6.9923 6.9799 0.0124 MBI 170 17.0 6.9552 6.8947 0.0605 MEK 193 17.0 7.0839 7.0936 -0.0097 YJI 194 16.0 6.8597 6.8680 -0.0083 AXI 202 19.0 7.0058 6.9728 0.0330 JLO 204 23.0 6.9286 6.9470 -0.0184 ZJG 230 16.0 6.9272 6.9393 -0.0121 LBO 231 25.0 7.2607 7.2192 0.0415 MNI 232 20.0 6.8479 6.8646 -0.0167 HWS 249 15.0 7.0333 7.0397 -0.0064 ROC 257 15.0 7.0599 7.0585 0.0014 YAJ 270 25.0 7.0619 7.0675 -0.0056 XSB 275 21.0 6.8928 6.8920 0.0008 BYD 277 22.0 6.8436 6.8274 0.0162 PWU 277 23.0 6.7735 6.7691 0.0044 JLI 277 16.0 6.9233 6.9382 -0.0149 YYC 281 21.0 6.8114 6.8102 0.0012 LZH 282 12.0 7.0510 7.0617 -0.0107 XCO 289 13.0 6.9893 6.9904 -0.0011 RTA 291 21.0 7.0672 7.0649 0.0023 
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表 2
地壳模型
Table 2.
Crust model
P波速度VP
(km·s-1)S波速度VS
(km·s-1)密度ρ
(g·cm-3)层厚度
(km)2.50 1.20 2.10 1.0 4.00 2.10 2.40 1.0 6.10 3.50 2.75 20.0 6.30 3.60 2.80 20.0 7.20 4.00 3.10 4.0 8.08 4.47 3.38
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表 3
点源机制解反演比较
Table 3.
Comparison of focal mechanisms
走向
(°)倾角
(°)滑动角
(°)深度
(km)矩张量
(N·m)恢复前结果 203 50 81 14 8.1×1019 恢复后结果 210 45 90 14 8.3×1019 参考机制解 213 45 94 15 7.5×1019
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表 4
震源参数对比
Table 4.
The comparison of source parameters
参数 Hao等(2013) 本文 标量地震矩 8.9×1018Nm 8.4×1018Nm 矩心深度 12.5 km 13.6 km 峰值滑动深度 15.1 km 13.8 km 主要滑动范围 ~5-20 km ~6-21 km 峰值滑动量 1.2 m 1.1 m 平均滑动量 0.4 m 0.4 m 破裂持续时间 10.3 s 9.6 s 破裂速度 2.0 km·s-1 2.1 km·s-1
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