P-wave velocity changes in the source area before and after two moderate strong earthquakes in Yingjiang， Yunnan in 2014
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摘要: 基于云南地震台网的地震记录，利用基于双差层析成像的时移层析成像方法，开展了2014年5月24日盈江MS5.6和5月30日MS6.1地震前后震源区地下P波速度变化的时空特征研究。结果表明：在盈江MS5.6地震后，震源区的P波速度轻微下降，在MS6.1地震后，相对于MS5.6地震，P波速度继续下降，并下降至最低，下降幅度约为1%，说明P波速度下降的幅度可能与主震的震级大小相关。另外，本研究还观测到P波速度下降与余震分布的时空变化相关，可能是由于余震的动态和静态应力变化造成震源区介质物理性质发生改变，从而导致地震波速度的变化。在两次地震发生后约五年内，震源区P波速度值上升，上升幅度小于震后总下降幅度，但这并不能表明震源区还处于愈合过程中。Abstract: The time-lapse tomography method based on double-difference tomography can obtain high-precision seismic wave velocity changes. In this study, we applied this method to the seismic data set that recorded by the Yunnan seismic network to obtain the spatiotemporal changes of P-wave velocity in the source area during the MS5.6 and MS6.1 Yingjiang earthquakes in 2014. The results showed that the spatial and temporal characteristics of the P-wave velocity of the crust at depth around the source area suggest co-seismic velocity reductions. After the MS5.6 earthquake occurred on May 24, the P-wave velocity of the crust dropped slightly. After the MS6.1 earthquake occurred on May 30, the P-wave velocity dropped definitely with an amplitude of about 1%. It showed that the amplitude of the reduction is related to the magnitude of the mainshock. We also reported that P-wave velocity is also related to the the spatio-temporal changes of aftershocks distribution. It is possible that of dynamic and static stresses changes of the aftershocks caused changes of the physical properties in the source area which leads to the velocity change. Finally, about five years after the Yingjiang earthquakes, a healing process is observed. And the rising level did not reach the previous falling level. However, this does not mean that the source area is still in the healing process.
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图 2 研究区内地震分布及网格划分
Figure 2. Seismic stations used in this study and grid nodes division in the studied area
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图 4 不同深度处的P波速度结构（a）和棋盘测试结果（b）
Figure 4. Distribution of P wave velocity structure （a） and checkerboard resolution test （b） at different depths
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图 5 四时段的参数信息
（a） 二维射线分布；（b） 不同阻尼参数的解的方差和数据方差的均衡曲线；（c） 不同平滑权重参数的模型方差和数据方差的均衡曲线；（d） 不同模型下的观测走时与理论走时的均方根拟合差变化
Figure 5. The parameter information of four periods
（a） Distribution of 2-D P wave ray paths；（b） The trade-off curves of solution variance and data variance for different damping parameters；（c） The trade-off curve of slowness model variance and data variance for a set of smoothing weight parameters；（d） The root-mean-square misfit improvement between observation travel times and theoretical ones based on 3D models
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图 6 四时段t1-t4 （a-d）不同深度处的P波速度棋盘测试结果
Figure 6. Checkerboard resolution test for the four periods t1-t4 （a-d）
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图 7 四个时间段t1−t4 （a−d）不同深度处的P波速度分布
Figure 7. The distribution of P wave velocity for the four periods t1−t4 （a−d）
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图 8 相邻两时间段的P波速度变化棋盘在不同深度的恢复图
Figure 8. Recovered checkerboard result of P wave velocity for the two adjacent time periods at different depths （a） t2－t1；（b） t3－t2；（c） t4－t3
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图 9 两个相邻时间段的P波速度变化在不同深度的分布图
Figure 9. Distribution of temporal P-wave velocity changes for the two adjacent time periods at different depths
（a） t2－t1；（b） t3－t2；（c） t4－t3
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图 10 两个相邻时间段的P波速度变化沿垂直剖面AA′（左）和BB′（右）的分布图
Figure 10. Temporal distribution P-wave velocity changes along the cross sections AA′ （left）， BB′ （right） for the two adjacent time periods
（a） t2－t1；（b） t3－t2；（c） t4－t3
表 1 初始一维速度模型
Table 1 The initial 1D velocity model
深度/km 速度/（km·s−1） 0 5.000 2 5.367 5 5.803 7 6.033 10 6.126 14 6.400 18 6.600 25 6.800 30 7.200 38 8.000 
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表 2 盈江两次中强震四个时段的数据分布
Table 2 Data of the four periods for two moderate strong earthquakes in Yingjiang
时间段 时间窗 台站个数 地震次数 绝对到时/条 相对到时/条 t1 2009−01−01—2014−05−23 12 679 4310 64208 t2 2014−05−24—2014−05−29 14 947 6054 67628 t3 2014−05−30—2014−07−31 16 3982 35139 160560 t4 2014−08−01—2019−12−31 21 1007 8517 78155
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表 3 4个时段反演前后到时差的均方根残差变化
Table 3 The RMS residuals between observed and predicted differential travel times after and before tomography for the four periods
时段 到时差的均方根残差/s 下降百分比 时段 到时差的均方根残差/s 下降百分比 初始三维速度模型 最终三维速度模型 初始三维速度模型 最终三维速度模型 t1 0.519 0.203 60.9% t3 0.318 0.052 83.6% t2 0.317 0.048 85.8% t4 0.404 0.117 71%
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表 4 时移层析成像所用到的数据
Table 4 The data of time-lapse tomography
时间段 地震次数 台站/个 P波到时差/个 t2－t1 1626 14 10364 t3－t2 4929 16 41193 t4－t3 4989 21 43656
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表 5 相邻两个时间段反演前后到时差的均方根残差变化
Table 5 The RMS residuals between observed and predicted differential travel times after and before tomography varies for two adjacent time periods
时间段 到时差的均方根残差/s 下降百分比 初始三维模型 三维速度变化 t2－t1 0.279 0.097 65.2% t3－t2 0.140 0.016 88.6% t4－t3 0.180 0.015 91.7%
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