Quantitative effect of regional one-dimensional velocity models on earthquake relocation：Take the 2017 Jiuzhaigou MS7.0 earthquake sequence as an example
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摘要: 利用2017年8月1日至2017年12月31日四川地震台网和甘肃地震台网记录到的发生在青藏高原东缘的731个地震事件的9 284条Pg震相到时数据，首先反演了该地区的“最小一维速度模型”，并将该模型和选取的速度模型建立对比模型，以九寨沟地震序列为研究目标，定量讨论了两种速度模型分别在绝对定位和相对定位方法中对定位结果的影响。所得定位结果表明：反演获得的“最小一维速度模型”在重定位中可以有效地减小地震走时均方根残差；绝对定位比相对定位更加依赖于一维速度模型，一维速度模型会直接影响绝对定位结果中的震源分布形态，但在相对定位结果中仅起到调整地震事件相对位置的作用；在地震绝对定位中，震级越大的地震对于速度模型越敏感，而这一特点在相对定位中表现得并不明显。通过本项研究可知，在地震定位研究中，联合采用绝对定位和相对定位方法是最佳策略。
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关键词:
- 2017年九寨沟MS7.0地震 /
- “最小一维速度模型” /
- 绝对地震定位 /
- 相对地震定位
Abstract: Using 9 284 Pg phase arrival times of 731 earthquakes occurred in the eastern margin of the Tibetan Plateau recorded by Sichuan and Gansu Seismic Networks from August 1, 2017 to December 31, 2017, this paper determine “the minimum one-dimensional velocity model” for the area by inversion. “The minimum one-dimensional velocity model” and the selected comparative velocity model are used to establish the comparative models. Taking the Jiuzhaigou earthquake sequence as the research object, we quantitatively discuss the effects of two velocity models on the positioning results in absolute seismic location and relative seismic location methods. The results show that “the minimum one-dimensional velocity model” obtained by inversion can effectively reduce the root-mean-square residuals of travel time in relocation. Absolute seismic location method is more dependent on one-dimensional velocity model than relative seismic location. Furthermore, one-dimensional velocity model will directly affect the distribution pattern of hypocenters in absolute location, but only adjust the relative position of seismic events in relative location. In absolute location of earthquakes, the earthquake with larger magnitude is more sensitive to velocity model, which is not obvious in relative location. Therefore, the combination of absolute location method and relative location method is the best strategy in earthquake relocation. -
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图 1 研究区域内地震台、地震震中及射线分布图（a），P波走时曲线图（b）及初始震源深度直方图（c）
Figure 1. Distribution of seismic stations，epicenters and seismic rays （a），travel time versus epicentral distance curve of P wave （b） and histogram of the initial focal depth （c）
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图 2 初始一维输入速度模型 （a），反演得到的一维输出速度模型 （b）及不同速度模型下地震走时均方根（RMS）残差随迭代次数增加对应的变化 （c）
Figure 2. Initial 1-D input P wave velocity model （a），1-D output velocity model obtained by inversion （b）， variation of root mean square （RMS） of travel time residuals corresponding to different velocity models with increasing iterations （c）
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图 3 模型Ⅲ与“最小一维速度模型”的对比图
Figure 3. Comparison of the model Ⅲ with “the minimum one-dimensional velocity model”
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图 4 速度模型反演稳定性检验
（a） 输入模型；（b） 输出模型
Figure 4. Stability testing of velocity model inversion
（a） Input model; （b） Output model
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图 5 使用“最小一维速度模型”及台站校正值进行绝对定位前后的震源分布图（a，b，c）及走时均方根（RMS）残差统计分布图（d）
蓝色圆圈表示初始震源位置，红色圆圈表示绝对定位后的震源位置，黄色星形和绿色星形分别表示九寨沟MS7.0主震初始位置和绝对定位后的位置；图（a）中AA′和BB′分别表示平行于、垂直于余震分布方向的两条剖面线，两条剖面线交于O点。图（b）和图（c）分别为震源沿经度和纬度方向的投影. 图6中相应符号的意思同此
Figure 5. Distribution of hypocenters before and after absolute relocation by using “the minimum 1-D velocity model” and station correction （a，b，c） and statistical distribution of travel time RMS residuals （d）
Blue solid circles denote the initial focal location，red solid circles denote the hypocenters after absolute relocation，yellow and green stars denote the initial location and absolute relocation of Jiuzhaigou MS7.0 mainshock，respectively. In Fig. （a） AA′ and BB′ represent two profile lines parallel to and perpendicular to the distribution direction of aftershocks，respectively. The two profiles intersect at O. Figs. （b） and （c） denote the distribution of focal depths along longitude and latitude directions，respectively. All these are the same in Fig. 6
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图 6 使用模型Ⅲ进行绝对定位前后的震源分布图（a，b，c）及走时均方根（RMS）残差统计分布图（d）
Figure 6. Distribution of hypocenters before and after absolute relocation by using model Ⅲ （a，b，c） and statistical distribution of travel time RMS residuals （d）
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图 7 AA′和BB′深度剖面上绝对定位后的震源分布图
图（a）和（b）表示使用“最小一维速度模型”及台站校正值进行绝对定位的结果，图（c）和（d）表示使用模型Ⅲ进行绝对定位的结果，红色星形表示九寨沟MS7.0主震绝对定位后的位置
Figure 7. Distribution of hypocenters after absolute relocation along profiles AA′ and BB′
Figs. （a） and （b） denote the hypocenters after absolute relocation by using “the minimum 1-D velocity model” and station correction，Figs. （c） and （d） denote the hypocenters after absolute relocation by using model Ⅲ，and the red stars denote the location of Jiuzhaigou MS7.0 mainshock after absolute relocation
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图 8 使用“最小一维速度模型”进行相对定位前后的震源分布图（a，b，c）及走时均方根（RMS）残差统计分布图（d）
蓝色圆圈表示相对定位前的震源位置，这里采用的是使用“ 最小一维速度模型”和台站校正值在初始位置上进行绝对定位之后的位置，红色圆圈表示相对定位后的震源位置，黄色星形和绿色星形分别表示相对定位前九寨沟MS7.0主震位置和相对定位后九寨沟MS7.0主震位置。图（a）中AA′和BB′分别表示平行于余震分布方向和垂直于余震分布方向的两条剖面线（剖面线位置同图5），两条剖面线交于O点；图（b）和（c）分别为震源沿经度和纬度方向的投影。图9中相应符号的意思同此
Figure 8. The distribution of hypocenters before and after relative relocation by using “the minimum 1-D velocity model” （a，b，c） and statistical distribution of travel time RMS residuals （d）
Blue solid circles denote the hypocenters before relative relocation，and here we use the data after absolute relocation at the initial position by using “the minimum 1-D velocity model” and station correction，red solid circles denote the hypocenters after relative relocation，yellow and green stars denote the location of Jiuzhaigou MS7.0 main shock before and after relative relocation，respectively. AA′ and BB′ represents the two profile lines parallel to and perpendicular to the distribution direction of aftershocks，respectively. The position of profile lines is the same as Fig. 5. The two profiles intersect at O. Figs. （b） and （c） denote the distribution of focal depths along longitude and latitude directions. All these are the same in Fig. 9
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图 9 使用模型Ⅲ进行相对定位前后的震源分布图（a，b，c）及走时均方根（RMS）残差统计分布图（d）
Figure 9. Distribution of hypocenters before and after relative relocation by using model Ⅲ （a，b，c） and statistical distribution of travel time RMS residuals （d）
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图 10 AA′及BB′深度剖面上相对定位后震源分布图
图（a）和（b）表示使用“最小一维速度模型”进行相对定位的结果，图（c）和（d）表示使用模型Ⅲ进行相对定位的结果，红色星形表示九寨沟MS7.0主震相对定位后的位置
Figure 10. Distribution of hypocenters after relative relocation at profiles of AA′ and BB′
Figs. （a） and （b） denote the hypocenters after relative relocation by using “the minimum 1-D velocity model”，Figs. （c） and （d） denote the hypocenters after relative relocation by using model Ⅲ，the red stars denote the location of Jiuzhaigou MS7.0 mainshock after relative relocation
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图 11 相对定位后不同震级的震源深度分布统计图
图（a）−（c）表示使用“最小一维速度模型”进行相对定位的结果，图（d）−（f）表示使用模型Ⅲ进行相对定位的结果
Figure 11. Histogram of depth distribution of hypocenters with different magnitudes after relative relocation
Figs. （a）−（c） denote the hypocenters after relative relocation by using “the minimum 1-D velocity model”，Figs. （d）−（f） denote the hypocenters after relative relocation by using model Ⅲ
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