Focal depth determination of Shanxi reservoir earthquakes from sPL phase with single station
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摘要: 稀疏台网下的传统走时定位难以确定中小地震的震源深度，而地震波深度震相蕴含着震源深度信息，为确定地震震源深度提供了新的途径。近震深度震相sPL和直达Pg波到时差与震源深度呈线性关系，可用以约束地震震源深度。本文以珊溪水库2014年震群事件为例，利用单台sPL震相测定了地震震源深度。结果表明：震源深度的测定结果与基于水库台网高密度台站下Pg和Sg走时定位Hyposat方法和全波形拟合CAP方法测定的震源深度高度一致，为4—6 km，与区域活动断层探测结果相符。sPL震相的优势震中距为30—50 km，区域台网范围内sPL与Pg的到时差与震源深度的线性关系相对固定，因此利用单台sPL震相即可快速获取可靠的地震震源深度，适用于稀疏台网下的中小地震震源深度的确定，且误差可控制在1—2 km范围内。Abstract: It is difficult to determine focal depth via phase onset time fitting method for the regions where only sparse seismic stations are available. we demonstrate that focal depth can be well constrained with depth phase sPL which is sensitive to focal depth but insensitive to epicentral distance. With 2014 Shanxi reservoir earthquake swarm as a case study, this paper determined the focal depths of earthquakes by sPL phase with single station. The result show that the focal depths derived from sPL phase are well consistent with the results from CAP method and Hyposat method. The depths of the main shock and six ML＞3.0 events are predominantly around 4−6 km, which is coincident with results of surveying and prospecting of active fault. Therefore sPL phase can be applied to small earthquakes for sparse network as it is well observed with small epicentral distances. Robust result of focal depth can be rapidly obtained by single broadband seismic station with bias of about 1−2 km.
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Keywords:
- Shanxi reservoir /
- focal depth /
- depth phase /
- sPL phase
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图 1 珊溪水库区域断裂、地震和台站分布图
灰色填充区为珊溪水库，三角形为台站，圆圈代表2014年震群，加号为2014年之前地震，黑线为断层
Figure 1. Fault structure and distribution of earthquakes and stations in Shanxi reservoir region
Gray region indicates Shanxi reservoir，and its surrounding fault system is depicted by line segments，open circles denote the swarm epicenters of the year 2014 while plus signs denote the swarm before the year 2014，seismic stations are indicated by triangles
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图 2 sPL射线路径示意图（a）和万阜台记录的三分向位移波形（b）
图（a）中黑色为sPL及其参考震相Pg，灰色射线示意相对sPL震相其它深度震相则需更远的震中距才发育
Figure 2. Schematic illustration of sPL ray path （a） and three-component displacement waveforms recorded at Wanfu station to view sPL phase （b）
Black ray for sPL-Pg pair and gray for other depth phases which appear in farther epicentral distance in Fig.（a）
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图 3 基于sPL和CAP方法确定2014年珊溪水库震群主震震源深度
（a） sPL震相敏感性测试，灰色波形表示不同深度三分向理论波形，红色波形表示万阜台显示的主震最佳拟合深度；（b） CAP反演所得的主震最优震源机制（左）和震源深度（右）
Figure 3. Focal depth determined by sPL and CAP method for the mainshock of the Shanxi reservoir swarm in 2014
（a） sPL phase depth sensitivity test，where gray waveforms represent three-component synthetics at different focal depths and red ones represent data recorded by Wanfu staion at best fitting；（b） Focal mechanism （left） and depth （right） determined by CAP method
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图 4 表1中万阜台水库震群事件1—6的位移波形图
Figure 4. Displacement records of six events labeled with one to six in Table 1 at Wanfu station with vertical （red），radial （blue） and tangential （green） components
表 1 珊溪水库震群七次地震事件震源深度的测定结果
Table 1 Focal depths of seven events from Shanxi reservoir seismic swarm
序号 发震时刻 震中位置 ML 震源深度/km 年−月−日 时:分:秒 东经 北纬 sPL结果 台网结果* CAP结果 0 2014−10−14 04:14:57 119.94 27.71 4.2 5 4 4 1 2014−09−17 20:47:31 119.95 27.71 3.5 4—5 4 4 2 2014−09−23 17:40:25 119.94 27.71 3.7 4—5 4 4 3 2014−10−15 15:49:27 119.95 27.71 4.0 5—6 5 5 4 2014−10−15 16:37:24 119.96 27.70 4.0 6 5 5 5 2014−10−23 08:35:02 119.93 27.72 3.7 5 4 4 6 2014−10−26 07:03:41 119.97 27.69 3.4 4 4 / *引自浙江省数字地震台网中心地震目录（内部资料）. 
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