Determination on the focal mechanism and central focal mechanism of Zhongwei ML3.4 earthquake on June 9, 2020
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摘要: 2020年6月9日宁夏中卫市沙坡头区发生ML3.4地震，该地震发生在1709年中卫南7½级地震的极震区内，且震中位于以往弱震相对偏少的地区。本文利用宁夏区域地震台网的波形记录，采用gCAP方法反演了2020年6月9日中卫ML3.4地震的震源机制解及震源矩心深度，并用Hash方法计算其震源机制解，且得出了两种方法的震源机制中心解。结果表明，gCAP方法的震源机制解为：节面I走向255°，倾角79°，滑动角−20°；节面II走向348°，倾角70°，滑动角−168°，震源矩心深度为12 km。而Hash方法的震源机制解为：节面I走向344°，倾角89°，滑动角176°；节面II走向74°，倾角86°，滑动角1°。两种方法的震源机制中心解为：节面I走向255°，倾角87°，滑动角−11°；节面II走向346°，倾角80°，滑动角−176°，主压应力轴走向主要为NE向，其中gCAP方法结果与震源机制中心解的最小空间旋转角相对最小，为12.09°。结合过去地质构造资料，推测2020年6月9日中卫ML3.4地震的主要错动方式为左旋走滑，且断层面为NEE向节面的可能性较大。
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关键词:
- 2020年6月9日中卫ML3.4地震 /
- gCAP方法 /
- Hash方法 /
- 震源机制 /
- 震源机制中心解
Abstract: On June 9, 2020, an earthquake measuring ML3.4 occurred in Shapotou District, Zhongwei City, Ningxia. The earthquake occurred in the meizoseismal area of the 1709 M7½ earthquake of South Zhongwei, and its epicenter is located in an area where there were relatively few weak earthquakes in the past. In this paper, based on the waveform records of the Ningxia regional seismic network, the focal mechanism solution and focal centroid depth of the Zhongwei ML3.4 earthquake on June 9, 2020 are inversed using the gCAP method. The focal mechanism solution is also calculated by the Hash method, and the central focal mechanism of these two methods is obtained. The result shows that the focal mechanism solution of gCAP method is strike 255°, dip 79°, rake −20° for nodal plane I, and strike 348°, dip 70°, rake −160° for nodal plane II, and the centroid depth is 12 km, and that the focal mechanism solution of Hash method is strike 344°, dip 89°, rake 176° for nodal plane I, and strike 74°, dip 86°, rake 1° for nodal plane II. The central focal mechanism solution of the two methods is strike 255°, dip 87°, rake −11° for nodal plane I, and strike 346°, dip 80°, rake −176° for nodal plane II, and the principal stress axis is mainly in NE direction. The minimum 3-D rotation angle between the gCAP method result and the central focal mechanism is relative small, which is 12.09°. Combing the past geologic structure data, we deduces that the main dislocation of Zhongwei ML3.4 earthquake on June 9, 2020 is the left-lateral slip, and that the fault is more likely to belong to the nodal plane in NEE direction. -
图 1 2020年6月9日中卫ML3.4地震震中及其周边区域M5.0以上历史地震（公元876—1969年）和ML3.0以上地震（1970—2020年）分布（白线为块体线，粗黑线为断裂）
Figure 1. The distribution of M≥5.0 historical earthquakes （AD 876—1969） and ML≥3.0 earthquakes （1970—2020） in the epicenter of the ML3.4 Zhongwei earthquake on June 9，2020 and its adjacent areas （the white lines represent the block border，and the thick solid lines represent the faults）
图 2 gCAP方法和Hash方法所使用的台站分布（a）和宁夏及邻区平均地壳速度模型（b）
Figure 2. The distribution of seismic stations used by gCAP and Hash method （a），and the average crustal velocity model in Ningxia and its adjacent areas （b）
图 3 gCAP方法测定的2020年6月9日中卫ML3.4地震的波形拟合误差随深度变化图（a）及观测波形（黑）和理论波形（红）的波形拟合图（b），拟合图波形左上角字母为台网及台站名，下侧为震中距（单位：km）和方位角（单位：°）；波形下侧的两行数字分布为理论波形相对观测波形的移动时间（单位：s）及其相关系数（单位：%）
Figure 3. The variation of waveform fitting error with depth （a） and the fitting figure （b） between observed （black） and synthetic （red） waveforms of the ML3.4 Zhongwei earthquake on June 9，2020 by gCAP method. Letters in the upper left corner are the network and station names，and epicentral distance （in km） and azimuth （in degree） are under the station names. Numbers under the waveforms are the time shifts （in second） of the synthetic waveforms relative to the observation waveforms and their correlation coefficients （in percentage）
图 4 2020年6月9日中卫ML3.4地震的震源机制（gCAP和Hash方法）及其中心解（即黑色弧线代表中心解的两个节面，绿色弧线覆盖区域代表其不确定范围。红、蓝和橙色的点分别代表中心解P、T和B轴，其周围对应颜色的封闭曲线代表其各自的不确定范围。绿点和黑点代表该两个方法得到的P轴和T轴，紫弧线为该两个方法得到的震源机制节面）
Figure 4. The focal mechanism of the ML3.4 Zhongwei earthquake on June 9，2020 by gCAP and Hash method and their central focal mechanism （the black curves are the two nodal planes of central focal mechanism，and the coverage areas are the uncertainty range. Red，blue and orange dots are respectively P，T and B axes of the central focal mechanism， and the corresponding color around closed curve are their uncertainty range. Green and black dots are the P，T axes from these two methods，and the magenta curves are the nodal planes of focal mechanism from these two methods）
图 5 2020年6月9日中卫ML3.4地震的震源机制中心解及其附近地质断层构造（白色线为块体线，地质断层数据来源于《天景山活动断裂带1∶50000地质图及说明书》 [13]）
Figure 5. The central focal mechanism of the Zhongwei ML3.4 earthquake on June 9，2020 and the adjacent geological fault structure （the white lines represent the block border，and geological fault data is from 《Geological map of Tianjingshan active fault zone （1∶50000） and its instructions》 [13]）
表 1 Hash方法测定的2020年中卫ML3.4地震震源机制解等参数结果
Table 1. Focal mechanism and other parameters of the Zhongwei ML3.4 earthquake in 2020 by Hash method
走向 倾角 滑动角 P波极性数目
（implusive，emergent）2020年6月9日中卫ML3.4地震的
震源机制（Hash方法）344° 89° 176° （5，5） 断层面
不确定度辅助面
不确定度初动矛盾比 解的质量
分级49° 50° 21% D 逼近最佳解
的概率台站分布比 S/P振幅比
的数量100*（平均log10（S/P）
矛盾比）30% 49% 6 88 
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表 2 gCAP方法和Hash方法给出的2020年中卫ML3.4地震震源机制解和得到的中心机制解及标准差
Table 2. Focal mechanism of the Zhongwei ML3.4 earthquake in 2020 by gCAP and Hash method and the center focal mechanism and its residuals
序号 震源机制解 计算方法 作为初始解所得的
中心震源机制作为初始解所得
的标准差S/°以gCAP方法作为初始解的中心
震源机制和其他震源机制
的最小空间旋转角/°走向/° 倾角/° 滑动角/° 走向/° 倾角/° 滑动角/° 1 255 79 −20 gCAP方法 255.16 86.53 −10.52 12.112393 12.09 2 344 89 176 Hash方法 345.78 79.51 −176.52 12.112456 12.13
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表 3 两种方法结果及最终中心解的两个节面、P、T及B轴参数
Table 3. The two nodals，P，T and B axes parameters of two method and the terminal center focal mechanism
方法 节面I 节面II P轴 T轴 B轴 走向/° 倾角/° 滑动角/° 走向/° 倾角/° 滑动角/° 方位角/° 倾伏角/° 方位角/° 倾伏角/° 方位角/° 倾伏角/° gCAP 255 79 −20 348 70 −168 210 21 303 5 47 67 Hash 344 89 176 74 86 1 29 2 298 3 149 85 中心解 255 87 −11 346 80 −176 210 10 301 5 57 79
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