Relocation and focal mechanisms of the 2021 Wuzhong-Lingwu ML3.6 seismic swarm
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摘要: 2021年7月18日—8月7日，宁夏吴忠—灵武地区发生ML3.6显著震群活动。本文利用多阶段定位方法对该震群进行了重新定位，并根据gCAP方法反演了2021年7月20日灵武ML3.6地震的震源机制及震源矩心深度，采用Snoke方法计算了震群中3次ML3.0以上地震的震源机制，测定了同一地震多个震源机制的中心解。结果表明，该震群中最大的地震即7月20日02时40分ML3.6地震的震源机制为节面Ⅰ走向289°，倾角72°，滑动角−22°，节面Ⅱ走向26°，倾角69°，滑动角−161°，震源矩心深度为12 km，初始破裂深度为12.5 km；7月20日03时15分ML3.2地震的震源机制为节面Ⅰ走向290°，倾角82°，滑动角−2°，节面Ⅱ走向20°，倾角88°，滑动角−172°，初始破裂深度为11.9 km；7月21日04时55分ML3.1地震的震源机制为节面Ⅰ走向285°，倾角53°，滑动角2°，节面Ⅱ走向194°，倾角88°，滑动角143°，初始破裂深度为11.6 km，这些地震震源机制的主压应力轴主要为NE向。该震群序列的震源深度主要相对集中在7—15 km之间，其中ML3.0以上地震的震源深度主要介于11—13 km，震源深度剖面显示震群相对集中的区域由深到浅大体呈现近似于陡立的展布。本文进一步研究发现区域应力场在灵武ML3.6地震震源机制NNE向节面产生的相对剪应力为0.393，而在NWW向节面产生的相对剪应力为0.945。结合地质构造和已有断层资料初步分析认为，若NNE向的崇兴隐伏断裂为灵武ML3.6地震的发震断层，则表明崇兴断裂可能是一条断裂薄弱带，地震破裂方式主要为右旋走滑；若NWW向的未知隐伏断裂为发震断层，则表明NWW向断裂可能为该地震在区域应力场下的剪应力相对最大释放节面，其破裂方式为左旋走滑。
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关键词:
- 2021年吴忠—灵武ML3.6震群 /
- 多阶段定位 /
- 震源机制 /
- 相对剪应力
Abstract: From July 18 to August 7 in 2021, a significant activity of ML3.6 earthquake swarm occurred in Wuzhong-Lingwu area, Ningxia. In this paper, the earthquake swarm are relocated using the multi-step locating method. The focal mechanism and the centroid depth of Lingwu ML3.6 earthquake on July 20, 2021 are calculated by gCAP method, and the focal mechanisms of three ML≥3.0 earthquakes of the swarm are calculated by Snoke method. Meantime, the center solution of several focal mechanisms for the same earthquake is also determined. The results indicate: The focal mechanism solution of the ML3.6 earthquake at 02:40 am on July 20, 2021 which is the largest earthquake in the swarm is strike 289°, dip 72°, rake −22° for nodal plane Ⅰ, strike 26°, dip 69°, rake −161° for nodal plane Ⅱ, and the centroid depth is 12 km while the initial rupture depth is 12.5 km. The focal mechanism solution of the ML3.2 earthquake at 03:15 am on July 20, 2021 is strike 290°, dip 82°, rake −2° for nodal plane Ⅰ, strike 20°, dip 88°, rake −172° for nodal plane Ⅱ, and the initial rupture depth is 11.9 km. The focal mechanism solution of the ML3.1 earthquake at 04:55 am on July 21, 2021 is strike 285°, dip 53°, rake 2° for nodal plane Ⅰ, strike 194°, dip 88°, rake 143° for nodal plane Ⅱ, and the initial rupture depth is 11.6 km. The principal stress axis of focal mechanism is mainly in NE direction. The focal depths of the earthquake swarm are relatively concentrated at 7—15 km, and the focal depths of ML≥3.0 earthquakes mainly varies from 11 km to 13 km. The focal depth profiles show that the distribution direction of earthquake concentrated area is approximately upright from deep to shallow. Furthermore, it is also found that the relative shear stress on NNE nodal plane of Lingwu ML3.6 earthquake under the regional stress field is 0.393, and the relative shear stress on NWW nodal plane is 0.945. Combing with the geological structure and known fault data, the preliminary analysis indicates that if the NNE-trending Chongxing buried fault is the seismogenic fault of Lingwu ML3.6 earthquake, the Chongxing fault may be a weak fault zone, and the seismic rupture mode is mainly right-lateral slip. Otherwise, if the unknown NWW-trending buried fault is the seismogenic fault, the NWW-trending fault may be the releasing nodal plane with relatively maximum shear stress of the earthquake under the regional stress field, and its rupture mode is left-lateral slip. -
图 1 2021年吴忠—灵武ML3.6震群分布、M-t和日频次图
F1：黄河断裂中段；F2：黄河断裂南段；F3：F1分支断裂；F4：崇兴断裂；F5：关马湖断裂；F6：牛首山东麓断裂；F7：新华桥断裂；F8：银川隐伏断裂
Figure 1. The distribution of the 2021 Wuzhong-Lingwu ML3.6 seismic swarm，M-t and daily frequency diagram
F1：the central segment of Huanghe fault；F2：the south segment of Huanghe fault；F3：the branch of F1 fault；F4：Chongxing fault；F5：Guanmahu fault；F6：Niushoushan eastern piedmont fault；F7：Xinhuaqiao fault；F8：Yinchuan buried fault
图 2 研究区内的震相和达曲线和筛选范围 （红色虚线为拟合直线的2倍均方差）
Figure 2. Wadadi diagram and the selected region of phase data in the research area （red dashed lines are the limits for 2.0 RMS of the fit line）
图 3 吴忠—灵武地区的一维速度模型 （彩色线为反演过程中扰动值，黑粗线为平均结果）
Figure 3. 1-D velocity model for the Wuzhong-Linwu region （colorized lines are the disturbed values during the inversion process，and thick black line is the average result）
图 4 多阶段定位、Snoke方法及gCAP方法所使用的台站分布（灰线为块体线，黑线为断裂）
Figure 4. The distribution of seismic stations used by multi-step locating method，Snoke and gCAP method （gray lines are block boundaries，and black lines are faults）
图 5 重定位后的2021年吴忠—灵武ML3.6震群序列的震中分布和AA'、BB' 垂直剖面分布图
Figure 5. Distributions of the earthquake epicenter after relocation of the 2021 Wuzhong-Lingwu ML3.6 swarm sequence，and cross-sections of AA' and BB'
图 6 2021年7月20日02时40分ML3.6地震（a）、7月20日03时15分ML3.2地震（b）和7月21日04时55分ML3.1地震（c）的Snoke结果和其多解的震源机制中心解
Figure 6. The Snoke results and the center focal mechanism for multiple solutions of the ML3.6 earthquake at 02:40 am on July 20, 2021 (a), the ML3.2 earthquake at 03:15 am on July 20, 2021 (b) and the ML3.1 earthquake at 04:55 am on July 21, 2021 (c)
图 7 gCAP方法得到的2021年7月20日灵武ML3.6地震的波形拟合误差—深度图及理论波形（红）和观测波形（黑）波形拟合图，拟合图波形左上角为台站代码和方位角（单位：°），其下侧为震中距（单位：km）和相对偏移时间（单位：s）；波形下方两行数字分别为理论波形相对观测波形的移动时间（单位：s）和相关系数（单位：%）
Figure 7. The waveform fitting error variation with depth and the fitting diagram between synthetic(red) and observed (black) waveforms of the ML3.6 Lingwu earthquake on July 20，2021 from gCAP method. Letters in the upper left corner are the station names and azimuth (in degree)，and under the names are epicentral distance (in km) and relative time shift (in second). 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 8 2021年7月20日灵武ML3.6地震的震源机制（Snoke方法和gCAP方法）及其中心解
Figure 8. The focal mechanism of the ML3.6 Lingwu earthquake on July 20，2021 by Snoke and gCAP method and their central focal mechanism
图 9 2021年灵武ML3.6地震所在区域应力体系下的震源机制以及相对剪应力(a)和正应力(b)分布
NS：正走滑型；SS：走滑型；NF：正断型；U：不确定型；TS：逆走滑型；TF：逆断型
Figure 9. The focal mechanism, distribution of relative shear stress (a) and normal stress (b) under the stress system for 2021 ML3.6 Lingwu earthquake area
NS：Normal and Strike-Slip；SS：Strike-Slip；NF：Normal Fault；U：Uncertainty；TS：Thrust and Strike-Slip；TF：Thrust Fault
表 1 根据图3得到的吴忠—灵武地区简化的一维速度模型参数
Table 1. The parameters of simplified 1-D velocity model for the Wuzhong-Linwu region from Fig.3
层号 1 2 3 4 5 6 7 8 9 10 11 顶层深度H/km 0.0 4.0 5.0 8.0 12.0 14.0 20.0 22.0 31.0 43.0 47.0 vP/km•s−1 5.14 5.92 5.97 6.14 6.21 6.32 6.33 6.46 6.82 7.33 7.60 
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表 2 Snoke方法和gCAP方法给出的2021年7月20日02时40分灵武ML3.6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 Lingwu ML3.6 earthquake at 02:40 am on July 20, 2021 by Snoke and gCAP method, the center focal mechanism and its residuals
序号 震源机制走向/°，
倾角/°，滑动角/°计算方法 作为初始解所得的中心
震源机制走向/°，
倾角/°，滑动角/°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/°1 294，65，−27 Snoke方法 289.18，72.21，−22.27 10.685764 10.69 2 284，79，−17 gCAP方法 289.18，72.21，−22.27 10.685744 10.68
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表 3 2021年吴忠—灵武震群中ML≥3.0地震多种方法结果的两个节面、P、T、B轴参数及震源机制类型
Table 3. The two nodal planes，P，T，B axes parameters of different methods for ML≥3.0 earthquakes in the 2021 Wuzhong-Lingwu seismic swarm and focal mechanism types
序
号节面Ⅰ 节面Ⅱ P 轴 T 轴 B 轴 类
型走
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/°1 294 65 −27 36 66 −152 255 36 165 0 74 54 走滑型 2 284 79 −17 17 73 −169 240 20 331 4 72 70 走滑型 3 289 72 −22 26 69 −161 247 28 338 2 72 62 走滑型 4 290 82 −2 20 88 −172 245 7 155 4 34 82 走滑型 5 285 53 2 194 88 143 246 24 143 26 12 53 不确定型 注：序号1—3分别为2021年7月20日02时40分ML3.6地震的Snoke、gCAP和其中心解结果；序号4和5分别为2021年7月20日03时15分ML3.2和2021年7月21日04时55分ML3.1地震的Snoke结果；最后一列类型根据震源机制类型标准进行划分[17]
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