Matching location for small events and seismogenic fault of Aohanqi earthquake swarm of Inner Mongolia
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摘要: 内蒙古敖汉旗地区在2018—2019年间曾发生多次小震丛集活动，不同地震事件的波形记录易相互交叠，导致地震目录缺失。针对以上问题，采用匹配定位（Match & Locate）方法，对台网遗漏地震进行识别、检测与定位，并通过CAP方法反演敖汉旗震群最大地震的震源机制解，利用匹配定位后的小震分布定量地拟合发震断层面参数，从而综合判定敖汉旗震群的发震断层面几何形态和发震构造。结果显示：通过匹配定位方法共识别、定位405个小震事件，是原有地震目录事件的5.4倍，震群主体沿NW−SE向展布于红山—八里罕断裂与赤峰—开源断裂相交区域的东侧，震源深度集中于8—10 km。断层拟合结果和最大地震震源机制解表明敖汉旗震群的发震构造应是一条左旋走滑型隐伏正断层，断层面走向为157°，倾角为84°。综合分析红山—八里罕断裂和赤峰—开源断裂的断层性质和活动特征，认为敖汉旗震群的发震断层可能是这两条深大断裂在不断活动中相互作用而形成。Abstract: In Aohanqi area of Inner Mongolia, there occurred several clusters of small earthquakes during 2018−2019 and the waveforms of different earthquakes are overlapped with each other, which leads to the incompleteness of earthquake catalog. In order to solve this problem, we adopt the match and locate （M&L） method to identify, detect and locate the missing events in the catalogs, and then the focal mechanism solution of the largest event is analyzed by the CAP （cut and paste） method. The matched and located small earthquakes are used to fit parameters of seismogenic fault plane of the largest earthquake so as to determine the geometry of fault plane and seismogenic structure of Aohanqi earthquake swarm. The results show that by using the M&L method, we identified and located 405 small earthquake events, which are 5.4 times as much as the number of the original detected earthquakes, and the earthquake swarm extended predominately along the direction of NW−SE in the east of the intersection area between Hongshan-Balihan fault and Chifeng-Kaiyuan fault with focal depth mainly concentrating on 8−10 km. Besides, according to the fault fitting result of small earthquakes and the focal mechanism of the largest earthquake, it is concluded that the seismogenic structure of Aohanqi earthquake swarm is a concealed normal fault with sinistral strike-slip characteristics, which has a strike of 157° and a dip of 84°. Combined with the fault properties and activity characteristics of the Hongshan-Balihan fault and the Chifeng-Kaiyuan fault, it is deduced that the seismogenic fault of Aohanqi earthquake swarm may be formed by the interaction of the two deep and large faults in the continuous activities.
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Key words:
- Aohanqi earthquake swarm /
- matching and locating /
- focal mechanism /
- fault plane fitting
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图 1 内蒙古赤峰市敖汉旗区域构造特征及2018年和2019年地震分布
（a） 研究区区域构造背景，F1：赤峰—开源断裂，F2：红山—八里罕断裂；（b） 震群地震分布；（c） M-t图
Figure 1. Regional tectonic settings and distribution of earthquakes in Aohanqi，Chifeng City，Inner Mongolia during 2018—2019
（a） The regional tectonic settings of the studied area；F1：Chifeng-Kaiyuan fault，F2：Hongshan-Balihan fault；（b） The earthquake distribution of the swarm；（c） M-t plot
图 2 双差方法重定位得到的敖汉旗震群分布图
（a） 震中分布图；（b，c） 震源深度分别沿纬度、经度方向的变化；（d） 震源深度分布图
Figure 2. The earthquake swarm distribution of Aohanqi obtained by double difference relocation method
（a） Epicentral distribution；（b，c） The variation of focal depth along latitude and longitude direction，respectively；（d） The distribution of focal depth
图 3 模板地震事件的空间分布
Figure 3. Spatial distribution of template events selected from Aohanqi earthquake swarm
图 4 不同互相关系数
$ \overline{c} $ 阈值下使用匹配定位方法检测出来的敖汉旗地震事件Figure 4. Aohanqi earthquake events detected by the method of match and locate with different threshold of cross correlation coefficient
$ \overline{c} $ 
图 5 （a） 匹配定位方法检测到的事件累积个数随互相关系数
$ \overline{c} $ 阈值的变化；（b） 事件检测个数、地震事件检测准确率和$ \overline{c} $ 阈值三者间的关系Figure 5. （a） The variation of the cumulative number of events detected by match and locate method with the threshold of cross-correlation coefficient
$ \overline{c} $ ；（b） The relationship among the number of detected events，the accuracy of earthquake event detection and the$ \overline{c} $ threshold
图 6 利用匹配定位方法检测到的地震事件波形（黑色）与模板事件波形（红色）的拟合图
（a） 模板事件的自检结果；（b） 在原有地震目录内的地震检测结果；（c） 原有目录遗漏的地震检测结果
Figure 6. Fitting of earthquake event waveforms （black） and template event waveforms （red） detected by the match and locate method
（a） Self-detection of a template event；（b） Detection result of an earthquake in the original earthquake catalogue； （c） Detection result of an earthquake missed in the original earthquake catalogue
图 7 匹配定位方法与双差定位方法对同一地震的震中定位结果偏差（a，b）及相应震源深度分布情况（c，d）
Figure 7. Deviations of the results of the same earthquake epicenter （a，b） and depth （c，d） obtained using the match and locate method and the double difference method，respectively
图 8 基于匹配定位方法与双差定位方法得到的赤峰—开源断裂北（a）、南（b）侧敖汉旗震群微震震中分布
Figure 8. The epicenter distribution of the Aohanqi earthquake events located on the north （a） and south （b） sides of Chifeng-Kaiyuan fault obtained by match and located method and double difference location method，respectively
图 9 微震检测前、后敖汉旗震群三维分布特征（a）和地震频次-震级分布图（b）
Figure 9. 3D distribution characteristics of Aohanqi earthquake swarm （a） and frequency-magnitude distribution （b） before and after microseismic detection
图 10 内蒙古东部地区一维地壳速度结构模型（赵艳红等，2018）
Figure 10. One-dimensional crustal velocity structure model of eastern Inner Mongolia （Zhao et al，2018）
图 11 敖汉旗震群最大地震的震源机制解反演误差随震源深度的变化（a）以及最佳反演深度7.5 km下的波形拟合（b）
拟合波形下方的两行数字分别为理论波形（红色）相对实际波形（黑色）的时移（单位：s）以及二者的相关系数，波形左侧第一行给出了台站名，第二行给出了震中距（单位：km）和相对偏移时间（单位：s），台站波形按震中距排列
Figure 11. Variation of inversion error of focal mechanism solutions for the largest earthquake of Aohanqi swarm with focal depth （a） and waveform fitting at the optimal inversion depth 7.5 km （b）
The numbers of two rows beneath the traces are the time shifts （in second） of synthetics （red） relative to the observations （black） and the corresponding cross-correlation coefficients，respectively. The upper-left corner are stations. The lower-left corner numbers represent the epicentral distance （in km） and the relative offset time （in second）. The waveforms of stations are sorted in epicentral distance
图 12 敖汉旗地震序列区小震（a）及其深度（b）的分布
Figure 12. Distribution of small earthquakes （a） and their depth in Aohanqi earthquake sequence region
图 13 经匹配定位方法精确定位的敖汉旗震群小震分布在水平面（a）、断层面（b）和垂直于断层面的横断面（c）上的投影以及小震距断层面距离的分布（d）
A′A为震群分布的优势方向；圆圈表示精确定位小震；红色方框表示拟合的断层面边界
Figure 13. Projection of small earthquakes of the Aohanqi earthquake swarm by using match and locate method in the horizontal plane （a），fault plane （b） and cross section perpendicular to the fault plane （c） as well as distribution of the distance between small earthquakes and the fault plane （d）
表 1 基于用匹配定位方法得到的敖汉旗震群小震定位结果计算出的断层面参数
Table 1. The fault plane parameters calculated based on the locations of small earthquakes of Aohanqi swarm by the match and locate method
走向/° 误差/° 倾角/° 误差/° 到原点距离/km 误差/km 顶点位置（北纬/°，东经/°，深度/km） 157 1.2 84° 2.9° 0 0.08 （42.24，119.95，13.0）
（42.36，119.88，5.9）（42.36，119.88，13.0）
（42.24，119.95，5.9）
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