Characteristics and Background of Shandong Rushan Earthquake Sequences
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摘要: 2013年10月1日在山东省威海市乳山市发生M3.2级地震，之后发生了一系列震群活动。截至2016年5月，山东台网已经记录到了1万多次余震，其中3级以上地震9次，4级以上地震3次。频繁的地震构造活动引起了乳山市及周边地区强烈震感。为研究乳山震群的发震机理，本文利用山东台网数字化地震波资料和新建的乳山台阵资料，通过双差精定位方法重新确定了震中位置。研究结果表明：余震序列呈现出NW向的条带分布；采用CAP方法（Cut and Paste）反演震群中9次3级以上地震的震源机制解，结果显示几次较大地震的震源深度平均约为5km，与台网编目定位的结果基本相同。从得到的精定位结果并结合震源机制解的结果来看，震群的走向是NW向，倾角是NE向，与最近的乳山断裂有一定距离。由此推断该区域可能是乳山断裂的分支，或者有一条或多条隐伏断裂。Abstract: An earthquake with MS3.2 occured in Weihai city of Shandong province on October 1st, 2013, followed by a series of earthquake sequences. Until now (May, 2016), the Shandong Seismic Network has recorded more than ten thousand aftershocks, among which there are nine events with magnitude over MS3.0 and three over MS4.0. Since then frequent seismotectonic activities have been perceived in Rushan and the surrounding areas. To understand the mechanism of Rushan earthquake swarms better, we collected and analyzed the digital seismic wave data from Shandong Seismic Network and the seismic array data from the newly-built Rushan station, and relocated the epicenter location and the sequence distribution with the Double-Difference method, and got a N-W stripe distribution from the aftershock sequence. The CAP (Cut and Paste) method was applied to inverse the focal mechanism solution three times of the earthquakes over MS4.0 and 6.0 times of the earthquakes over MS3.0-4.0. The results showed that the average focal depth of several great earthquakes was 5km, about the same with that of the seismic network. The MS3.2 earthquake in Rushan happened on January 7, 2014, was analyzed with the detailed result of nodal planeⅠ(strike angle 299.5°, dip angle 64ånd rake angle 16.7°) and nodal planⅡ(strike angle 202°, dip angle 75° and rake angle 162°). The strike distribution of the earthquake swarms is NW while the dip distribution NE. Because the certain distance between the distribution and the Rushan fault, it can be deduced that this area may belong to the part of Rushan fault, or there may be some undiscovered faults.
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Key words:
- Rushan /
- Accurate relocation /
- Focal mechanism
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图 1 乳山序列附近断层、发生位置和乳山台阵台站分布
F1乳山断裂；F2海阳断裂；F3朱吴断裂；F4米山断裂
Figure 1. Location of nearby faults, Rushan earthquake sequence and seismic stations
图 4 乳山震群地震序列台网原始震中分布图
Figure 4. Epicenter distribution of Rushan catalog earthquake sequences
图 5 乳山震群地震序列重新精定位后震中分布图
Figure 5. Epicenter distribution after relocation of Rushan earthquake sequences
图 6 乳山震群精定位深度剖面分布图
Figure 6. The depth profiles of Rushan earthquake sequences after relocation
图 7 乳山震群较大地震的震源机制解
蓝色沙滩球为3次4级以上震源机制解，红色填充的沙滩球为6次3—4级地震的震源机制解，白色区域的蓝点代表压缩P轴，红色和蓝色区域的黄点代表拉张T轴
Figure 7. Focal mechanisms of earthquakes with relatively large magnitude of Rushan earthquake sequences
图 8 CAP方法的最佳拟合深度和波形拟合
Figure 8. Tendency chart of focal depth variation with different depth errors and focal mechanism
图 9 乳山4.2级地震的理论地震图和观测地震图的对比
Figure 9. The comparison of theoretical seismogram and observed seismogram of Rushan MS4.2 earthquake
表 1 地壳速度模型
Table 1. Crustal velocity model
深度/km Vp/km•s-1 Vp/VS 0 4.00 1.73 5 6.10 1.73 10 6.20 1.73 20 6.40 1.73 30 6.78 1.73 40 8.20 1.73 
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表 2 乳山震群9次3级以上地震震源机制解
Table 2. Focal mechanisms of nine events over MS3.0 of Rushan earthquake sequence
发震时刻 经度/°E 纬度/°N 震级/M 节面Ⅰ/° 节面Ⅱ/° 最佳拟合震源深度/km 走向 倾角 滑动角 走向 倾角 滑动角 2013-10-01 12：07 121.70 36.83 3.2 297 68 －13 32 78 －158 5.5 2014-01-07 22：24 121.67 36.82 4.2 299.5 64 16.7 202 75 153 8.6 2014-04-04 00：12 121.70 36.83 4.0 289 90 27 199 63 180 8.3 2014-07-16 00：40 121.69 36.84 3.0 260.1 55.1 －41.6 17 57 －137 4.9 2014-09-16 14：42 121.65 36.83 3.5 317.6 61 －61.2 89 40 －131 3.8 2014-09-16 14：43 121.67 36.83 3.3 270 69 －52 24.6 42.6 －148.1 4.1 2015-05-22 00：05 121.69 36.83 4.6 295 85 －19 27 71 －175 10.2 2015-06-09 22：29 121.69 36.84 3.1 290 90 22 200 68 －180 7.6 2015-07-01 19：31 121.67 36.80 3.0 281 71 36 178 56 157 6.3
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