Anomalous characteristics of geomagnetic vertical strength polarization before the 2017 Milin MS6.9 earthquake in Tibet
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摘要: 基于中国大陆65 个地磁台站秒采样观测资料，利用地磁垂直强度极化方法提取了2017年西藏米林MS6.9 地震震中及附近区域超低频电磁异常信号并分析其时空演化特征。结果显示：米林MS6.9 地震前在青藏高原出现了大范围的地磁垂直强度极化高值异常，异常过程共持续9天；高值异常开始于2017 年10 月30 日，高值持续3 天之后出现短时间下降，随即转折上升，再次出现高值异常并持续了4 天，在此过程中各高值台站的时序曲线呈现出单峰或双峰的形态。空间分布图显示高值异常在中国大陆西部区域反复出现，尤其是在青藏高原巴颜喀拉地块与羌塘地块的交界处。异常呈“出现—扩大—收缩—消失—扩大—消失”的演化过程，10月31日异常面积达到最大值。异常过程结束后10天发生了西藏米林MS6.9地震，震中距离10 月31 日异常阈值线5 km。综合分析认为，此次异常与米林地震具有较强的时空相关性，是可靠的地震电磁前兆异常。
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关键词:
- 地震磁扰动异常 /
- 西藏米林MS6.9地震 /
- 电磁辐射信号 /
- 地磁垂直强度极化方法
Abstract: Based on the second sampling data from 65 geomagnetic stations in Chinese mainland, we extracted the ultra-low frequency electromagnetic anomaly signals from the epicenter of the 2017 Milin MS6.9 earthquake in Tibet and its vicinity using geomagnetic vertical strength polarization method, and analyzed their spatio-temporal evolution characteristics. The results show that a large range of geomagnetic vertical strength polarization high value anomalies appeared in the Qinghai-Xizang （Tibetan） Plateau before the MS6.9 earthquake, and the abnormal process lasted for nine days. The high value anomaly began on October 30, 2017. The high value lasted for three days, followed by a short period of decline, a turning point and an increase. Afterwards the high value anomaly appeared again and lasted for four days. In this process, the time-series curves of each high-value station show single peak or double peak. The spatial distribution map shows that high value anomalies occur repeatedly in the western part of Chinese mainland, especially at the junction of Bayankhara and Qiangtang blocks on the Tibetan Plateau. The anomaly appeared, expanded, contracted, disappeared, expanded and disappeared, and the abnormal area reached its maximum value on October 31. Ten days after the end of the abnormal process, the Milin MS6.9 earthquake occurred, with its epicenter 5 km from the anomaly threshold line of October 31. Comprehensive analysis indicates that this anomaly has strong temporal and spatial correlation with the Millin earthquake and can be regarded as reliable seismic electromagnetic precursor. -
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图 2 地磁垂直强度极化方法计算过程（以青海都兰台为例）
（a） 极化值；（b） 极化值的傅里叶拟合；（c） 极化值与其傅里叶拟合的残差值；（d） 残差的5日滑动平均值
Figure 2. The calculation process of geomagnetic vertical intensity polarization method （Data from Dulan observation station）
（a） Original polarization values；（b） Fourier fitting results of Fig. （a）；（c） The difference between Fig. （a） and Fig. （b）；（d） Five-day moving average of Fig. （c）
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图 3 2017年10—12月部分地磁垂直强度极化异常台站时序曲线
Figure 3. Time series of some stations with geomagnetic vertical strength polarization anomaly from October 1，2017 to December 31，2017
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图 4 2017年10月28日至11月8日全国地磁垂直强度极化异常空间演化过程
Figure 4. Spatial evolution process of geomagnetic vertical intensity polarization anomaly from October 28 to November 8，2017
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图 4 2017年10月28日至11月8日全国地磁垂直强度极化异常空间演化过程
Figure 4. Spatial evolution process of geomagnetic vertical intensity polarization anomaly from October 28 to November 8，2017
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图 5 异常参数统计
（a） 异常台站占比；（b） 异常面积；（c） 异常幅值最大值；（d） 震中距
Figure 5. Statistical graph of abnormal parameters
（a） The proportion of abnormal stations；（b） Anomalous area；（c） Maximum anomalous amplitude；（d） The epicentral distance
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图 6 极化高值异常期间Dst指数（a）和Kp指数（b）的变化情况
Figure 6. Variation of Dst and Kp indices during polarization high value abnormal period
表 1 异常参数统计表
Table 1 Statistical table of abnormal parameters
异常
日期异常面积
/（104 km2）异常台
个数计算台
个数异常
占比异常最大
幅值震中距
/km10月28日 0 1 64 1.56% 0.02 1540 10月29日 0 0 64 0 - - 10月30日 165.2 12 64 18.75% 0.40 380 10月31日 378.9 31 64 48.44% 0.87 1980 11月1日 70.9 8 65 12.31% 1.06 390 11月2日 0 0 65 0 - - 11月3日 0 0 65 0 - - 11月4日 2.3 1 63 1.59% 0.86 2300 11月5日 51.1 10 64 15.63% 1.26 970 11月6日 285.3 22 63 34.92% 1.22 2060 11月7日 0 1 64 1.56% 0.07 1540 
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