Abstract:

On April 25, 2015, a strong earthquake with magnitude 7.8(M_W)and focal depth of 8.2km(according to USGS)occurred in Nepal(28.15°N, 84.71°E), which caused heavy casualties and property losses to Nepal, China, and some neighboring countries. The focal mechanism solution shows that the earthquake is a low-angle thrust earthquake, resulting from the collision and following long-term compression between the Indian Plate and Eurasian Plate. In this paper, we analyze the magnetic disturbance characteristics before the Nepal M_W7.8 earthquake using polarization method based on the geomagnetic second data of 48 stations in mainland China.
Polarization method uses the ratio of the vertical component(Z)to the horizontal component(H)of the geomagnetic field, i.e. Z/H. This method can suppress external interference and thus extract seismo-magnetic anomaly information. Previous geophysical studies have shown that the perturbation in geomagnetic field during the earthquake preparation has a far greater impact on Z than H. The high polarization anomaly may contain some magnetic field information related to earthquake preparation. The calculation of geomagnetic second data includes three main parts: spectrum analysis, polarization calculation and abnormal signal extraction. The dominant frequency band(0.01~0.2Hz)is selected for subsequent calculation and analysis.
(1)By analyzing the relationship between the polarization and the perturbation in the geomagnetic field through spectrum analysis, we find that the polarization value at each stage is obviously negatively correlated with the geomagnetic K index, indicating that the high polarization anomaly is almost not related to the geomagnetic activity and therefore can be used to analyze the pre-seismic anomaly. In order to ensure the reliability of the calculation results, it is recommended that the data length should exceed 1 year, based on the annual variation characteristics of polarization value.
(2)The polarization value of the Lhasa station(epicentral distance of 628km)and the Shiquanhe station(epicentral distance of 652km)had risen for 3 days before the Nepal earthquake, and the anomaly amplitude of the Lhasa station, which is closer to the epicenter, is significantly higher than that of the Shiquanhe station. We analyze the polarization value of stations within 1 500km from the epicenter of the Nepal earthquake, which reveals a synchronous increase of polarization value starting about 98 days before the earthquake. Considering the regional synchronization of the perturbation in the geomagnetic field, a daily correlation analysis method is proposed to analyze the polarization of stations within 1 500km from the epicenter. We find that there is a significant increase in polarization correlation during earthquake preparation, and the earthquake occurred at the synchronization transition phase. It is suggested that the synchronization may be attributed to the additional effect of the source field associated with earthquake on the regional geomagnetic field. Certainly, this method requires higher data quality, and some certain interference factors need to be eliminated to reduce the influence of individual data on the overall results.
(3)The pre-seismic magnetic disturbance changes of each station are different in anomaly amplitude, which is associated with the spatial position, tectonic setting, and signal source of the abnormality. Subsequently, spatial analysis based on the relative variation of polarization value is necessary. The results show that continuous polarization anomalies exceeding 3 days before the earthquake occurred in more than 20% of stations, the spatial scope and abnormal amplitude experience a change trend from increasing to reducing, however, the spatial distribution of anomalies which has obvious regional characteristics always revolves around the epicenter. The time-space changing process of polarization anomalies really reflects the dynamic changes of the regional geomagnetic field, which is the result of external influence with strong dynamic characteristics. Plate movement is the main driving force of the perturbation in the regional geomagnetic field, while a large amount of melting fluid substances provide good channel for preparation and propagation of geomagnetic field. Thus, the generation and distribution of polarization anomalies are closely related to the geodynamic evolution of geological structures. Stress accumulation caused by geological activities is the main reason for the perturbation in the geomagnetic field.
(4)The study suggests that multiple synchronous polarization abnormality and turning in daily correlation have important indications for strong earthquakes, which will provide a new approach for monitoring and prediction. However, the quantitative relationship between anomaly amplitude and epicentral distance is not obvious, which is affected by tectonic environment of station, seismogenic background, complexity of changes of spatial geomagnetic field and fewer seismic examples. Therefore, in order to obtain more evidence and improve reliability, more seismic examples and theoretical analysis is necessary.

Key words: seismogeomagnetism, polarization, the Nepal M_W7.8 earthquake, daily correlation

摘要：

文中基于中国大陆地区48个台站的地磁秒值资料, 采用极化法对2015年尼泊尔M_W7.8地震前的磁扰动特征进行了分析。极化法的逐步计算结果与地磁K指数的对比分析表明极化高值异常与空间磁场活动之间没有相关性, 可用于提取震磁异常信息。距震中1 500km范围内台站的极化值在震前98d出现了同步性高值异常, 考虑到磁场变化具有区域同步性, 提出了极化逐日相关分析。研究发现地震孕育过程中各台站间的极化相关性明显增强, 而地震发生在同步性转折阶段。基于极化值相对变化量进行的空间异常分析显示, 尼泊尔地震前 >20% 的台站出现持续3d以上的异常, 且具有明显的区域性特征。文中从构造动力演化角度对地磁极化异常的孕育和发展进行了解释, 对磁场变化机理进行更进一步的探索。研究认为, 多台同步性极化异常和逐日相关转折对强震具有重要的指示意义, 该成果能为强震监测预测提供新的技术途径。

关键词: 地震地磁, 极化法, 尼泊尔M_W7.8地震, 逐日相关