Statistical analysis of ELF/VLF magnetic field disturbances before major earthquakes
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摘要: 本文利用DEMETER卫星记录的变化磁场数据统计研究了2005—2009年北半球7级以上强震前后空间磁场的扰动特征.在震中±10°范围内利用震前2个月至震后1个月的5年同期观测的ELF/VLF[370~897 Hz]频段磁场功率谱密度数据构建了背景场,定义了表征空间磁场在地震时段相对于背景场的扰动幅度指标.统计研究发现26个强震中42%的地震主要表现为震前磁场扰度幅度逐渐上升,超过3倍标准差,随后在扰度幅度下降过程中发震.35%强震在地震前10天内最大扰动幅度超过3倍标准差,在扰度幅度处于最高值期间发震,震后磁场扰动幅度逐渐回落.23%的强震在震前震后无明显规律.随机事件测试发现在空间天气平静且无强震条件下随机地点上空的磁场无明显的时空演化规律,且随机地点上空的空间磁场相对于背景场的平均扰度幅度很小,最大扰度幅度从未超过2倍标准差,并且没有明显的随时间变化特征.Abstract: The magnetic field data recorded by the microsatellite DEMETER is utilized to carry out statistical analysis about the characteristics of ELF/VLF emission disturbances during MS≥7.0 earthquakes which occurred from 2005 to 2009 in the north hemisphere. For each earthquake, in the area of ±10° of latitude and longitude from the epicenter, the normal background map were constructed during the same period (from 2 month before the earthquake to 1 month after the earthquake) for the years from 2005 to 2009 using the power spectrum density data of the magnetic field in the ELF/VLF frequency band [370~897 Hz]. A formula is defined to characterize the perturbation amplitude of the magnetic field during earthquake periods relative to the background, and the perturbation amplitude is normalized by the standard deviation of background data. Statistical analysis of perturbation amplitude during major earthquakes shows that for the 42% of these earthquakes, the perturbation amplitude increased 30 days before the earthquake, more than 3 times the standard deviation, and began to decline 20 days before the earthquake, and the earthquake occurred during the descent stage of the perturbation amplitude. However, for the 35% of the major earthquakes, the perturbation amplitude began to increase 20 days before the earthquake, and reached more than 3 times the standard deviation 10 days before the earthquake, and the earthquake occurred at the high perturbation stage. The remaining 23% of the major earthquakes shows no obvious spatial and temporal patterns. Random tests confirm that the magnetic field intensity recorded by DEMETER satellite at random locations during quiet phases (quiet space weather and no major earthquake occurred)shows no obvious spatial and temporal evolution pattern. It is demonstrated that the average perturbation amplitude of the magnetic field relative to its background change slightly over random locations, and its maximum perturbation amplitude was never more than two times the standard deviation of its background.
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Key words:
- DEMETER /
- ELF/VLF /
- Magnetic field /
- Statistical analysis /
- Major earthquake /
- Random events
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