2005年3月28日苏门答腊8.6级地震前的电离层扰动(英文)

IONOSPHERIC PERTURBATIONS ASSOCIATED WITH THE M8.6 SUMATRA EARTHQUAKE ON 28 MARCH 2005

2005年3月28日苏门答腊发生8.6级地震,通过分析处在地磁活动平静期的震前15天的数据,发现3月20、22、23和28日在赤道区域多个参量上出现电离层扰动,如电场和等离子体参量,认为这些电离层扰动可能与本次强震有关.震前电子密度异常有两种形态,一种是在原来的峰值变化区电子密度单调增加,幅度超过1σ,如3月20日和28日的异常;另外一种是改变了原来在赤道区的单峰变化形态,而呈现了双峰值和赤道谷值的变化形态,如3月22日和23日的扰动.分析结果显示,这两种形态的电子密度扰动都分布在一个比较大的范围内,经度上跨越100°,纬度范围30°,显示了比较长的异常持续时间和全球变化特性.VLF频段电场频谱的数据分析表明双峰形态的扰动不是磁共轭效应.同时低频电场扰动相对电子密度而言集中在一个相对小范围内,当N_e单调上升时,电场扰动却只出现在离震中最近的轨道上.分析认为震源产生的电场信号直接传播进入电离层是非常困难的,但是它可以通过岩石层-大气层-电离层圈层耦合机制造成电离层扰动.结合离子成分及其变化形态的分析,震前氧离子变化形态与氢、氦离子的形态相反,这与其他地震前的电离层异常扰动形态是类似的.综合分析认为,对于与地震相关的电离层异常的识别需要结合多个观测参量的联合分析.

During 15 days in a relative quiet geomagnetic time around the M8. 6 Sumatra earthquake on 28 March 2005, ionospheric perturbations occurred before this event on March 20, 22, 23, and 28,2005 in many parameters such as the electric field and plasma parameters in the equatorial area. Since it was a quiet magnetic time, those four days referred above may be related to the preparation for this strong earthquake. The electron density showed two types of anomalies, one being a monotone increase in the single peak values with amplitudes exceeding lo, such as on March 20 and 28 in 2005; the other one changing the normal single peak to double crests and a trough in the equatorial area, which occurred on March 22 and 23, 2005. Both the anomalies are distributed over a very large area of 100°longitude and 30°latitude, presenting a long duration and global feature. The study of VLF electric field spectrums demonstrated that those anomalies were not presented in the geomagnetic conjugate feature. Electric field perturbations were concentrated in a relative smaller area than those of Ne, occurring only in the nearest orbit to the epicentre with only monotone increasing Ne anomalies. It is thought that it is difficult for an electric field generated close to the seismic preparation area in the equatorial area to penetrate into the ionosphere directly, but it may occur through the coupling mechanism of lithosphere-atmosphere-ionosphere. Combined with the analysis of ion composition and varying shape, this shows that prior to this earthquake the variation in oxygen density reversed to that of hydrogen and helium density, similarly to other ionospheric precursors before other earthquakes. It is concluded that conjoint analysis among different parameters should be applied in order to distinguish the precursors related to the earthquakes.