| Long-term observations have shown that non-mechanical electromagnetic methods have brought new hope to short-term earthquake predictionin geophysics,geology and geochemistry.The study of space electromagnetic field observation data shows that there is a strong correlation between electromagnetic anomalies in the ionosphere and seismic activity.If fully applied the large dynamic,wide viewing angles and all-weather characteristics of the earth observation from space,maked up the insufficiency of ground observations,formed space-earth integration system of theearthquake monitoring,it will greatly improve the ability and level of the earthquake monitoring.The first successful satellite to detect seismic ionospheric disturbances specially in the world is the DEMETER satellite of France.It was launched into sun synchronous orbit on 29 June 2004 and expired on 29 November 2010.Six years observation data have been accumulated at an altitude of 660 to 710km.After the successful launch of the DEMETER satellite,the researchers did a lot of research on the background field of the ionosphere.Their background fields are built in different ways for example the seasons,frequencies,geomagnetic activity and grid division are all very different according to the needs of each person's research purpose,but the basic principle is the same because they all calculated the grid average and standard deviation and the results are used as background values for ionospheric disturbances.There have been no relevant reports on the background feature analysis of the ionospheric electric field in the local area.And the background data is urgently needed to the earthquake precursor anomalies.So this paper will select the local area to carry out the research to fill the gap in this fieldThe research area of this paper is located at the juncture area of Eurasian plate,Pacific plate and north Americans plate(105°E-145°E,38°N-58°N).The area has a strong seismic active now and it covers the only deep seismic area in northeastern China.It also covers tancheng-lujiang seismic belt,shanxi seismic belt,yanshan seismic belt and hebei plain seismic belt where have been occurred some earthquakes with M>5 magnitude in recent years.The region has recorded several earthquakes with M>7 in its history,including the sanhe-pinggu M8 earthquake in 1679,the tangshan M7.8 earthquake in 1976,and the haicheng M7.5 earthquake in 1975.And these major earthquakes have brought huge economic losses and casualties.Some power spectrum density(PSD)data of the electric field in the ELF/VLF frequency band recorded by the ICE on the microsatellite DEMETER is utilized to carry out statistical analysis about the spatial and temporal distribution rules of the day and night,season and year changes and try to analyze the physical mechanism in nearly half a solar cycle.After that,screen out typical earthquakes with Ms>6.0 in the region based on method of the background field.The abnormal feature rule is classified and summarized and the following conclusions are drawn after studing the spatial and temporal evolution of the electric field from 90 days before earthquake to 30 days after earthquake.(1)The annual variation characteristics of the electric power spectrum density(PSD)background field in ionosphere are as follows:spatial distributions of the nightside electric field in the same month of different years and in the same frequency band have good similarity,and the similarity of dayside is not as good as that of nightside;The intensity range of both dayside and nightside background is consistent in the same month of different years and in the same frequency band,and the electric field intensity in the ELF/VLF frequency band is decreasing year by year from 2005 in Northeast Asia.The electric field intensity is the strongestin2005 and is the weakest in 2009.(2)The seasonal variation characteristics of the electric power spectrum density(PSD)background field in ionosphere are as follows:The nightside background field has seasonal variation characteristics,and the summer is strongest and then from strong to weak are winter,spring and autumn.And the seasonal variation of dayside electric field is not obvious.(3)The diurnal variation characteristics of the electric power spectrum density(PSD)background field in ionosphere are as follows:The frequency band intensity analyzed of the nightside background field is stronger than that of the dayside background field except the band[371Hz-879Hz].(4)Three earthquakes of the six earthquakes screened outoccured when the disturbance reached its maximum and then falled.The other three earthquakes occurred when the disturbance amplitude increased to the maximum before the earthquake,and then recovered gradually.The electric field maximum perturbation amplitude of the space-time evolution characteristics is mainly concentrated in the ± 4 ° beyond the epicenter during the earthquake.It shows the seismic abnormal signals have a certain offset effect.The results are similar in time and space to those of previous studies means the results are credible. |
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