Study Of Ionospheric Anomalies During The Earthquake Based On The SVD Technique

The frequent occurrence of the disastrous earthquakes shows that earthquake is one of the most serious natural disasters. However, earthquake prediction is still a scientific puzzle all over the world nowadays. For a long time, experts in multi disciplines have put their attention and great efforts into the earthquake prediction research. Since the middle of twentieth Century, many scientists in different fields have carried out a lot of researches on the issue of the catastrophic earthquake prediction. Especially during the 70 s and 80 s of the twentieth Century, with the development of modern technology and space science, it is confirmed continuously that the pre-earthquake precursor phenomena do really exist, and earthquake can be forecasted in the end(Li, 1966; Ma, 1974; Chen et al, 2008).Predecessors have done pioneering work, for the monitoring and prediction of earthquake, and precious knowledge and experiences have been left. With the frequency of seismic activity, the monitoring density of seismic stations is far from the demand, large areas are still in blank state. Therefore, seismic satellite monitoring technology has become the focus of the current research. Seismic electromagnetic signals are sensitive to the observed parameters, and in the ionosphere, where is easy to obtain and extract the disturbed signals, obvious seismic precursor anomalies are observed(Pulinets, 1998; Liu et al, 2000; Zhang et al, 2010).Ionosphere is the region that is partial ionized in the atmosphere at the height of 70-1000 km, which locates between the non-ionized atmosphere and the fully ionized magnetosphere. As an important portion connecting the near earth space, ionosphere is the focus region during the spheres coupling processes, and it is also a main part to study and understand the solar-terrestrial space environment due to its sensitivity to electromagnetic disturbances. Combined with satellite observation technology, the space electromagnetic abnormal phenomena related to earthquakes has been found as electromagnetic emissions, plasma density perturbations, high-energy particle precipitation, etc. Considering that rock failure will cause local variations of the electromagnetic field during earthquake preparation, the ionospheric electromagnetic emissions, as one of the sensitive parameters, has attracted people's attention. Although the seismic electromagnetic emissions cover a very wide frequency band, the most sensitive band at low frequency has been analyzed emphatically.About the extracting technique of ionospheric anomaly, proposing a variety of algorithms(Molchanov, 2006; N?mec, 2008; Zeren, 2012), experts do relative researches for ionospheric physical parameters, and make a lot of progresses. However, there are still some deficiencies in the analysis of multiple parameters and the characteristics of seismic electromagnetic wave. Based on former researches, in this paper we firstly construct the stable background field observation model, and calculate the magnetic field and electric field variations around the earthquakes, and then the abnormal electromagnetic emissions are extracted accordingly. Furthermore, the properties of abnormal electromagnetic waves are obtained by using the wave vector technique, which are expected to provide example support for the study of the formation of the seismo-ionospheric electromagnetic anomaly and propagation mechanism of the earthquake electromagnetic wave.Utilizing the electromagnetic field observations onboard DEMETER satellite, we statistically analyze the spatio-temporal evolution of electromagnetic field during 45 global strong earthquakes with MS?7.0. Using the power spectral density data of electromagnetic field in the ELF/VLF frequency band(370-897Hz), in the area within ±10° in latitude and longitude from the epicenter, the background model is built up with the same period in five years of 2005-2009(from 90 days before the earthquake to 30 days after the earthquake). At meantime, a typical event is selected with obvious eletromangetic perturbations and rich waveform data, in which the detailed time and position of electromagnetic emissions are obtained to explore the relationship of abnormal electromagnetic waves and the earthquake. The main conclusions are as follows.(1) Statistical analyses on the perturbations amplitude of magnetic field for all 45 earthquakes show that the maximum of perturbations amplitude exceed 2.2 times of the standard deviation in 35 cases, while maximum perturbations in the electric field are higher than twice standard deviation in 39 cases. The occurrence probabilities of the abnormal magnetic, electric and electromagnetic perturbations are 78%, 87% and 60%, respectively. The maximum perturbations mainly appear within ±4° to ±10° in latitude and longitude, which displays deviation from the epicenter positions.(2) The electromagnetic perturbations related to the earthquakes, three types can be classified into according to the temporal variation patterns, in which the maximum abnormal perturbations amplitude occur 30 to 15 days before the earthquake, 15 days before the earthquake to earthquake time, and earthquake time to 15 days after the earthquake, respectively. And the epicenters of the cases with precursory electromagnetic perturbations have some association with certain latitude. There is a positive proportional relation between the maximum perturbation amplitude of electric field, but it will disappear if the magnitude is largerthan 8.0.(3) The study on the random events proves that perturbations amplitude of electromagnetic field over the non-earthquake region are weak, and the probability of abnormal perturbations of random events is lower than earthquakes, illustrating the abnormal electromagnetic phenomena may be caused by earthquakes.(4) Within one month before the 2005 Miyagi, Japan earthquake, abnormal electromagnetic radiations appeared, and before 2 days of the earthquake, over the epicenter, the magnetic and electric field disturbances were enhanced simutaneously. According to the results of wave vector analysis, the abnormal electromagnetic wave was revealed under the local proton cyclotron frequency, showing clear right hand polarization characteristics, in the direction of the Earth, and spreading along the local magnetic meridian and continuously propagating equator-ward. According to the position and direction of the abnormal electromagnetic wave, it is deduced that this abnormal electromagnetic wave may be caused by this earthquake.