Background Variation Of SR And Its Application In Earthquake Monitoring

Schumann resonances (SR) are global electromagnetic (EM) resonances excitedby lightning discharges in the cavity (waveguide) formed by the earth’s surface andits ionosphere. In the normal mode description of SR, the fundamental mode is astanding wave with a wavelength equal to the circumference of the Earth. And thelowest-frequency (and highest-intensity) mode of the SR occurs at a frequency ofapproximately7.83Hz，and the higher resonance modes are spaced at about6Hzintervals, a characteristic attributed to the atmosphere’s spherical geometry. The SR isin the extremely low frequency (ELF) range of the Earth’s EM spectrum. Because ofthe resonance interaction, EM energy of SR is stronger than the other frequencies’.Due to the field source and propagation path, the observations of SR have been usedto track global lightning activity and monitor global temperature variations. In recentyears, studies have found that earthquakes would disturbance the stable parametersand spectra of Schumann resonances, so motiving the study of application of SRanomalies to earthquake monitoring.To find out the EM field background variation is the basis of distinguishing theseismic electromagnetic (SEM) anomalies. So in this thesis, taking advantage of the12ELF electromagnetic monitoring stations’ natural EM field data, I processed andanalysised EM field components’ power spectrum curve of SR frequencies. And Iobtained Schumann resonance background change with seasons and latitudes. Then Itake several stations for example to analyze the curve shape and the electromagneticfield intensity of SR before and after earthquakes.The EM method using controlled-source ELF waves is a new technology that isbased on the large-power alternating EM field generated by an artificial procedure. Inorder to describe the EM anomalies before earthquakes better, I use the artificialemissive source to simulate earthquakes, and calculate the electric and magnetic fieldauto-power spectrum of SR in the different electric dipole moments. The main workof this thesis is summarized as follows:(1)Obtain the variation with seasons and latitudes of SR observed in low-latitude regions of the Northern hemisphere.I aggregate and process the data observed in the twelve ELF stations, and use theEM field’s auto-power spectrum to describe the intensity of the EM field. I choose thestations which recorded for longer time with better data, and utilize the interpolationmethod to write software to extract Schumann resonances frequencies’ power spectraldata. Then I obtain the electric and magnetic field intensity distributions of thestations. After a series of processing of the data preprocessing and the moving averagenoising, I know the background variation of SR is as follows. Comparing SR’electromagnetic field curves of one station, I see the EM field intensity changes withseasons obviously. It is weaker in winter and stronger in summer’s sinusoidalvariation. Afterwards I contrast the same frequency’s graphs of different stations. It isobserved that the EM field intensity is enhanced with the decreasing of the latitude,and this phenomenon is more pronounced in summer and autumn.I explain this phenomenon from the perspective of field sources. Schumannresonances (SR) are excited by lightning discharges, and propagate in the cavityformed by the Earth surface and the ionosphere. The resonance activity reflects theglobal lightning activity, and the more thunderstorm activities occurred in summerand near-equatorial regions. So the horizontal electric and magnetic fields intensity ofSR are stronger in summer and near-equatorial regions.(2)Analysis the seismic electromagnetic (SEM) anomalies measured atstations.I collect the detailed information of earthquakes which occurred in China(Ms>5.0) and other countries (Ms>6.5) since2009when the data has recorded.Taking into account the influence of the epicentral distance, I calculate the distance ofthe epicenters and the ELF stations. Then I choose the earthquakes which meet thefollowing conditions:(a). Ms=5-6and the epicentral distance is less than500km;(b).Ms=6-7and the epicentral distance is less than1500km;(c). greater than Ms=7andthe epicentral distance is less than3000km. And I put the earthquake information intothe power spectrum curve to observe the EM anomalies before and after the earthquakes. I found when earthquakes are frequently, the EM background field willnot be of one-year sinusoidal variation. And this shows that the earthquakes canindeed cause EM anomalies.The current SEM method is mainly empirical, which needs a mass of reliableobservational data for statistical analysis of the phenomenon.(3) Use the artificial emissive source to simulate the EM radiation ofpre-earthquake, and calculate the electric and magnetic field auto-powerspectrum of SR in the different electric dipole moments.I use the software to calculate the controlled-source ELF EM field. And I utilizeartificial emission source to simulate the EM radiation which is released bypre-earthquakes, then I calculate the EM field intensity of SR frequencies in the near,far and waveguide zones. Afterwards through the unit conversion and fast Fouriertransform processing.I calculate and plot the field intensity radiation pattern of each component in near,far and waveguide zone respectively. And I select three points in each fieldcomponent: the zero line, the maximum region and the farthest point. Then I find thefollowing characteristics: the emission current increases, and the radiation intensity isgreater. Even the observation point is close to the emission source, and it is in the zeroline, the observed EM field intensity is much weaker than the farthest point and themaximum value region.(4) Comparison the numerical simulation with the SR background fieldwhich is observed in the ELF stations.Because the ELF stations observe the four horizontal components of the electricand magnetic fields, and use the Cartesian coordinate system for data processing, Ican only compare the results of the near and far zones. If the earthquake can producethe EM radiation of4000Akm～20000Akm, I put the intensity of EM field about150km from the emission source into the stations’ background field, more than fourorders of sudden jump can also be seen. However, if the observation point is in thezero line, even the distance is very close, any anomaly can’t be found. This shows that the EM radiation has strong directional nature, and it may be one of the reasons thatthe EM anomalies and the earthquake can’t always correspond to each other.These phenomena will have the guiding significance for the judgment of SEManomalies, station location and data processing. And it presents another point ofunderstanding view through estimating the earthquake current flow direction for thestudy of SEM mechanism.