Seismogenic magnetic activity study

The earthquakes (EQ) are one of the most devastating natural hazards and their study is a permanent challenge of geophysics. The EQ often occur in Carpathian-Balkan region and any progress in the study of the possibility to predict them is welcome. It is known that seismic, in spite of numerous experimental attempts and theoretical considerations, is not efficient for short-term prediction of EQ. Much more chances have the registration of electromagnetic (EM) radiation. It is experimentally confirmed that the most advantageous for study of magnetic variations accompanying the EQ preparation process is ultra-low frequency (ULF) band (0.001-0.5 Hz), and namely the monitoring of ULF signals is believed to be efficient for the EQ forecasting problem. There are numerous observations of ULF magnetic field enhanced activity before EQ as well as many approaches to construct a credible physical model of this phenomenon. The greatest problem which arises at the attributing of the observed ULF activity to the EQ under preparation is the necessity to separate the seismogenic signals from the natural fluctuations of ionospheric origin which fully coincide as to the frequency band with seismogenic ones being much more powerful and often occurred. Several methods of their separation proposed in the literature proved to be not enough efficient. This work describes still one attempt to select the candidates for EQ precursors, basing on the experimental data, collected in India and China by multi-point synchronized observation magnetometer network. The temporary network in India was formed by specially developed for EQ-related magnetic signals measurement low-noise magnetometers LEMI-30, installed near the EQ focal area. For study of pre EQ magnetic activity in China the stationary flux-gate magnetometer network was used. The data from these magnetometers spaced by distances 50-100 km collected during observation campaigns have been analyzed. The wave forms, dynamical Fourier spectra and polarization ellipse parameters of signals from magnetometers pairs have been studied and compared with seismic activity and natural magnetic field variations data. A complete analysis of these multi-points data allowed us to propose a new criterion for the extraction of seismogenic ULF signals from the interference background. It was shown that the controlled by the orientation of seismogenic faults resulting seismo-EM field would have definite orientation in comparison to the isotropic direction distribution of highly variable natural signals arising from complex ionospheric-magnetospheric interactions. Basing on these physical considerations, it was revealed that the intersection lines formed by the planes of polarization ellipses calculated for the magnetic fields measured at minimum two sites, define the azimuth to seismo-EM source. Further, ratio of major axes of these polarization ellipses above certain threshold was taken as second selection value helping to distinguish ULF signals dominated by seismo-EM origin from those associated with ionospheric origin. The details of the method and obtained experimental results for two EQ occurred in India and China are discussed in the report. These works were partially supported by STCU grant 3165.