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Abstract : Measurements of ULF electromagnetic disturbances were carried out in Japan before and during a seismic active period 1 February 2000 to 26 July 2000. A network consists of two groups of magnetic stations spaced apart at a distance of ?140 km. Every group consists of three, 3-component high sensitive magnetic stations arranged in a triangle and spaced apart at a distance of 4?7 km. The results of the ULF magnetic field variation analysis in a frequency range of F = 0.002?0.5 Hz in connection with nearby earth-quakes are presented. Traditional Z-G ratios Z is the vertical component, G is the total horizontal component, magnetic gradient vectors and phase velocities of ULF waves propagating along the Earth-s surface were constructed in several frequency bands. It was shown that variations of the RF = Z-G parameter have a different character in three frequency ranges: F1 = 0.1 ± 0.005, F2 = 0.01 ± 0.005 and F3 = 0.005 ± 0.003 Hz. Ratio RF3-RF1 sharply increases 1?3 days before strong seismic shocks. Defined in a frequency range of F2 = 0.01 ± 0.005 Hz during nighttime intervals 00:00?06:00 LT, the amplitudes of Z and G component variations and the Z-G ratio started to increase ? 1.5 months before the period of the seismic activity. The ULF emissions of higher frequency ranges sharply increased just after the seismic activity start. The magnetic gradient vectors ? B ? 1 ? 5 pT-km, determined using horizontal component data G ? 0.03 ? 0.06 nT of the magnetic stations of every group in the frequency range F = 0.05 ± 0.005 Hz, started to point to the future center of the seismic activity just before the seismoactive period; furthermore they continued following space displacements of the seismic activity center. The phase velocity vectors V ? 20 km-s for F = 0.0067 Hz, determined using horizontal component data, were directed from the seismic activity center. Gradient vectors of the vertical component pointed to the closest seashore known as the -sea shore- effect. The location of the seismic activity centers by two gradient vectors, constructed at every group of magnetic stations, gives an ? 10 km error in this experiment.

Autor: V. S. Ismaguilov - Yu. A. Kopytenko - K. Hattori - P. M. Voronov - O. A. Molchanov - M. Hayakawa -

Fuente: https://hal.archives-ouvertes.fr/


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