HR: 0830h
AN: T51E-0209    [PDF]
TI: Pre Earthquake Phenomena in Phase Velocities and Gradients of ULF Geomagnetic Disturbances
AU: * Raspopov, O
EM: oleg@orb074.spb.ru
AF: SPBF IZMIRAN, Muchnoy 2 POB 188, St Petersburg, 191023 Russian Federation
AU: Kopytenko, Y
EM: galina@gh5667.spb.edu
AF: SPBF IZMIRAN, Muchnoy 2 POB 188, St Petersburg, 191023 Russian Federation
AU: Ismaguilov, V
EM: Valery.Ismag@pobox.spbu.ru
AF: SPBF IZMIRAN, Muchnoy 2 POB 188, St Petersburg, 191023 Russian Federation
AU: Hattori, K
EM: hattori@iord.u-tokai.ac.jp
AF: RIKEN IFREQ, Chiba University, Chiba, 3-20-1 Japan
AU: Hayakawa, M
EM: hayakawa@whistler.ee.uec.ac.jp
AF: The Univ. of Electro-Communications, 1-5-1 Chofugaoka, Chofu-city, Tokyo, 182-8585 Japan
AB: Tree high-sensitive three-component torsion magnetometers MVC-2DS were operating in Japan before and during a seismic active period started 26.06.2000. A seismic active region was situated to the southeast from Izu peninsula. The strongest seismic shock with magnitude MS=6.4 had taken place 01.07.2000 under a sea bottom at a distance ~85 km from the magnetic stations. The magnetometers were installed at the Izu peninsula in tops of a triangle at a distance 4-6 km from each other. This configuration of the magnetic stations allows determining of gradient and phase velocity vectors along the earth's surface using a phase-gradient method. The vectors of the gradient are usually directed to a source of the ULF EM waves, the vectors of the phase velocity - from the source. A time evolution of the phase velocity and gradient values of the pulsations with periods T=2-512 s (data were preliminary filtered by narrow pass-band filters) was investigated before and during the seismic active period (from 1.02 to 25.07.2000). An increasing of the phase velocity and gradient values was observed ~1.5 months before first seismic foreshock. The greatest values in Z component were reached close to the moment of the EQ. The increasing of the phase velocity values was observed only in direction from the future seismic activity region. There was no change of the phase velocities in the opposite direction. A sharp increasing of the gradients was distinctly seen ~2-3 days before the seismic shocks with Ms$>$6. The effect was most clearly observed in the short-period pulsations in the total horizontal component. A new direction of the gradient vectors in the total horizontal component aroused 2-3 weeks before the seismic activity beginning - just the direction to the future seismic activity region. The phase velocity vectors shown a new direction (just the direction from the future seismic activity region) in the total horizontal component the same 2-3 weeks before the seismic activity start. The phase velocity and gradient vectors in Z component were mostly connected with the seawater induction currents ("sea coast" effect) and only during the seismic active period the vector directions were connected with ULF EM sources situated in the seismic active region. A comparison of the ULF magnetic field variations at the Kakioka magnetic station, located ~200 km to the northwest from the magnetic stations, with the gradient values at the Izu peninsula shows a low correlation between a magnetic activity and the time evolution of the magnetic pulsation gradients.We suppose that the gradients and phase velocities features observed before the seismic activity start is probably connected with an arising of a local geoelectric anomaly connected with the future seismic region. The increasing of the gradients just before the strong seismic shocks is probably connected with the ULF EM emissions exited in the seismic activity region situated in the Earth's crust.
DE: 2400 IONOSPHERE
DE: 2487 Wave propagation (6934)
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2003 Fall Meeting