Variations of phase velocity and gradient values of ULF geomagnetic disturbances connected with the Izu strong earthquakes

Abstract. Results of study of anomaly behavior of amplitudes, phase velocities and gradients of ULF electromagnetic disturbances (F = 0.002 – 0.5 Hz) before and during a seismic active period are presented. Investigations were carried out in Japan (Izu and Chiba peninsulas) by two groups of magnetic stations spaced apart at a distance ~ 140 km. Every group (magnetic gradientometer) consists of three 3-component high sensitive magnetic stations arranged in a triangle and spaced apart at distance 4–7 km. Kakioka magnetic station (>200 km to the North from Izu) was used as a reference point. Available data (only night-time intervals 00:00–07:00 LT) were preliminary filtrated by narrow pass-band filters (16 frequency bands – periods T = 2–512 s). The amplitude, gradient and phase velocity values and probabilities of directions of gradient and phase velocity vectors were constructed for the every frequency band. Apparent resistivities of the Earth’s crust in the every frequency band were calculated using the phase velocity values. It was found that Z component amplitudes of the ULF magnetic disturbances increased at Izu peninsula 2–4 weeks before the seismic active period and 2–4 days before the strongest seismic shocks (M > 6). Ratio of Z component amplitudes of Kamo (Izu) and Kakioka data (Z_k/Z_kk) increased during 2–4 weeks before the seismic activity start (27 June 2000) and reached a maximum just before a moment of the strongest seismic shock (EQ with M = 6.4). The gradient and the phase velocity values had an anomaly behavior during the same 2–4 weeks before the start of seismic active period. The gradient vectors of the total horizontal component of the ULF magnetic pulsations were probably directed to the regions with increased conductivity. New additional direction of the gradient vectors appeared 2–3 weeks before the seismic activity start – the direction to the seismic active area which appeared due to a magma rising.