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 earthquakes 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 R(F ) = Z/G parameter have a different character in three frequency ranges: F 1 = 0.1 ± 0.005, F 2 = 0.01 ± 0.005 and F 3 = 0.005 ± 0.003 Hz. Ratio R(F 3 )/R(F 1 ) sharply increases 1-3 days before strong seismic shocks. Defined in a frequency range of F 2 = 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, conCorrespondence to: V. S. Ismaguilov (galina@gh5667.spb.edu) structed at every group of magnetic stations, gives an ≈10 km error in this experiment.

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 passband 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.

No abstract

One of the strongest earthquakes, with magnitude M 8.9, occurred at the sea bottom near to the east coast of Japan on March 11, 2011. This study is devoted to the investigation of anomaly disturbances in the main magnetic field of the Earth and in ultra-low frequency magnetic variations (F <10 Hz) observed before this earthquake. Secular variations of the main geomagnetic field were investigated using three-component 1-h data from three magnetic observatories over the 11-year period of January 1, 2000, to January 31, 2011. The Esashi and Mizusawa magnetic stations are situated northwest of the earthquake epicenter, at distances of around 170 km to 200 km, and the Kakioka observatory is situated southwest of the earthquake epicenter, at a distance of about 300 km. During this period, there were four local anomalies in the secular variations. The last anomaly was the biggest, which began around 3 years prior to the earthquake moment. All of the anomalies can be most distinctly recognized, in the form of differences in the corresponding magnetic components at these remote magnetic stations. For investigations of the ultra-low frequency magnetic field disturbances, three-component 1-s data at two magnetic stations (Kakioka and Uchiura) were used. The Uchiura station is situated 119 km south of Kakioka, at a distance of about 420 km from the earthquake epicenter. Data from the time interval of February 18, 2011 to March 10, 2011 01:00 to 04:00 local time) were investigated in a wide frequency range. In the frequency range of 0.033 Hz to 0.01 Hz, there was the clearest anomaly, seen as a decrease in the correlation coefficients of the corresponding magnetic components at these two stations, from February 22, 2011. Differences in the Z components showed an increase, and became positive after this date. This might suggest that the ultra-low frequency lithospheric source appeared north of the Kakioka station. Outside this specified frequency range, the anomalies were not well defined.

Results of investigation of ULF electromagnetic disturbances recorded in Japan during a seismic active period are discussed. Observations were carried out at two magnetic stations located at a distance ~4 km along a geomagnetic meridian. A central zone of the seismic activity was situated ~35 km East of the magnetic stations at the depth ~3 km under sea bottom. Methodological aspects of data processing are considered in connection with large industrial noise observed in the territory of Japan. A digital filtration and spectral method were used to calculate RMS values of H, D, and Z components of magnetic variations. The data were filtered in 4 frequency ranges f 1 =0.2-2.5 Hz, f 2 =0.05-0.2 Hz, f 3 =0.01-0.05 Hz, and f 4 =0.002-0.0l Hz for 2, 4 and 8-hours nighttime intervals of each day during the period 21.03-02.06.1998. It is found that RMS values of raw data increase during the bursts of the seismic activity. Ratios of RMS values of Z component to the horizontal ones (Z/H, Z/D, ZIG) increase ~ 15 days before a start of the seismic active period, then the they decrease and reach their minimum at the moment of an earthquake (strongest shock Ms=5. 7), followed by a smooth relaxation of these ratio values. The availability of two magnetic stations gives an opportunity to define gradients of the electromagnetic disturbances. The RMS values of the gradients and the ratios of Z component gradients to the horizontal ones are found to exhibit the same temporal evolutions as the raw data.