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The characteristics of the ULF magnetic field emissions measured at two magnetic observatories in the Republic of Georgia prior to and after the Ms = 6.9 earthquake that occurred near Spitak, Armenia, on December 7, 1988, are compared with the apparently similar emissions associated with the Ms = 7.1 earthquake that occurred near Loma Prieta, California, on October 17, 1989. The main features of the Spitak measurements, according to observations made at the Dusheti station (128 km to the Spitak epicenter), as compared with the Loma Prieta measurements, which were made at Corralitos, California (7 km to the Loma Prieta epicenter), are the following: (1) The intensity of ULF background activity started growing 3 to 5 days before the Spitak earthquake, whereas the corresponding increase in activity began 12 days before the Loma Prieta earthquake; (2) a substantial ULF emission burst was recorded at Dusheti starting 4 hours prior to the main shock; a similar large burst of ULF activity commenced 3 hours before the Loma Prieta event, and continued until the occurrence of the main shock; (3) ULF activity remained high for about two weeks after the Spitak earthquake, and for several months after the Loma Prieta earthquake; (4) ULF noise bursts were observed 1 to 6 hours before powerful aftershocks at Spitak during the period of enhanced activity, but there was no conclusive link between the ULF noise at Corralitos and the after‐shocks. A major difference in the ULF activity preceding the two earthquakes is a difference in amplitude (0.2 nT at Spitak and 5 nT at Loma Prieta), but this is easily explained as being caused by the different distances of the observation stations from the epicenters.

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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 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.

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