Seismo-ionospheric depression of the ULF geomagnetic fluctuations at Kamchatka and Japan

We study preseismic behavior of the ULF geomagnetic variations at Karymshino (E158.13, N52.83), Russia, and Matsukawa (E140.94, N39.88), Japan. A depression of ULF power around local midnight is registered several days before strong isolated earthquakes. The relative depression, i.e. δ D = ( 1 G - 〈 1 G 〉 ) / 〈 1 G 〉 has been analyzed, where the absolute depression is defined as a sum of inversed values of spectral power densities of horizontal components (where 1 G = 1 / P hh and P dd are the mean spectral densities of H and D components) in a sliding window ±(10–15) days. At Karymshino the most evident effect is found 3 days before an earthquake for the 0.5 h vicinity of local midnight in the frequency range 0.02–0.05 Hz. The data from Matsukawa are used to estimate the spatial scale of the effect and its local nature has been confirmed. Furthermore there is a definite correlation between the depression value and index of seismic activity. Thus, the analysis supports a hypothesis on seismic origin of the observed ULF geomagnetic field depression preceding an earthquake.