Ultra-low-frequency magnetic field depression for three huge oceanic earthquakes in Japan and in the Kurile islands

摘要

The depression (reduction in amplitude) of ULF (ultra-low-frequency) magnetic field variations observed on the Earth's surface is found for the recent huge Japan earthquake (EQ) (magnitude(M) ~ 9.0) on March 11, 2011 which is a typical oceanic EQ of the plate type. In order to confirm the presence of such a ULF depression, we have also studied the additional two huge oceanic EQs in the Kurile islands (M≅8), and similar ULF depressions have also been detected. This suggests that such a ULF depression seems to be a universal phenomenon for huge (M=8-9) EQs even occurred in the sea bed of subduction region.