Gradients and phase velocities of ULF magnetic disturbances (F = 0.1-0.4 Hz) before and during strong earthquakes in 2003 year at Bosso peninsula (Japan)

Measurements of ULF EM variations (T = 1-1000 s) were carried out in Japan before and during a seismically active period in 2003. Three of the same type 3-component high-sensitivity torsion magnetometers, MVC-2DS, were located at Bosso peninsula at the vertices of a triangle at a distance 4-5 km from each other (a magnetic gradiometer). The magnetic gradiometer gives an opportunity to construct gradient and phase velocity vectors of the ULF magnetic disturbances. The gradient vectors are usually directed to the source of the magnetic disturbances. The phase velocity vectors usually have the opposite direction. Seismic activity was observed in this region during the whole year. We observed anomalous behavior of the gradients and phase velocities before strong earthquakes.