Criticality features in ULF magnetic fields prior to the 2011 Tohoku earthquake

The criticality of ULF (Ultra-low-frequency) magnetic variations is investigated for the 2011 March 11 Tohoku earthquake (EQ) by natural time analysis. For this attempt, some ULF parameters were considered: (1) F(h) (horizontal magnetic field), (2) F(z) (vertical magnetic field), and (3) D(h) (inverse of horizontal magnetic field). The first two parameters refer to the ULF radiation, while the last parameter refers to another ULF effect of ionospheric signature. Nighttime (L.T. = 3 am ± 2 hours) data at Kakioka (KAK) were used, and the power of each quantity at a particular frequency band of 0.03–0.05 Hz was averaged for nighttime hours. The analysis results indicate that F(h) fulfilled all criticality conditions on March 3–5, 2011, and that the additional parameter, D(h) reached also a criticality on March 6 or 7. In conclusion, criticality has reached in the pre-EQ fracture region a few days to one week before the main shock of the Tohoku EQ.