LAUSR

Abstract The method of critical fluctuations (MCF) is a time series analysis method which provides a way of both identifying the critical state and the departure from it. Its application to the ground-based ultra-low frequency (ULF) magnetic field fluctuations observed prior to the very strong earthquakes (EQs) of magnitudes M W = 6 . 2 , M W = 6 . 0 and M W = 7 . 0 which sequentially took place within a two days period (April 14–15, 2016) in Southwest Japan under the city of Kumamoto reveals when the underlying process reached critical state, as well as when it departed from critical state. Specifically, critical dynamics was identified in the raw total geomagnetic field intensity (F) ULF recordings of the Kanoya (Japan) station ∼ 4 d before the M W = 6 . 2 and ∼ 5 d before the M W = 7 . 0 event. Moreover, the progressive development of the symmetry breaking phenomenon, which signifies departure from critical state, was identified in later recordings. Specifically the phenomenon evolved from a few hours before the M W = 6 . 0 event until a few hours after the occurrence of the specific EQ, which was a few hours before the M W = 7 . 0 event. The detected combination of phenomena, i.e., the detection of critical state followed by the detection of the departure from the critical state by means of symmetry breaking, indicates that the main event was the third and strongest EQ, while the two preceding EQs were strong foreshocks.