Criticality features in ultra-low frequency magnetic fields prior to the 2013 M6.3 Kobe earthquake

Abstract

<p>The nonlinear criticality of ultra-low frequency (ULF) magnetic variations is investigated before a particular earthquake (EQ) occurred in Kobe on April 12, 2013, by applying the “natural time” analysis on a few ULF parameters: <em>F</em><span><em>_h</em></span>, <em>F</em><span><em>_z</em></span> and <em>D</em><span><em>_h</em></span>. The first two refer to radiation from the lithosphere, and the last parameter corresponds to depression of horizontal component as a signature of ionospheric perturbation. A recent paper of our team has indicated, using the same data as in this paper but by means of conventional statistical analysis, a clear effect of depression in the horizontal component as an ionospheric signature. But there seems to be no convincing signature of lithospheric ULF radiation according to the specific analysis, so this paper aims at extending our study on the electromagnetic data recorded prior to the specific EQ by trying to find any significant phenomenon in ULF effects (both lithospheric radiation and the depression of horizontal component) using the critical, natural time analysis. The natural time analysis has yielded that criticality at Shigaraki (SGA), as the station closest to the EQ epicenter, is reached on March 27-29 for <em>F</em><span><em>_h</em></span> and March 27 to April 1 for <em>F</em><span><em>_z</em></span> (about two weeks before the EQ). But, the criticality for <em>D</em><span><em>_h</em></span> was not observed at SGA probably due to high noise, on the other hand such criticality was observed at Kanoya (KNY) because of its known property of a wider range of detection of ULF depression.</p>