The unusual case of the ultra-deep 2015 Ogasawara earthquake (MW7.9): Natural time analysis

Abstract

On 30 May 2015, a powerful earthquake (EQ) (M W 7.9, Japan Meteorological Agency reported magnitude M8.1) struck west of Japan's remote Ogasawara (Bonin) island chain, which lies more than 800 km south of Tokyo. It occurred at 680 km depth in an area without any known historical seismicity. It was the first EQ felt in every Japanese prefecture since intensity observations began in 1884. Here, by applying natural time analysis, which unveils hidden properties in complex time series, we find that almost three and a half months before this powerful EQ, which is the strongest one after the M W 9.0 Tohoku EQ on 11 March 2011, the fluctuations of the order parameter of seismicity were minimized around 17 February 2015. Remarkably, such a behavior has been also observed 1–3 months before all shallow EQs in Japan of magnitude M7.6 or larger that occurred since 1984 until the M W 9.0 Tohoku EQ. We also find that upon the Ogasawara EQ occurrence the change of the entropy of Japanese seismicity upon changing the direction of the time arrow (i.e., under time reversal) exhibited a minimum. This minimum, which may appear when a system approaches a dynamic phase transition, is not equally deep with that observed on 22 December 2010 along with the occurrence of a M7.8 EQ in Bonin islands. The fact that the latter EQ was followed almost 3 months later by the appreciably stronger Tohoku EQ, while the Ogasawara EQ did not, is discussed in the frame of the Lifshitz-Slyozov-Wagner theory of phase transitions and subsequent work by Penrose and coworkers.