Natural Time Analysis of Seismicity: Recent Results

Natural time analysis introduced almost a decade ago[1] may uncover novel dynamic features hidden in the time series of complex systems and has been applied[2] to diverse fields. For a time series comprising N events, the natural time for the occurrence of the k-th event of energy Q_k is defined by χ_k=k/N and the analysis is made by studying the evolution of the pair (χ_k,p_k ), where p_k=Q_k/ΣQ_n is the normalized energy. In natural time analysis of seismicity, the variance κ₁ of natural time χ weighted for p_k calculated from seismic catalogues serves as an order parameter [2]. The Japan seismic catalog was analyzed in natural time by employing a sliding natural time window of fixed length comprised of the number of events that would occur in a few months. This is a crucial time scale since it corresponds to the average lead time of the observed Seismic Electric Signals (SES) activities [2]. The following results are obtained: First, the fluctuations of the order parameter of seismicity exhibit [3] a clearly detectable minimum approximately at the time of the initiation of the pronounced SES activity observed [4] almost two months before the onset of the volcanic-seismic swarm activity in 2000 in the Izu Island region, Japan. This is the first time that before the occurrence of major earthquakes, anomalous changes are found to appear almost simultaneously in two different geophysical observables. Second, these two phenomena were shown to be also linked in space[3]. Third, minima of the order parameter fluctuations of seismicity were observed [5] a few months before all shallow earthquakes of magnitude 7.6 or larger that occurred from 1 January 1984 to 11 March 2011 (the day of the M9 Tohoku earthquake) in Japanese area. Among these minima, the minimum before the M9 Tohoku earthquake was the deepest. Additional recent results are forwarded which shed more light on the importance of the aforementioned minima for earthquake prediction purposes. [1] Varotsos, P. A.; Sarlis, N. V. and Skordas, E. S. Phys. Rev. E (2002) 66, 011902 [2] Varotsos, P. A.; Sarlis, N. V. and Skordas, E. S. Natural Time Analysis: The new view of time. Precursory Seismic Electric Signals, Earthquakes and other Complex Time-Series Springer-Verlag (2011) [3] Varotsos P.A.; Sarlis N.V.; Skordas E.S. and Lazaridou M.S., Tectonophysics (2013) 589, 116-125 [4] Uyeda, S.; Kamogawa, M; and Tanaka, H. J. Geophys. Res., (2009) 114, B02310 [5] Sarlis, N. V.; Skordas, E. S.; Varotsos, P. A.; Nagao, T.; Kamogawa, M.; Tanaka, H. and Uyeda, S. Proc. Natl. Acad. Sci. U.S.A. (2013) DOI: 10.1073/pnas.1312740110