Precursory variations of Tsallis non-extensive statistical mechanics entropic index associated with the M9 Tohoku earthquake in 2011Efthimios S. Skordas, Nicholas V. Sarlis and Panayiotis A. VarotsosEur. Phys. J. Special Topics, 229 5 (2020) 851-859DOI: https://doi.org/10.1140/epjst/e2020-900218-x