Seismic Regime in the Vicinity of the 2011 Tohoku Mega Earthquake (Japan, Mw = 9)

The 2011 Tohoku mega earthquake ( M w = 9) is unique due to a combination of its large magnitude and the high level of detail of regional seismic data. The authors analyzed the seismic regime in the vicinity of this event using data from the Japan Meteorological Agency catalog and world databases. It was shown that a regional decrease in b -value and of the number of main shocks took place in the 6–7 years prior to the Tohoku mega earthquake. The space–time area of such changes coincided with the development of precursor effects in this area, as revealed by Lyubushin (Geofiz Prots Biosfera 10:9–35, 2011 ) from the analysis of microseisms recorded by the broadband seismic network F-net in Japan. The combination of episodes of growth in the number of earthquakes, accompanied by a corresponding decrease in the b-value and average depth of the earthquakes, was observed for the foreshock and aftershock sequences of the 2011 Tohoku earthquake. Some of these anomalies were similar to those observed (also post factum) by Katsumata (Earth Planets Space 63:709–712, 2011 ), Nanjo et al. (Geophys Res Lett 39, 2012 ), and Huang and Ding (Bull Seismol Soc Am 102:1878–1883, 2012 ), whereas others were not described before. The correlation of the periods of growth in seismic activity with the decrease of the average depth of earthquakes can be explained by the growth of fluid activity and the tendency of a penetration of low density fluids into the upper horizons of the lithosphere. The unexpectedly strong Tohoku mega earthquake with a rather small rupture area caused an unexpectedly high tsunami wave. From here it seems plausible that M9+ earthquakes with a large tsunami could occur in other subduction zones where such cases were suggested before to be impossible.