Spatio-temporal distribution of seismic moment release near the source area of the 2011 Tohoku-Oki earthquake

To understand the generation mechanism of the 2011 Tohoku-Oki earthquake, we analyzed the spatial and temporal distribution of the cumulative seismic moment (ΣM₀) for all earthquakes along the Japan Trench listed in the global centroid moment tensor catalog between January 1976 and November 2011. Three areas with distinct characteristics of ΣM₀ are identified: (1) in the 2011 Tohoku-Oki source area, the ΣM₀ released in the down-dip portion (≥30 km) was greater than that in the up-dip portion (<30 km) before the mainshock; (2) the ΣM₀of the up-dip portion in the region where slow slip activity prevails is greater than that of the down-dip portion throughout the study period; (3) in the surroundings of the source area, where interplate coupling is relatively low and the largest foreshock occurred, values of ΣM₀ for the down-dip and up-dip portions are both intermediate. Our results show that a megathrust rupture could be generated by large accumulations of strain energy at the subduction interface, resulting from the differential strain energy released in the down-dip and updip portions during the interseismic interval. We propose that the variation pattern of ΣM₀ may offer spatial constraints for seismic hazard assessment in the future.