Abstract
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 Σ M0 of 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 up-dip portions during the interseismic interval. We propose that the variation pattern of ΣM0 may offer spatial constraints for seismic hazard assessment in the future.
Highlights
The Japan Trench, where the Pacific plate subducts beneath the Okhotsk plate at an average rate of approximately 8 to 8.5 cm/year (DeMets et al, 2010), has experienced numerous interplate earthquakes with a moment magnitude (MW) of 7 to 8 (Sagiya, 2004; Ishibe and Shimazaki, 2009; Ozawa et al, 2011)
The M0 of the down-dip area reveals a reverse distribution pattern, with a high M0 observed in the area of 36–38◦N (Fig. 4(b)). This seismic moment release pattern indicates that a large amount of strain energy was released in the down-dip portion of the source area, and a small amount of energy was released in the up-dip side during the interseismic period
A large amount of energy was released by the 1994 mainshock and slow slips followed subsequently in the rupture area (Kawasaki et al, 2001), as expressed by the substantial seismic moment release in the up-dip portion shown in category 2 (Fig. 5(a))
Summary
The Japan Trench, where the Pacific plate subducts beneath the Okhotsk plate at an average rate of approximately 8 to 8.5 cm/year (DeMets et al, 2010), has experienced numerous interplate earthquakes with a moment magnitude (MW) of 7 to 8 (Sagiya, 2004; Ishibe and Shimazaki, 2009; Ozawa et al, 2011). This seismic moment release pattern indicates that a large amount of strain energy was released in the down-dip portion of the source area, and a small amount of energy was released in the up-dip side during the interseismic period.
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