Earth Planets Space, Vol. 63 (No. 7), pp. 719-723, 2011
Masahiro Kosuga and Kazutoshi Watanabe
Earthquake and Volcano Observatory, Graduate School of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan
(Received April 7, 2011; Revised June 2, 2011; Accepted June 3, 2011; Online published September 27, 2011)
We investigated the seismic activity around the northern neighbor of the 2011 off the Pacific coast of Tohoku Earthquake (MW 9.0) with special attention to a potential large aftershock in the area. We obtained a combined data set by adding our manually-picked locations to the catalog locations by the Japan Meteorological Agency. The hypocenter distribution delineates active and inactive bands of seismicity. The band of low seismicity corresponds to a zone of a large seismic slip, indicating that aftershocks occurred in peripheral neighbors of the mainshock asperity. The broad band of active seismicity along the coast corresponds to the zone of a large postseismic slip, suggesting the enhancement of the aftershock activity by the slip. Although the northern neighbor of the mainshock fault is a favored region of increased seismicity, as shown from a Coulomb stress calculation, no significant seismic activity is observed within the potential source area except along the Japan Trench and the SW corner. This implies that the zone of interplate moment release by previous large earthquakes and the subsequent slow slip acted as a barrier to the migration of both the mainshock rupture and aftershock activity. However, an aftershock area in the zone may reflect inhomogeneous moment release by past seismic and aseismic sequences.
Key words: 2011 off the Pacific coast of Tohoku Earthquake, hypocenter distribution, aftershock activity, postseismic slip.