Earth Planets Space, Vol. 63 (No. 7), pp. 659-662, 2011
Building Research Institute, Tsukuba, Ibaraki 305-0802, Japan
(Received April 7, 2011; Revised May 19, 2011; Accepted May 20, 2011; Online published September 27, 2011)
We relocated foreshocks, mainshock, and aftershocks of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.1) using the modified joint hypocenter determination (MJHD) method in order to obtain their precise hypocenters and to identify fault planes of larger earthquakes. We used P-wave arrival times at stations worldwide reported by the U.S. Geological Survey (USGS). We confirmed by relocated hypocenters that the mainshock and aftershocks had occurred along the plate boundary between the North American and Pacific plates. We also confirmed that the Mw 7.5 foreshock, which occurred two days before the mainshock, and the largest aftershock (Mw 7.9), which occurred half an hour after the mainshock, were thrust earthquakes along the plate boundary. The second largest aftershock (Mw 7.6), which was a normal-faulting earthquake and was a bending-stress intra-plate event caused by the strain reduction on the subduction thrust, occurred at the outer rise of the Japan Trench and was well relocated with its aftershocks. We found that its fault plane dipped westward and it bounded the aftershock distribution on the seaward side. This implies that the western side of the fault plane had subsided, corresponding with the westward plate subduction.
Key words: 2011 off the Pacific coast of Tohoku Earthquake, relocation, fault plane, joint hypocenter determination.