Fig. 1.

Location map of ocean bottom seismometers with bathymetric features as of April 8th, 2011. Triangle and hexagons indicate the positions of pop-up type OBSs deployed before the mainshock. Circles and squares denote the positions of OBSs deployed for an urgent observation after the mainshock. Various types of OBSs are used for the aftershock observation. See an inset legend. Yellow hexagons indicate the position of the OBSs which were recovered on March 30th, 2011 and were used for a preliminary analysis. Large and small stars indicate the epicenters of the mainshock and the largest aftershock, respectively. Thick line indicates the profile of the seismic survey using OBSs, an airgun array and explosives in 2008 (Nakahigashi et al., in preparation).