Hypocenter locations and focal mechanisms obtained from the OBS observations. (a) Map showing hypocenter locations and error ellipsoids obtained from the OBS observations. The hypocenters are color-coded by the depth. The OBS locations (diamonds) and centroid moment tensor solutions of December 7, 2012 intraplate doublet earthquakes by the Global CMT Projects (GCMT) are also shown. The arrow indicates the epicentral lineation coinciding with the GCMT solution of the shallow normal-faulting earthquake. The red solid line is the MY102 seismic survey line (Ito et al. 2005). The dashed line on the extension of MY102 indicates the area where the velocity structure was not obtained by the seismic survey. The dotted rectangle indicates the grid-search area for the hypocenter locations. (b) Hypocenters and error ellipsoids of earthquakes within the dotted rectangle in (a) were projected on the P-wave velocity model along MY102. The dashed line is the top of the subducting oceanic crust (Ito et al. 2005). (c) Focal mechanisms obtained from the OBS observations. The red and green focal mechanisms correspond to normal and strike-slip faulting earthquakes grouped by rake angles, respectively. No reverse-faulting focal mechanisms were obtained. The P, B, and T axes of the earthquakes are indicated in the inset. (d) Focal mechanisms projected on the cross section along MY102.