Fig. 1

a Study area with the slip distribution of the 2011 Tohoku-oki earthquake within a grey square (Lay et al. 2011), as well as the 2011 Ibaraki-oki earthquake within a black square (Kubo et al. 2013). The grey triangles show the stations of the OBS array in the off Ibaraki region. The blue circle shows the subducting seamount (Mochizuki et al. 2008). The green line shows the northeastern limit of the Philippine Sea Plate (Uchida et al. 2010). b Distribution of seismicity and OBS stations. The white triangles show OBS stations. The circled white triangles show the stations which were recovered approximately 2 weeks after the 2011 Tohoku-oki earthquake occurred. The colored circles show the earthquake hypocenters determined by JMA with their color indicating their depth. The red rectangle represents the region, where the Green’s functions were calculated. c Seismic velocity structure model along profile A–A’ in (b). The region shallower than ~ 10 km was constructed by Yamaya et al. (2021), and deeper by Koketsu et al. (2012)