Fig. 1

Tectonic setting and bathymetric map of the study area. a Tectonic setting of the study area. Brown contours show depths of the upper boundaries of the Pacific plate (PAC) obtained by Nakajima and Hasegawa (2006) and Nakajima et al. (2009), with dashed contours estimated by interpolation. See text for details. Blue contours show the depth of the upper boundary of the Philippine Sea plate (PHS) estimated by Uchida et al. (2010). Blue dashed line denotes the NE limit of the PHS reported by Uchida et al. (2009). The source areas of the 1703 Genroku Kanto earthquake and the 1923 Kanto earthquake are indicated by gray and green lines, respectively (e.g., Matsu’ura et al. 2007; Sato et al. 2016). The area of slow slip events is shown in yellow (e.g., Ozawa et al. 2003; Sagiya 2004; Hirose et al. 2014). Black lines with triangles represent trench axes. Black arrows indicate the motion of the PAC and the PHS with respect to the North American plate (NA) (DeMets et al. 2010). b Bathymetric map showing the locations of stations used in this study, with orange and yellow triangles indicating deployed broadband ocean-bottom seismometers (BBOBSs) and short-period ocean-bottom seismometers (SPOBSs), respectively, and gray triangles and squares indicating OBSs of the Earthquake Research Institute (ERI) of the University of Tokyo and stations operated by the National Research Institute for Earth Science and Disaster Prevention, respectively. Small gray dots are epicenters derived by the Japan Meteorological Agency (JMA) during our observation period of March 2012 to March 2013. Earthquake selection areas are delineated by orange, yellow, and blue lines, and white dots are the epicenters determined by JMA that were analyzed in this study. See text for details