Fig. 1

Tectonic map in and around the Hyuga-nada region. The thick barbed black line denotes the Nankai Trough. The black dashed lines in Kyushu are the boundaries of the prefectures. The thin black solid lines represent isodepth contours of the geometry of the upper surface of the PHS plate subducting beneath Kyushu: the 10 km isodepth by Baba et al. (2002), the 20–50 km isobath by Hirose et al. (2008), and the isodepth contours deeper than 60 km by Nakajima and Hasegawa (2007). The light green circle represents the approximate location of the Bungo Channel. The green bold line indicates the outer edge of the subducted Kyushu-Palau Ridge (KPR) by Yamamoto et al. (2013). The red and blue solid circles indicate the GNSS observation stations analyzed in this study whose detailed information is listed in Additional file 1: Table S1, and the blue solid squares represent the reference stations used in this study. The yellow–green box represents the horizontal projection of the source region of the assumed model on the 3D plate boundary. The red solid star represents the epicenter of the Mw7.0 Kumamoto earthquake on April 14, 2016. Processed time series data for all the stations (red and blue solid circles) with station codes are shown in Fig. 2. The gray boxed area in the inset shows our study area