Fig. 3

Time series of estimated horizontal displacements of array centroid at (a) BOSS and (b) SAGA; the reference frame is the ITRF2005 (Altamimi et al. 2007). The open and solid red circles indicate the results from the drift observation and the sailing observation, respectively. The linear fitting using the M estimation method (a robust regression technique) and the 90 % two-sided confidence intervals are shown with the red and blue lines, respectively. The green dashed lines indicate the occurrence time of the 2011 Tohoku-oki earthquake (M9.0)