Fig. 4

GPS measurements in Sumatra and its forearc. The black arrows show the annual movement vectors relative to Southeast Asia estimated by Prawirodirdjo et al. (1997) for the period 1989–1993, the blue arrows the vectors from measurements made by Bock et al. (2003) in 1991–1997 and 2001, close to the beginning of a series of megathrust earthquakes spanning a period of seven years. Red circles mark the epicentres of earthquakes listed in the catalogues of the National Earthquake Information Center as having magnitudes (Mw) of more than 7.5. Magnitudes and dates are shown to the left of the margin. Events named in blue (e.g. ACEH) are those with magnitudes of more than 8, cited in the text. The earthquake search period was from 2000 to 2020, but there were no events satisfying the search criteria during its second half. More recent GPS studies (Feng et al., 2015) have concentrated on estimating short-term post-seismic displacements that are unlikely to be identifiable as such in the geological record. The thick brown line oriented roughly NE-SW south of Nias marks the location of the seismic section shown in Fig. 5. The dashed blue line marks the trace of the Mentawai Fault imaged in Fig. 5. Illustration modified from Fig. 2.4 of Milsom (2005a)