Data from yearly GPS campaigns (unlike continuous GPS) after the earthquake are less suitable for the accurate detection of vertical coseismic displacements, since they typically have noisier position estimates, and also are affected by a strong post-seismic signal. This makes the use of GPS data from post-2004 campaigns at BANH and
CHON less reliable andmeasurements fromCGPS are more appropriate. Most of the CGPS sites are in Malaysia (Vigny et al., 2005). Fig. 1 shows the 4 CGPS nearest to the tidal stations. Co-seismic vertical displacements at these locations, determined from GPS processing (the difference between the first weekly averaged static solution after and
before the earthquake), are PHKT (20.4±14.5 mm), NTUS (−8.1±5.2 mm), ARAU (12.1±13.0 mm) and BNKK (−5.9±9.2 mm). These co-seismic rates are derived from a data set, of which the horizontal component is analyzed and presented in Satirapod et al. (2011).
Both uplift and subsidence are present at PHKT and NTUS, contradicting the model that predicts only subsidence in the far field. At BNKK and ARAU, standard deviations are too large to be useful for co-seismic corrections.Besides, none is close enough to any group 1B tide gauges. Vertical coseismic displacement is comparable to a shift of reference and could cause a bias in the determined trend. The unavailability of reliable coseismic
magnitudes is a reason for us to exclude post-2004 data from the analysis.