A major obstacle preventing the use of complete tide gauge data
time series in the determination of long-term sea level trend are the
co- and post-seismic motions caused mainly by the 2004 Mw 9.2
Sumatra-Andaman earthquake. Co-seismic vertical land shifts at tide
gauges along the coast of Gulf of Thailand are still largely unknown
but magnitudes obtained from CGPS at other places in the region
show that they are not negligible. Further, magnitudes of non-linear
post-seismic motions indicated by GPS measurements near Sattahip
and Ko Mattaphon are roughly estimated to be several times larger
than rising rates of sea level (on the order of−10 mm/yr). At present,
the availability of co- and post-seismic motion corrections are not of
paramount importance as the number of years after the earthquake
is much shorter than the total years of the record. As a result, no
significant change in absolute sea level rates is expected and the
1940–2004 tide gauge relative rates derived in this study were
successfully corrected using steady vertical land motions as detected by
GPS (1994–2004). In the longer run, impact of co-seismic shifts will
decrease but the cumulative effect of post-seismic tectonic plate
subsidence,which could last for several decades,willmake it imperative
for sea level change studies to estimate reliable subsidence rates.