Vertical motion of Phuket Island (1994–2018) due to the Sumatra-Andaman mega-thrust earthquake cycle: impact on sea-level and consequences for coral reefs

Macro-tidal coral reefs are particularly sensitive to medium to long-term changes in sea-level. Vertical motions of the seabed contribute to both lower or higher relative sea-level changes, particularly in tectonic plate boundary deformation zones along active subduction trenches. Phuket Island in Southern Thailand is subject to both horizontal and vertical land deformations during the seismic cycle of mega thrust earthquakes along the Sumatra and Andaman trenches. The relative sea-level changes in this region were historically monitored using the space geodetic techniques GPS and satellite altimetry alongside the traditional tide-gauge measurements over a period of almost 25 years. The GPS results show that the south of Thailand is still undergoing post-seismic deformations from the 2004 Mw 9.2 Sumatra-Andaman earthquake, after a significant change in the vertical motion of Phuket: from stable quasi-linear uplift at 2.5 ± 0.2 mm/yr, to (temporary) non-linear subsidence rates of 5–10 mm/yr in the past 14 quake aftermath years. The satellite altimetry data estimates the absolute sea-level rise in the Andaman Sea around the island at 3.9 ± 0.5 mm/yr. Therefore relative sea-level changes in Phuket appear to have been slightly positive until the end of 2004, followed by a significant increase averaging ~9 mm/yr that accumulated in 12 ± 1 cm by the end of 2018. Tide-gauge data gives a very similar result. The increased sea-level rise from 2005 onwards, due to tectonic land subsidence, correlates with the enhanced shallow coral reef growth and swift recoveries from bleaching events reported by long-term coral studies in this area.