The Samoan Islands are sinking faster after earthquakes caused a deformation in Earth鈥檚 mantle beneath the island chain. Projections of sea level rise that fail to account for this additional sinking substantially underestimate the risk of flooding on the islands.
In 2009, a pair of magnitude-8 earthquakes created a tsunami that killed nearly 200 people and caused hundreds of millions of dollars in damages on the Samoan Islands and the island of Tonga. The earthquakes also released stress on the Tonga subduction zone fault, which caused the mantle and parts of the crust beneath the islands to relax.
In 2019, researchers reported this was causing some of the Samoan Islands to sink as much as six times as fast as the global average, accelerating the rate of sea level rise relative to the land. The current rate in Tutuila, the largest island in American Samoa 鈥� a US territory that contains five of the nine Samoan Islands 鈥� is about 16 millimetres per year, four times as fast as the global average.
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at the University of Hawai鈥檌 at M膩noa and her colleagues compared different projections of flooding related to sea level rise on Tutuila. They found the area at risk of flooding in 2030 with intermediate sea level rise is around 45 per cent greater in scenarios that account for this sinking than in scenarios that don鈥檛. The gap falls to 30 per cent by 2080 as the effects of the earthquakes wear off. 鈥淏y the end of the century, most of the sea level rise is coming from other processes,鈥� says Baizeau, who presented these results at the American Geophysical Union Ocean Sciences Meeting in New Orleans, Louisiana, on 20 February.
Baizeau says as with other islands threatened by sea level rise, there is a lack of data on flooding hazards in American Samoa, and the area at risk of flooding may be even greater than the researchers鈥� initial estimates, which were based on rough elevation maps. Her team is working on more detailed .
鈥淲hat matters to people is 鈥榬elative鈥� sea level rise that is a measure of sea level seen from ground,鈥� says at the University of Newcastle, Australia, whose research linked the rapid rates of subsidence with the earthquakes. 鈥淭he ground is not steady.鈥� He says in many parts of the seismically active region known as the Ring of Fire 鈥� a belt of volcanoes that rim the Pacific Ocean 鈥� the vertical motion of the land is a larger driver of flooding risk than sea level rise due to climate change.
