Big earthquakes might make sea level rise worse. Here's how.
University of Newcastle, AUSTRALIA — A geologic one-two punch rocked the South Pacific in September 2009, as a magnitude 8.1 earthquake struck off the coast of the island nation of Samoa, followed mere moments later by a similarly intense tremor. A towering tsunami soon crashed onto the shores of islands nearby, leaving more than 180 dead and communities in ruins in Samoa, the neighbouring U.S. territory of American Samoa, and the surrounding islands.
But a new study, published in the Journal of Geophysical Research: Solid Earth, reveals that the quakes also sparked a slow-burning danger for the more than 55,000 residents of American Samoa: a sea level rise that is five times as fast as the global average.
Like other island and coastal regions around the world, Samoa and American Samoa are facing encroaching waters as our warming world sends sea levels soaring at accelerating rates. In the wake of the mega-quakes, though, the researchers discovered that these Pacific islands are also sinking. The situation is particularly concerning for American Samoa, where the team estimates that, over the next fifty to a hundred years, local sea levels could rise by roughly a foot in addition to the anticipated effects of climate change.
While the contributions of big earthquakes won’t be the same everywhere, the discovery emphasises the sometimes overlooked effects that geology can have on the increasing number of people around the world who call coastlines home. (Also find out how powerful quakes are priming the region around Mount Everest for a huge disaster.)
“Everybody is talking about climate change issues ... but they overlooked the impact of the earthquake and associated land subsidence,” says study leader Shin-Chan Han of the University of Newcastle, Australia, referring to documents from regional governments on sea level rise.
“This is a really important thing to point out,” says geophysicist Laura Wallace of the geoscience consultancy firm GNS Science, Te Pū Ao, in New Zealand, who was not involved in the study. “It obviously has a big impact on the relative sea level changes people are going to see in places like [the Samoan islands].”
The 2009 event was a particularly unusual earthquake that initially baffled scientists, since the pair of powerful temblors ripped through the Earth nearly at the same time. One broke along a so-called normal fault, created due to the flex of the oceanic crust as it plunges under another tectonic plate in what’s known as a subduction zone. Another quake broke within the subduction zone due to the compressive forces of the colliding plates.
The researchers investigated the lingering impacts of these quakes using a combination of GRACE data and local GPS and tidal gauge records. They then built a computer model to tease apart the complex interplay between the temblors and what is happening at the surface.
This data showed slow sinking of the landscape, driven primarily by the normal-fault quake. This particular earthquake causes one side of the landscape to fall in relation to the other, which sent the nearby islands sinking downward.
The team found that nearly a decade after the event, the island of Samoa has sunk by roughly 0.4 inches a year. The situation is particularly acute for American Samoa, which has seen more than 0.6 inches of subsidence each year, and it doesn’t look like it’s stopping anytime soon.
The pace outstrips the estimated rate of global sea level rise, which is creeping upward at some 0.13 inches a year. Flooding and seawater intrusion in freshwater aquifers are already grave concerns for residents of American Samoa, Han says, and the latest find only adds to the worry.