A geological shock of one to two shook the South Pacific in September 2009, while an earthquake of magnitude 8.1 had shook the shoreline of the island island of Samoa, closely followed by an earthquake of comparable intensity. A gigantic tsunami soon crashed on the coasts of neighboring islands, killing more than 180 people and communities in Samoa, on American Samoa's neighboring American territory and in the surrounding islands.
But a new study, published in the Journal of Geophysical Research: Solid Earth, reveals that earthquakes have also caused a slow burning hazard for more than 55,000 people in American Samoa: sea level rise is five times faster than the global average.
Like other island and coastal regions around the world, Samoa and American Samoa are facing increasingly invasive waters as the warmer world is causing a rapid rise in sea level. Mega-earthquakes, the researchers found that these Pacific islands were also sinking. The situation is particularly worrying for American Samoa, where the team estimates that in the next 50 to 100 years, sea level at the local level could increase by about one foot in addition to the anticipated effects of climate change.
Although contributions from major earthquakes are not the same everywhere, the discovery highlights the sometimes neglected effects of geology on the growing number of people in the world calling for shorelines. (See also how powerful earthquakes are preparing the region around Mount Everest for a major disaster.)
"Everyone is talking about climate change issues … but they have neglected the impact of the earthquake and the associated land subsidence," said study leader Shin-Chan Han of the University of Newcastle, Australia, referring to regional government documents on the rise in sea level.
"This is a very important thing to point out," says geophysicist Laura Wallace of GNS Science geoscience consultancy, Te Pū Ao, New Zealand, who did not participate in the event. study. "This obviously has a big impact on the relative changes in sea level that people will see in places like [the Samoan islands]. "
Plate tectonics constantly reshapes the surface of our planet, a particularly obvious role during an earthquake. In general, these events occur when tectonic plates collide or slide against each other, creating geological stress. When this accumulated energy is released suddenly, blocks of the earth's crust can move. (Find out how smaller "hidden" earthquakes are affecting California.)
But all the change of a big earthquake is not immediate. Unlike hard crust, the mantle rocks below flow like cold molasses and gradually adapt to the sudden shaking of the surface, says Wallace. This can cause subsidence or uplift that can last for decades after an earthquake.
This prolonged deformation of the landscape is what intrigues Han. For years, he skimmed the satellite data of the Gravity Recovery and Climate Experiment, or GRACE, to look for the rise and fall of earth after an earthquake. This pair of satellites orbited Earth online from 2002 to 2017 and accurately tracked the gap between the spacecraft. Passing over slightly more massive areas and therefore more gravitational, the leading machine would feel the tug just before the one who followed. This altered the space between the two and recorded a wobble in the gravitational field of the planet that may reveal changes in the continental landmass below.
In the case of the 2009 earthquake, such changes have been painstaking day-to-day. But ultimately, the effects were significant enough that Han would see something strange in Samoa while examining GRACE data.
A rare coincidence
The 2009 event was a particularly unusual earthquake that first puzzled scientists, as the pair of powerful shakers tore the Earth almost at the same time. One of them pierced along a so-called normal fault, created by the crease of the oceanic crust as it dives under another tectonic plate in a so-called subduction zone. Another earthquake erupted in the subduction zone due to the compressive forces of the colliding plates.
The researchers studied the lingering effects of these earthquakes using a combination of GRACE data and local GPS and tide gauges. They then built a computer model to highlight the complex interaction between shakers and what is happening on the surface.
These data show that the landscape is sinking slowly, mainly because of the normal fault earthquake. This earthquake causes one side of the landscape to fall in relation to the other, which has caused the sinking of neighboring islands.
The team found that nearly 10 years after the event, the island of Samoa had sunk about 0.4 inches a year. The situation is particularly critical for American Samoa, which has suffered more than 0.6 inches of subsidence each year, and it does not seem to stop that soon.
The pace exceeds the estimated global sea level rise rate, which climbs 0.13 inches per year. Flooding and intrusion of seawater into freshwater aquifers are already serious concerns for residents of American Samoa, Han said, and the latest discovery is only one. Add to the concern.
Oceans of the bathtub
It is possible that similar effects occur on other islands near the collision of the plates, but many factors have an influence on the consequences of the post-earthquake. The general importance of the new work shows that the causes of sea-level change in a given place are much more complex than melting ice and sea-warming.
"People think that the average global sea level is rising when the tub gets full and empty," says Don Chambers of the University of South Florida, an expert in the use of satellite gravimetric data for the study of water levels. the sea. But a host of other factors are disrupting our vast ocean basins. Some of these causes are of human origin, such as the extraction of groundwater or the compression of sediments by expanding cities, which cause the earth to sink. This effect is a culprit along the coast of Louisiana, where waters rise one inch every two years in some parts.
Tectonic damage, like that seen in the Samoan Islands, is also a common factor, but the effects depend on the geometry of the faults, says Jeffrey Freymueller, a geophysicist from Michigan State University, who did not participate in the work. In many places, tectonic conflicts cause an uplift rather than a subsidence. For example, Han followed the evolution of post-earthquake changes in Japan and New Zealand, both of which have been rising for years.
But around Samoa and elsewhere, subsidence due to earthquakes is a real concern for the rise in sea level. Freymueller points to a recent study in Marine geology the dramatic rise in sea level for Phuket Island, Thailand, following the magnitude-9.2 Sumatra-Andaman earthquake in 2002.
This latest study underscores the need for greater awareness and ongoing monitoring to mitigate the potential effects of mega-earthquakes, Wallace said. However, it is impossible to predict such effects on sea level until an earthquake occurs, as earthquake prediction itself remains difficult to achieve.
"This could be a problem that suddenly causes you heartburn next week," says Freymueller, "or it might not be a problem for the next century."