On Sept. 29, 2009, an 8.1-magnitude earthquake struck near American Samoa, Samoa, and Tonga, triggering a tsunami that caused human casualties and $200 million in property damage on the islands. The earthquake also exacerbated another problem in American Samoa: subsidence, or the sinking of land. When combined with relative sea level rise, land sinking can increase the frequency and amount of coastal flooding. Protecting against flooding on islands requires reliable measurements of how much the ground is sinking and where, said Jeanne Sauber, a geophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “You need to know in detail where the land is going down the fastest,” she said. Sauber and several NASA colleagues are combining remote sensing tools to figure that out.
Historically, subsidence measurements on small tropical islands have been difficult to make for two reasons. Islands often have few resources for acquiring detailed measurements at the land surface, and dense midday clouds and vegetation can make good satellite data difficult to get. Using the island of Tutuila in American Samoa as an example, a team of NASA scientists last year published a study on how to better map ground changes on earthquake-prone islands.They found that using a combination of satellite and ground-based observations could result in a more nuanced and comprehensive map.
In the past, scientists had used data from two points of measurement on Tutuila: a GPS station and the island’s one tide gauge. They typically coupled those points with satellite altimetry, which allows scientists to broadly monitor the surface height of the ocean. But these data provided only a limited picture.
In the study, the researchers added InSAR, or interferometric synthetic aperture radar, which allowed them to see where the ground was changing. InSAR is a technique that involves comparing satellite radar images of the same area collected at different times to spot movement on Earth’s surface and track changes in ground height.
The study found that Tutuila sank an average of 0.24 to 0.35 inches (6 to 9 millimeters) per year between 2015 and 2022 compared to 0.04 to 0.08 inches (1 to 2 millimeters) per year before the 2009 earthquake.
“We knew how much the ground is deforming at this one point because of the GPS station there, but with the radar remote sensing technique, we can get a much denser map of what’s going on across the island,” said Stacey Huang, a fellow with NASA’s Postdoctoral Program at NASA Goddard and the study’s lead author.
Huang and Sauber’s study used data from the ESA (European Space Agency) Copernicus Sentinel-1A satellite.
The researchers also used satellite altimeter data to assess sea level and correlate it with measurements from the island’s Pago Pago tide gauge station. The gauge measured sea level relative to Tutuila, while the altimeter measured the absolute sea level. The difference between them shows, among other signals, Tutuila’s land motion, or movement, relative to Earth’s center.
The study found that Tutuila sank an average of 0.24 to 0.35 inches (6 to 9 millimeters) per year between 2015 and 2022 compared to 0.04 to 0.08 inches (1 to 2 millimeters) per year before the 2009 earthquake.
The researchers also used satellite altimeter data to assess sea level and correlate it with measurements from the island’s Pago Pago tide gauge station. The gauge measured sea level relative to Tutuila, while the altimeter measured the absolute sea level. The difference between them shows, among other signals, Tutuila’s land motion, or movement, relative to Earth’s center.