11/29/2009 earthquake also exacerbated sea level rise, land sinking

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. When combined with relative sea level rise, land sinking can increase the frequency and amount of coastal flooding.

NASA reported that 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 combine remote sensing tools to figure that out.

Historically, subsidence measurements on small tropical islands have been challenging for two reasons. Islands often need more resources to acquire detailed measurements at the land surface, and dense midday clouds and vegetation can make good satellite data challenging.

Using the island of Tutuila in American Samoa as an example, a team of NASA scientists last year published a study on how to map ground changes on earthquake-prone islands better.

They found that combining satellite and ground-based observations could create a more nuanced and comprehensive map. In the past, scientists had used data from two measurement points on Tutuila: a GPS station and the island’s tide gauge.

They typically coupled those points with satellite altimetry, which allows scientists to monitor the surface height of the ocean broadly.

However, these data provided only a limited picture. 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 highest sinking rates occurred right after the earthquake, especially along the coastlines.