On September 29, 2009, an 8.1 magnitude earthquake struck near American Samoa, Samoa, and Tonga, triggering a tsunami that caused loss of life on the islands and $200 million in property damage. The earthquake also exacerbated another problem in American Samoa: subsidence or ground subsidence. Land subsidence combined with relative sea level rise can increase the frequency and amount of coastal flooding.

Jeanne Sauber, a geophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said reliable measurements of how much and where the ground sank are needed to protect the islands from flooding. “You have to know in detail where the soil moves fastest,” she said. Sauber and several NASA employees are combining remote sensing devices to solve this.

Historically, measuring depression on small tropical islands has been difficult for two reasons. Islands often have few resources to obtain detailed ground surface measurements, and dense midday clouds and vegetation make good satellite data difficult to obtain.

Using Tutuila Island in American Samoa as an example, a team of NASA scientists published a study last year on how to better map soil changes on earthquake-prone islands. They found that using a combination of satellite and ground observations could lead to a more detailed and complete map.

In the past, scientists used data from two measurement points on Tutuila: a GPS station and the island’s only tide meter. They often combined these points with satellite altimetry, which allows scientists to monitor the height of the ocean’s surface widely. But these data gave only a limited picture.

In the study, the researchers added InSAR, or synthetic aperture interferometric radar, that allows them to see where the soil has changed. InSAR is a technique that involves comparing satellite radar images collected at different times of the same area to detect motion on the Earth’s surface and monitor changes in ground altitude.

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) in the year before the 2009 earthquake. The highest collapse rates occurred immediately after the earthquake, especially along the coastline.

“Thanks to the GPS station, we knew how deformed the ground was at that point, but with radar remote sensing we were able to get a much denser map of what was going on on the island,” said Stacey Huang, a postdoctoral research fellow at NASA. Goddard and lead author of the study.

Creating a better map

Synthetic aperture radar data is collected from aircraft or satellites. It works by sending microwave pulses from a satellite to the Earth’s surface, then measuring the time it takes for the pulses to bounce off and the strength of that reflection, or “backscatter.” Unlike many satellite instruments, such radars can penetrate clouds and dense vegetation, allowing researchers to accurately measure relative altitude and changes in land surface. 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 estimate sea level and correlate it with measurements from the Pago Pago tide measuring station on the island. The altimeter measured absolute sea level, while the transducer measured sea level relative to Tutuila. The difference between the two shows the movement of Tutuila’s country, or its movement relative to the center of the Earth, among other signals.

One of the challenges in predicting land subsidence on remote islands is understanding how island movements may be affected by wider movements of tectonic plates. By combining measurements from the Tutuila GPS station, the researchers were able to track the vertical movement velocity.

“So we can not only say what a point is doing relative to another point on an island, but we can also tell what that island is doing relative to other places in the world,” said Sauber, one of the study’s authors.

Why is the world sinking?

Land subsidence in this part of the western Pacific Ocean is a result of the movement of the Pacific and Australian tectonic plates. When one plate passes under another, a phenomenon called subduction occurs along the Tonga Trench, a deep canyon in the Pacific Ocean. Earthquakes often create a vertical movement on the island’s surface as a result of this process, along with changes in the land surface.

To understand how much the earth has changed after each earthquake, scientists measure what is called vertical ground motion, the up and down movement of the earth due to the removal and rearrangement of materials on the earth’s surface.

“Over hundreds of thousands or even millions of years, these volcanic islands tend to sink as they cool,” said Eric Fielding, a geophysicist at NASA’s Jet Propulsion Laboratory in Southern California. “This long-term geological process applies to the Samoan Islands, and the earthquake cycle also contributes to that.”

Richard Ray, third author of the study and geophysicist at NASA Goddard, said sea-level rise exacerbates the problem. For example, according to a previous study by Ray and Sauber, the relative sea level in Tutuil is rising five times the global average.

Average global sea level rose by 0.11 inches (2.7 millimeters) between 2021 and 2022, according to NASA analysis of satellite data. In this 2019 study, scientists found that before the earthquake, sea level rise in the region was between 0.04 and 0.08 inches (2-3 millimeters) per year relative to land, but now the relative rise in sea level is several times the global average. .

“Three millimeters may not seem like a lot, but it increases over time,” Ray said.

Many islands around the world are facing rising sea levels and have features similar to Tutuila. The researchers hope to apply what they learned from Tutuila to other islands for coastal resilience planning, including a joint effort between NASA and the United Nations to inform decision-making processes in Pacific island nations.

Scheduled to launch in early 2024, NISAR – short for NASA-ISRO Synthetic Aperture Radar – developed jointly by NASA and ISRO (Indian Space Research Organization) will track and help identify and detect the movement of Earth’s land and ice surfaces in extremely fine detail. will be. Follow the vertical movements of the world around the world.

Coastal resilience planning is essential to protecting people living on smaller islands and requires reliable data.

“We really need to know how fast this land is sinking so that policy decisions can be based on science,” Sauber said. said. “You don’t want to push people out of their homes unless they’re really in trouble.” Source