Tectonic Trouble: NASA Measures Sinking Land in American Samoa

NASA Detects Land Subsidence in American Samoa Following 2009 Earthquake

A seismic event of magnitude 8.1 in 2009 exacerbated the issue of land subsidence in American Samoa. NASA researchers have introduced InSAR technology, which has illuminated post-earthquake escalation in subsidence rates. This study underscores the necessity of precise data for global coastal resilience planning.

In September 29, 2009, a formidable earthquake measuring 8.1 in magnitude struck the vicinity of American Samoa, Samoa, and Tonga. This event triggered a tsunami, causing both human casualties and approximately $200 million in property damage across the islands. Concurrently, the earthquake amplified an ongoing concern in American Samoa – the gradual sinking of land, commonly referred to as subsidence. When this phenomenon is coupled with the rise in relative sea levels, the susceptibility to coastal flooding is significantly heightened.

The Challenge of Quantifying Subsidence

Efforts to safeguard island regions against flooding entail accurate measurements of ground subsidence rates and their geographic distribution. Jeanne Sauber, a geophysicist based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, explains that comprehending the locations with the most rapid subsidence is crucial. To address this challenge, Sauber and her NASA colleagues are utilizing a combination of remote sensing tools.

Historically, measuring subsidence on small tropical islands has been arduous due to two primary factors. These islands often lack the resources required for comprehensive surface measurements, while obstructive factors such as thick daytime clouds and dense vegetation hinder the acquisition of reliable satellite data.

Enhancing Measurement Precision

Taking the example of Tutuila in American Samoa, a team of NASA scientists conducted a study aimed at refining the mapping of ground changes on seismically active islands. Their findings indicate that by amalgamating satellite and ground-based observations, a more intricate and comprehensive map can be achieved.

Formerly, scientists relied on data from two measurement points on Tutuila: a GPS station and the island’s sole tide gauge. These points were generally coupled with satellite altimetry, which broadly monitors ocean surface height. However, this approach provided only a limited depiction.

Key Insights from the Study

In the study, researchers introduced InSAR, or interferometric synthetic aperture radar, enabling them to identify areas of ground alteration. InSAR involves the comparison of satellite radar images of a specific region captured at different times, allowing for the detection of Earth’s surface movements and variations in ground elevation.

The study unveiled that between 2015 and 2022, Tutuila experienced an annual subsidence of 0.24 to 0.35 inches (6 to 9 millimeters), compared to the pre-2009 earthquake rate of 0.04 to 0.08 inches (1 to 2 millimeters) per year. The most pronounced sinking occurred immediately following the earthquake, particularly along the coastlines.

Stacey Huang, lead author of the study and a participant in NASA’s Postdoctoral Program at NASA Goddard, emphasizes that while the GPS station provided insights into localized ground deformation, radar remote sensing offers a far more comprehensive island-wide perspective.

Refined Techniques

Synthetic aperture radar data is collected via aircraft or satellites. The technique involves transmitting microwave pulses from the satellite to Earth’s surface, measuring the time taken for the pulses to return, and assessing the strength of the reflected signal or “backscatter.” Unlike many satellite instruments, this radar has the capacity to penetrate through clouds and dense vegetation, allowing for accurate measurement of relative elevation and land surface changes. The study utilized data from the ESA (European Space Agency) Copernicus Sentinel-1A satellite.

The researchers also integrated satellite altimeter data to assess sea level, correlating it with measurements from Tutuila’s Pago Pago tide gauge station. By comparing sea level measurements relative to Tutuila with absolute sea level data from the altimeter, variations in Tutuila’s land motion in relation to Earth’s center were discerned.

Understanding the Broader Context

Understanding the influence of tectonic plate movement on island motions is vital for evaluating land subsidence on remote islands. By incorporating data from Tutuila’s GPS station, researchers monitored the rate of vertical motion.

Sauber explains that this technique not only provides insight into local island dynamics but also enables the assessment of an island’s motion relative to other global locations.

Geological Underpinnings

Land subsidence in the western Pacific Ocean results from the interaction of the Pacific and Australian tectonic plates. The phenomenon of subduction, where one plate slides beneath another, occurs along the Tonga Trench, a deep trench in the Pacific Ocean. This process often leads to earthquakes and vertical movement of the island’s surface, thus contributing to ground-level changes.

Eric Fielding, a geophysicist from NASA’s Jet Propulsion Laboratory in Southern California, highlights that over extended periods – thousands to millions of years – volcanic islands tend to sink as they cool down. The earthquake cycle exacerbates this long-term geological process.

The Impact of Rising Sea Levels

The challenge is compounded by rising sea levels, particularly in the Tutuila region. Richard Ray, a geophysicist at NASA Goddard and a co-author of the study, points out that relative sea levels around Tutuila have risen up to five times faster than the global average. This is in contrast to the global average sea level rise of 0.11 inches (2.7 millimeters) from 2021 to 2022, as analyzed from satellite data by NASA. In comparison, the region’s sea level rise was previously estimated at 0.04 to 0.08 inches (2 to 3 millimeters) annually prior to the earthquake.

Long-Term Implications and Future Endeavors

Numerous islands worldwide share similar challenges with Tutuila, confronting rising sea levels and related vulnerabilities. The insights gained from Tutuila are anticipated to guide coastal resilience planning on other islands. Collaborative initiatives between NASA and the United Nations are underway to inform decision-making across Pacific Island nations.

Scheduled for launch in early 2024, NISAR (NASA-ISRO Synthetic Aperture Radar), developed jointly by NASA and ISRO (Indian Space Research Organization), will provide detailed tracking of Earth’s land and ice surface movements. This technological advancement will play a pivotal role in identifying and monitoring vertical land motion globally.

The criticality of reliable data for coastal resilience planning is underscored by the need to make informed policy decisions that ensure the well-being of island inhabitants.

“Accurate knowledge of the rate at which land is sinking is pivotal for formulating policy decisions rooted in scientific evidence,” Sauber emphasizes. “Relocating individuals from their homes should only be considered in genuinely dire circumstances.”

Frequently Asked Questions (FAQs) about subsidence rates

More about subsidence rates

• NASA Earth Observatory
• Interferometric Synthetic Aperture Radar (InSAR)
• Tonga Trench and Subduction
• Rising Sea Levels and Coastal Resilience
• NISAR – NASA-ISRO Synthetic Aperture Radar