A new wave of seismic science is emerging from the ocean floor—and it could help us predict the next big disaster. Using a remotely operated vehicle (ROV), researchers have mapped the geology of the Mona Rift, a deep underwater fault zone northwest of Puerto Rico, revealing hidden seismic hazards that could impact millions along the island’s coast.
The findings, published in Geosphere, mark a breakthrough in how we monitor tectonic activity in remote, inaccessible regions, and offer a powerful reminder that the most dangerous fault lines aren’t always on land.
A Robotic Dive into the Unknown
To explore the ocean floor where traditional drilling isn’t feasible, the research team, led by Dr. Uri S. ten Brink of the U.S. Geological Survey, deployed ROVs to depths of nearly 13,000 feet. These robotic explorers collected high-resolution images and seven rock samples across the seafloor of the Mona Rift.
Instead of drilling, the team used “virtual outcrop” modeling, building 3D geological reconstructions from the images. This innovative approach allowed them to extract detailed data from a previously unstudied underwater fault, creating a new geological timeline from footage that ten Brink compared to “driving by an outcrop on land.”
The Hidden Rift Beneath the Waves
The Mona Rift is a seismically active zone located between Puerto Rico and the Dominican Republic. It has long been suspected of playing a role in the region’s frequent tremors—but until now, data was scarce.
In 1918, a magnitude 7.2 earthquake triggered a tsunami that devastated parts of Puerto Rico. While earlier research dismissed the Mona Rift as the tsunami’s source, this new study reveals it may still pose a serious seismic threat.
The team’s analysis dated the rift at roughly 3.3 million years old—much younger than surrounding formations—suggesting that tectonic rifting in the area is happening faster than previously believed. That means the Mona Rift is not just active—it’s evolving.
A 330-Foot Gap in the Geologic Record
By comparing onshore and offshore rock formations, the scientists found a 330-foot gap in the stratigraphic sequence, meaning entire layers of geologic history are missing from the land-based record. This discrepancy highlights just how little we truly understand about underwater fault zones—and how crucial deep-sea exploration is for filling those gaps.
The data also revealed variable rock thicknesses, further supporting the theory that the Mona Rift has its own distinct tectonic evolution, separate from nearby land masses.
Why This Matters Now
Puerto Rico remains highly vulnerable to earthquakes and tsunamis, with coastal populations and infrastructure at risk. The ability to detect early signs of fault movement in places like the Mona Rift could be critical for disaster preparedness.
The research also offers a blueprint for using ROVs to study other submarine fault zones around the world, especially in regions where traditional drilling is too expensive or impractical. As climate change intensifies coastal risks and sea levels rise, understanding how the seafloor behaves is becoming more urgent than ever.
