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Scientists Map Tectonic Structure Below the Seafloor of Puerto Rico

The USGS is working to identify and map faults in southern Puerto Rico, to estimate the location and magnitude of potential earthquakes.

Marine technician deploy a hydrophone off Puerto Rico
USGS marine technicians Alex Nichols (L) and Eric Moore (R) deploy the hydrophone array on a seismic research cruise off the southwest coast of Puerto Rico aboard the R/V Sultana in March 2020.

Starting in December 2019, residents of southern Puerto Rico were startled by a sequence of earthquakes that included a magnitude 6.4 quake on January 7, 2020. Aftershocks are expected to continue for years, including some relatively strong ones, like a more recent May 2 magnitude 5.4 temblor.

USGS seismologists were surprised too—not by the fact that the earthquakes occurred but by where they occurred. Puerto Rico lies on an active tectonic boundary between the Caribbean and North American plates, with the northeast corner of the Caribbean plate moving eastward about two centimeters per year along a strike-slip fault. There is geologic evidence of earthquakes that probably took place millennia ago, whereas history records earthquakes and tsunamis in Puerto Rico as far back as the 1500s. But most seismic activity has been on the north side of the island, not the south side, where previously unknown undersea faults may have triggered this latest earthquake series.

The USGS is working to identify and map faults in the region to estimate the location and magnitude of potential earthquakes. With more than 3 million U.S. citizens in Puerto Rico and the Virgin Islands, the risk to lives and property from earthquakes and tsunamis is significant. USGS seismic research can help inform better building codes, safer zoning, and public education about earthquake hazards.

When USGS research geophysicist Uri ten Brink, learned about the Puerto Rico quakes, he quickly made plans to launch a seismic research cruise off the island’s south coast. In the past 15 years, as project lead for a marine geohazards project focused on documenting undersea tectonic processes and assessing the landslide, tsunami, and earthquake hazards they pose, his team has mapped faults off the island’s north coast. But the seafloor close to the south coast was largely unexplored.

“This data will eventually help seismologists develop a clearer picture of tectonic activity in the area,” ten Brink said. “Ultimately, we hope the USGS’ work in this region will help give the public a clearer sense of the potential for future earthquakes. The USGS’ research findings are being used to improve building codes that will help Puerto Rico better withstand future earthquakes and to better prepare for tsunamis.”

Map of seismic research site on seafloor off Puerto Rico
Locations of multichannel seismic reflection profiles (in black), collected during cruise FA2020-14. The profiles are overlain on colored and shaded multibeam bathymetry, Lidar topography (green and white) and near-shore bathymetry (darker blue), and NOAA coastal relief model (light blue and white). Dashed purple lines – hypothesized faults from the distribution of earthquake epicenters. Red markers – Preliminary identification of possible faults on the seismic reflection data.

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