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Most residents of southern Puerto Rico were startled by the sequence of earthquakes that began Dec. 28, 2019 and included a magnitude 6.4 quake on Jan. 7, 2020. Aftershocks are expected to continue for years, including some relatively strong ones, like a May 2 magnitude 5.4 temblor. 

Many islanders have lived through hurricanes, but the last major earthquake to be felt on Puerto Rico, a magnitude 7.2 that occurred northwest of the island in 1918, was beyond the living memory of most islanders.

USGS seismologists were surprised too – not by the fact that the earthquakes occurred, but by where they occurred. Puerto Rico lies on an active boundary between the Caribbean and North American plates, with the northeast corner of the Caribbean plate moving eastward about two centimeters (less than an inch) per year along a strike-slip fault. There is geologic evidence of earthquakes that probably took place millennia ago, while 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.

Marine technician deploy a hydrophone off Puerto Rico
In early March, before USGS field researchers were practicing social distancing, marine technicians Alex Nichols (L) and Eric Moore (R) deployed a hydrophone array on a seismic research cruise off southwest Puerto Rico. Credit: Uri ten Brink, USGS. Public domain.

The USGS is working to identify and map faults in the region, in order 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.

Caribbean quake experts quickly launch a research cruise

When he learned about the Puerto Rico quakes, USGS research geophysicist Uri ten Brink made plans to quickly launch a seismic research cruise off the island’s south coast. Ten Brink heads a marine geohazards project, based at the USGS Woods Hole Coastal and Marine Science Center, that focuses on documenting undersea tectonic processes and assessing the landslide, tsunami and earthquake hazards they pose. In the past 15 years he has headed USGS and NOAA teams that mapped faults off the island’s north coast, but the sea floor close to the south coast was largely unexplored.

“It seemed likely that a previously unknown fault system off the south coast was involved in the earthquake sequence, and there should be evidence of that on the sea floor,” ten Brink said. “We thought that if that were true, the information would be potentially very valuable to science, and to the people of southern Puerto Rico. They are still recovering from the effects of Hurricane Maria, and now they are facing a completely different type of hazard, and they are hungry for information about it.”

With the help of scientists from the University of Puerto Rico Mayaguez, who provided logistical support including the use of the 43-foot R/V Sultana and its crew, ten Brink and colleagues ran a series of one-day research cruises out of the University of Puerto Rico Marine Station in La Parguera, west of Ponce on the island’s south coast, from March 7 to 13.

Newly discovered deep sea faults near quake epicenters

Preliminary results bear out ten Brink’s suspicions: There is evidence of at least one undersea fault in Guayanilla Bay, which may be an extension of a fault previously mapped on land. The team tentatively identified several more faults lying seven and fifteen kilometers (about four and nine miles) offshore, in water up to 1,000 meters (about 3,300 feet) deep and within areas identified as the epicenters of some of the recent earthquakes, ten Brink said.

To find and map these faults, the team aboard Sultana towed a device called a “sparker,” which uses an electrical charge to generate an air bubble in the water. When the bubble expands it creates a sound wave, which travels through the water and into the seabed, where geologic layers reflect some of the wave’s energy back to the sea surface.

USGS technicians launch "sparker" used in seafloor mapping
In early March, before USGS field scientists began practicing social distancing, USGS research geologist Jason Chaytor (L) and marine technicians Alex Nichols (center) and Eric Moore (R) deployed the “sparker” sound source off southwest Puerto Rico. Credit: Uri ten Brink, USGS. Public domain.

The research vessel also towed an array of 32 hydrophones just below the sea surface– devices that pick up pressure changes underwater. The hydrophones measure the time it takes the signal to bounce off the ocean bottom and geologic structures lying beneath the seabed, and then return to the sea surface. By measuring differences in the signals’ return, researchers can detect differences in the elevation of the rock lying beneath the ocean floor. These differences, called offsets, are considered to be signs of faults that have been recently active.

Working in deep waters roiled by spring storms, the USGS team collected 135 nautical miles, or 250 kilometers, of seismic reflection data, imaging the seabed and undersea structures at resolutions of just a few meters.

From the ship, the team identified several fresh scars in shore-facing cliffs from Punta Montalva to Guayanilla Bay, which were probably caused by rock falls resulting from the strong earthquake shaking. The team also looked for evidence of a fault breaking the sea floor at Punta Montalva, but at first look, the seismic data do not clearly indicate such a fault there. In the coming months, ten Brink said, more sophisticated data processing is likely to reveal additional faults, and the team expects to be able to measure ruptures along these faults.

“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
The black lines on this map show the locations where a USGS team collected multichannel seismic reflection profiles during a series of research cruises in March 2020. The dashed purple lines are hypothesized faults, based on the distribution of earthquake epicenters. The red marks are preliminary indications of faults, based on the seismic reflection data gathered during the cruise. The seafloor image is drawn from multiple sources: colored and shaded multibeam bathymetry, Lidar topography (green and white), near-shore bathymetry (darker blue), and NOAA coastal relief model (light blue and white). Credit: Uri ten Brink, USGS. Public domain.

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