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.
Now, USGS researchers have newly discovered deep sea faults near the quake epicenters…
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.
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.
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.”
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.
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.
“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.”