First a killer hurricane, now an earthquake: Why the ground shook in Puerto Rico
It’s common knowledge that the Caribbean is a magnet for destructive hurricanes. The catastrophic passage of Hurricane Maria across Puerto Rico two years ago was grim reminder of the region’s vulnerability to atmospheric disturbances.
What’s less well known is that the Caribbean is also highly prone to natural hazards of a different, earthbound sort — earthquakes. The entire island chain is an active seismic zone because it sits at the juncture of two major tectonic plates, the North American and Caribbean plates.
Puerto Rico, which experienced a strong series of shocks culminating in a damaging early Tuesday quake measuring 6.4 in magnitude, sits in an especially precarious spot. The mountainous island, just 100 miles long and 35 miles wide, sits on a mini plate of sorts. Both the North American and Caribbean plates slide under Puerto Rico at that point, and the island floats above that unstable borderline, said Grenville Draper, professor of geology at Florida International University and an expert on the geology of the Caribbean.
The upwards pressure exerted on the island as the plates grind against each other means it’s actually been “arching upwards” for some 10 million years, Draper said.
“Puerto Rico is caught in this vise-like grip between these two plates,” Draper said. “It’s very vulnerable to earthquakes because it’s essentially being squeezed between these two plates.”
As a consequence, the island and the surrounding sea bottom are riddled with ancient and newer, active faults, some buried deep in the earth and others near the surface. Scientists suspect that a shallow formation just off the island’s southwest coast was the source of the series of tremors emanating from the area since late December — possibly an offshore extension of the Punta Montalva fault, said Elizabeth Vanacore, a professor at the University of Puerto Rico-Mayagüez and a contributor to the Puerto Rico Seismic Network.
Those hundreds of tremors led up to two strong quakes on consecutive days, a significant 5.8 quake on Monday that was followed, unusually, by the even stronger 6.4 shock at 4:24 a.m. local time on Tuesday. The first quake was considered moderate, while the second was rated as strong. Because the scale is logarithmic, a difference of one point means the second quake was 10 times the magnitude of the first. That first tremor was still potent enough to destroy a famed natural stone arch jutting into the ocean that was likely weakened by the preceding shocks.
Scientists won’t know the precise source and the dynamics at play for months, Vanacore said. Complicating matters to an extent is the fact that the southwestern area has not historically been very active seismically, so there is a thin record to analyze, said Shimon Wdowinski, a professor at FIU’s Department of Earth and Environment. And though earthquakes are always hard to predict, the relative lack of a record in this area also makes it especially hard to say what might happen next, he added.
“This particular area wasn’t known to be one of the major faults,” Wdowinski said. “In California, we can see patterns and we see major earthquakes. We can say what the probability is of a major earthquake. Here, we don’t see patterns.”
What is certain, he added, is that seismic pressure had been building in the area for hundreds if not thousands of years, the indirect result of the slow, ceaseless movement of the North Atlantic plate against the Caribbean plate. The sequence of quakes began “in earnest,” according to the U.S. Geological Survey, with a magnitude 4.7 earthquake late on December 28 and a 5.0 quake a few hours later.
The two more potent quakes this week happened as the pressure accumulated in the earth suddenly popped, the experts say.
“As the plates move with respect with one another, they push, they accumulate stress, and this pressure is released by earthquakes,” Wdowinski said. “Energy has been building for a very long time, and it’s released very quickly.”
It’s likely, though not certain, that the two potent quakes are it for now, the scientists say. As is typical after a quake, there have been numerous aftershocks as the earth’s crust settles, including some fairly strong ones. But sometimes one quake can trigger another, Wdowinski said. The U.S. Geological Survey’s forecast rates the probability of another strong or even more potent quake as relatively low, though FIU’s Draper cautioned that the possibility can’t be ruled out.
“The tectonics in that region are very, very complicated,” Draper noted.
Two major deformations around the island occur where the North American plate dives down, forming the Puerto Rico Trench, the Atlantic Ocean’s deepest spot, and where the Caribbean plate makes its downward plunge to the south, forming the Muertos Trough. But there are numerous secondary faults running across the island and nearby ocean bottom.
The area is so active seismically, in fact, that tremors occur daily in Puerto Rico, though most are too weak to be felt. But strong quakes are far less common, Draper said.
“That’s one of the biggest they’ve had for decades,” he said, referring to Tuesday’s temblor.
The last major earthquake struck Puerto Rico in 1918. The 7.5-magnitude quake, originating in the Mona passage west of the island, produced a tsunami and killed 116 people. In 1946, the island was hit by a tsunami produced by an 8.1 magnitude quake originating on the Dominican Republic, which shares the island of Hispaniola with Haiti.
Haiti was devastated by a 7.0 quake a decade ago that claimed more than 300,000 lives and toppled homes and buildings across the country. The seismic system affecting Hispaniola, though also emanating from the meeting of the North Atlantic and Caribbean plates, is separate from the system that affects Puerto Rico, Draper said. Those major quakes occur only about every 100 years on average, he noted.
This story was originally published January 7, 2020 at 6:41 PM.