Why so many earthquakes in Puerto Rico?

Puerto Rico is experiencing an unusual and prolonged period of seismic activity causing some homes to collapse and structural damage to roads and bridges, as well as spreading fears of tsunamis along the south coast of the island.

At a press conference Tuesday morning, the island’s Governor, Wanda Vázquez, said federal agencies monitoring the activity say the tremors and quakes could continue for the next few days.

"We've never been exposed to this kind of emergency in 102 years," she said, adding that the island is in an official state of emergency.

So, why is the island experiencing so many earthquakes?

"Buffer zone"

Puerto Rico is surrounded by geological faults and sits in a “buffer zone” between the North American and Caribbean plate, making it a high risk zone for earthquakes, according to Alberto López, a geologist with the Puerto Rico Seismic Network.

“Puerto Rico forms a micro-plate within the Caribbean plate,” López told Univision. While tremors are common, its highly unusual for the island to feel such intense earthquake activity, he said. The island experienced smaller magnitude 4 quakes, in 1991 and 1999, “but never have we seen the kind of intensity we are having today,” he added.

The latest quakes in Puerto Rico's southern region began the night of Dec. 28. The latest earthquake with a magnitude of 6.4 struck off the coast of the island at 4:24 a.m. local time on Tuesday, according to the United States Geological Survey.

That came after a week of smaller tremors in the same area off the south coast. Several strong aftershocks followed Tuesday’s big quake, one registering 5.8 magnitude. The quakes knocked out power to much of the island, seriously damaging homes and buildings and leaving at least one person dead. That followed a 5.8-magnitude quake on Monday.

The last major earthquake was in 2014, measuring 6.3 on the Richter scale, but it caused much less impact as the epicenter was 50 miles off the north coast.

More than 1,280 earthquakes have hit Puerto Rico’s southern region since Dec. 28, more than two dozen of them magnitude 4.5 or greater, according to the US Geological Survey. The latest was a 5.9 magnitude aftershock that occurred Jan 11.

The quake of 1918

One of the largest and most damaging earthquakes to hit Puerto Rico occurred in October 1918, when a magnitude 7.3 quake struck near the island’s northwest coast, unleashing a tsunami and killing 116 people.

Seismologists say that shallow quakes were occurring along the Guayanilla Canyon, a sub-terranean area of the south-west corner of the island where the North American plate meets the Caribbean plate.

When the tectonic plates in the region push up against each other they create friction and energy builds up until there is a rupture, unleashing an earthquake. The earthquakes redistribute stresses along the fault for a time allowing the plates to settle back down, until those stresses build up again and new tremors occur. “It can take hundreds or thousands of years to accumulate the energy,” said López.

Scientists only began measuring the activity with instruments about 40 years ago so it’s hard to make predictions. “As we have observed the life of the fault for so little time what we are seeing is only an instant in the life of the fault,” he said.

Experts say they don’t know at this stage if the island is over the worst of the current activity or if there are larger earthquakes to come. Usually, the larger the earthquakes the longer the period of tremors can last. They can last days, or months, even years, depending on the local conditions, such as the type of rock.

“When there is a sequence like we have now it's hard to tell when the next big one will come. You can ask, what awaits us tomorrow? On the other hand, what we had today could be the last one,” López explained.

Fortunately, Puerto Rico’s tectonic displacement is small, only 1-2 millimeter a year, compared to larger ones like the San Andreas fault in California, which slips between 4 to 5 centimeters a year.

The Caribbean has had it's fair share of quakes, most recently in Haiti in 2010, when a 7.0-magnitude earthquake struck the capital, Port-au-Prince on Jan. 12, killing 100,000 to 200,000 people.

Uncertainty

"You cannot predict when you will see a strong earthquake in Puerto Rico, but they have occurred in the past and will occur in the future," said Puerto Rican geomorphologist Jose Molinelli, in a Facebook interview with the newspaper El Nuevo Dia.

The ‘Guayanilla Canyon’ is an undersea fault zone, creating the potential for tsunamis when cracks in its walls cause landslides.

The United States National Tsunami Warning Center said there was no tsunami threat from Tuesday morning’s earthquake. The local authorities initially issued a tsunami watch before canceling it, according to Puerto Rico’s emergency management agency.

But the risk has not gone away, says Molinelli. “Fortunately, that has not happened so far but the possibility cannot be ruled out because the seismic activity is continuing,” he added.

He said there was a 17 percent probability of another quake between 5 and 6 magnitude on the Richter scale and a 3 percent probability of a quake of magnitude 6 to 7.

“The important thing is that we will continue for the next few days feeling earthquakes that could be almost as strong as the initial one. They are coming in clusters. People who live on the coast, where they are in the area susceptible to tsunamis, should review the maps for evacuation routes,” he said.

Major earthquakes in the southwestern part of the island are unusual in recent history, he pointed out, using as an example the collapse of a popular tourist landmark, known as the Punto de Ventana (Window Point), a rocky outcrop with hole shaped like a round window located on the south coast near Guayanilla.

It's collapse "indicates from a geomorphologist point of view that over hundreds of years, or possibly thousands of years, there had not been an earthquake strong enough to destroy it,” said Molinelli.