By Ron Shillingford
The Caribbean seems to be blighted by earthquakes at present with the Cayman Islands being hit last month along with many other countries in the region. Puerto Rico seems to be having the worst of it though, the latest was a 4.5 magnitude earthquake near Puerto Rico Saturday night.
According to the United States Geological Survey, the quake was 44 miles northwest of the island and 6 miles deep. No tsunami threat was administered at the time.
Tremors and aftershocks have been rocking Puerto Rico, weeks after a magnitude 6.4 earthquake toppled buildings, killed at least one person and injured another eight on Jan. 7. Families have begun leaving the island because it won’t stop shaking.
For many on the island, the devastation is a reminder of September 2017 when Hurricane Maria killed 3,000 people and as many as 200,000 Puerto Ricans were forced to hastily relocate to the mainland United States.
These major disasters have ravaged the island’s cultural heritage, too. Numerous historic landmarks – including a 2,000-year-old archaeological site containing priceless evidence of the island’s earliest dwellers, the Taíno people – have been destroyed.
The fact that Puerto Rico seems to be inordinately hit by earthquakes is because it is squeezed between the border of the North American and Caribbean tectonic plates.
The Puerto Rico Trench, north of Puerto Rico and the United States Virgin Islands, is an undersea fault zone. The North American plate is sliding under the Caribbean plate there, creating the potential for earthquakes and undersea landslides that can set off tsunamis.
“We’re just as likely to have earthquakes as a place like California, Japan, New Zealand, Alaska,” said Elizabeth Vanacore, a seismologist with the Puerto Rico Seismic Network.
When the tectonic plates in the region slide past each other and squeeze together, energy and stress build up until one side of a fault pops up, unleashing an earthquake. The earthquakes redistribute stresses along the fault for a time, until those stresses build up again and new tremors occur.
The North American plate is edging south and sinking underneath the Caribbean plate at about two centimetres per year, according to Gavin Hayes, a research geophysicist at the United States Geological Survey. To the south, the Caribbean plate is moving in the opposite direction, northward, at a similar rate.
“They are converging at a relatively low rate in comparison to plate boundaries around the world,” he said. In South America, it can be as high as six centimetres per year, while in the southwest Pacific it is up to 10 centimetres per year. Slower convergence rates mean strain builds up more slowly, making earthquakes either smaller or rarer, he said.
Mayita Meléndez, mayor of Ponce, said people living near beaches are desperate to get out of their homes for fear of tsunamis. “It’s not safe,” Meléndez said. “The earth is moving constantly.”
A tsunami happens when a quake on the seabed suddenly pushes water upward, producing a perilously towering wave. In 1918, an earthquake near the island’s north-western coast triggered a tsunami that killed 118 people, according to the geological service.
Puerto Rico has seen damaging quakes before, but major earthquakes in the south-western part of the island are unusual in recent history.
The last significant temblors recorded in that area, in 1991 and 1999, had a magnitude of about 4.1, according to the Puerto Rico Seismic Network, whose data dates to 1986.
Most of the seismic activity has occurred in the underwater band to the north of the island, close to where the tectonic plate boundary is. The geological survey has recorded 61 earthquakes of 4.5 magnitude or within about 50 miles of the northern shoreline since 2009.
Major earthquakes have regularly devastated cities in the Caribbean, including Kingston, Jamaica, which was destroyed twice in three centuries. Ten years ago, a 7.0-magnitude earthquake struck Haiti, killing more than 200,000 people and destroying tens of thousands of buildings.
“Puerto Ricans should be aware that they live in a tectonically active region,” Hayes said. “Earthquakes can be expected any time.” He emphasised the importance of building to seismic codes.
Seismologists said that more earthquakes are expected to be felt on the island for a few more days because the seismic activity is occurring close to land. But the frequency of aftershocks is expected to slow down over time, which means the chances of another intense earthquake will also fall.
Arrivals and departures at the international airport in San Juan, which was running on generators, have returned to schedule.