Here’s what makes earthquakes so devastating in Haiti

More than a decade after a powerful quake devastated Haiti in 2010, the region's complex geology has sent the island into yet another spate of deadly convulsions. An intense magnitude 7.2 earthquake rocked Haiti in the morning hours of August 14, some 46 miles west of the 2010 temblor.

Both quakes are part of Haiti’s long history of shakes, which results from the island nation’s position at the edge of the slowly shifting Caribbean plate. The movements build stresses in a network of fractures that crisscross the island, which occasionally release pent-up stress in ground-rattling earthquakes. While the region's quakes are not the most powerful in the world, their deadliness is magnified by Haiti's abundance of concrete and masonry buildings that were not built to withstand earthquakes.

The full impacts from this latest event are not yet clear, but the quake likely wreaked havoc in communities that are already struggling from multiple pressures. The country is still recovering from the 2010 earthquake, which struck closer to the capital city Port-au-Prince, flattening many buildings and causing more than 200,000 deaths. It will take time for officials to determine how many people were killed in this latest event, but the U.S. Geological Survey estimates fatalities could number in the thousands.

The shaking was intensely felt in the cities of Les Cayes and Jeremie—both of which are still recovering from Hurricane Matthew in 2016, which battered the island with 145-mile an hour winds, significant floods, and downpours of rain. This new disaster also strikes as the country is still reeling from the assassination of President Jovenel Moïse on July 7.

"They just have one thing after the other," says Susan Hough, a geophysicist at the United States Geological Survey.

Constantly shifting tectonics

The island of Hispaniola, which includes the countries of Haiti and the Dominican Republic, sits atop the Caribbean tectonic plate, which is surrounded by a sea of other plates. Between the jostling of the North American, Cocos, South American, and Nazca plates, the Caribbean plate is constantly shoved and squashed by tectonic movements.

The key juncture that sparks shaking on the surface in Haiti lies just to the north of the island nation, where the Caribbean plate creeps eastward roughly three-quarters of an inch each year relative to the North American plate. Yet the boundary between the plates is not one straight fracture.

As the plates grind against each other, the forces produce a series of fractures that crisscross the region. Both the 2010 event and this latest quake—as well as multiple older quakes—occurred within one set of these breaks, which are known collectively as the Enriquillo-Plantain Garden fault zone.

Scientists believe that the 2010 event is likely connected to today's temblor. "An earthquake releases stress, but it also nudges—pushes—nearby faults in a way that makes other earthquakes more likely," Hough explains.

Analyses of the region after the 2010 temblor suggested that the shifting of the surface increased stresses both eastward toward Port-au-Prince and westward toward the epicenter of today's magnitude 7.3 quake, says Newdeskarl Saint Fleur, a geophysicist at the University of Haiti currently based in Paris, who is the lead author of a 2015 study that modeled these stress changes. Hough adds that a similar increase in stress on faults in this area was seen during the 1700s, when a spate of earthquakes struck in 1701, 1751, and 1770.

Stress also tends to accumulate most at bends or curves in the faults, Saint Fleur says, and today's event seemed to strike at one such bend. The epicenter is near the site of the 1770 quake, which, at an estimated magnitude 7.5, is the largest known to ever strike within this fault zone.

Yet even with this information, it's still not possible to predict quakes, Hough notes. "We see the patterns after the fact, and we say okay this domino nudged this domino," she says. But "there's no way to know which domino might go next."

A nation under strain

While the region has a history of shakes, the temblors in Haiti are not as large as those that occur where one plate plunges beneath another in what’s known as a subduction zone. One such quake, clocking in at a magnitude 8.2, recently shook Alaska's Aleutian Islands.

But the Aleutian Islands are sparsely populated, so that quake caused little damage. The deadliness of quakes in Haiti is the result of the structures on the surface as much as the shaking underground. Years of exploitation from outside countries, dating back to the enslavement of the island’s people after Christopher Columbus arrived in 1492, and political unrest within Haiti have resulted in the country’s current standing as the poorest in Latin America.

The unrest and poverty have translated to the development of the region, which is rife with substandard structures and building materials. Many structures use concrete, which is inexpensive and can be used to create heavy walls and roofs that resist hurricane winds, Hough says. But much of the region's concrete is unreinforced, and it readily crumbles under the shaking of earthquakes. While rebuilding after the 2010 devastation led to some structures built with earthquakes in mind, corruption and political turmoil have stymied many efforts to rebuild.

"The construction in Haiti is just like a perfect storm of unfortunate factors," Hough says. "It's everything you don't want in an earthquake."

The blow of this latest event, however, may have been blunted by the direction in which the earthquake traveled. The quake appears to have traveled westward, which means the most intense shaking was directed away from the densely populated city Port-au-Prince. Still, modeling from the U.S. Geological Survey suggests that landslides will be a significant hazard, barreling into structures, blocking roads, and limiting access for recovery crews.

Aftershocks will continue to rattle the region, and scientists will be keeping close watch on the sequence of shakes. After the 2010 earthquake, Hough was part of a team of scientists that traveled to Haiti to install seismometers. She returned in the subsequent years to help establish an earthquake monitoring network and, she says, is working now to figure out "what we can do to support this fledgling network and this community."