Scientists in Puerto Rico have tracked 15,000 quakes over the last year. Here’s how they did it.

When a magnitude 6.4 earthquake struck Puerto Rico one year ago, Elizabeth Vanacore pulled a pillow over her head and held onto her bed until the shaking stopped.

Then, the seismologist got to work.

Within minutes, she was in her car navigating traffic as people fled tsunami evacuation zones. She arrived at 5 a.m. Analysts were already logging data from seismic and tide gauge stations.

“It’s like an emergency code in the hospital,” she said.

Thursday marks one year since the quake, which killed one and injured nine. The tremor was the peak of a seismic sequence that began in December 2019 in southwestern Puerto Rico and has continued ever since. There have now been almost 15,000 quakes across the island and the Virgin Islands, compared to the annual average of 4,000 tremors.

And they may not stop any time soon.

For the scientists charged with tracking every temblor, the last year has been like no other.

Staff at the Puerto Rico Seismic Network, which works with the U.S. Geological Survey and is part of the University of Puerto Rico Geology Department, have worked day and night during the pandemic to record and investigate the largest amount of quakes ever detected by the organization.

And they’ve done it all while experiencing the nerve-rattling quakes up close themselves.

“At the end of the day, the heart of the network is the people who work in it,” said Vanacore, an associate research professor at the University of Puerto Rico. “It’s not the instruments, it’s not the computers. It’s the people.”

Life in southwest Puerto Rico interrupted by quakes

Victor Huérfano, the network’s interim director, told the Miami Herald that the scientists have been in emergency mode since September 2019, when an unrelated magnitude 6.0 earthquake shook the Mona Passage, a water channel that divides Puerto Rico and Hispaniola.

“Everything suggested that something could happen there,” he said.

Scientists tracked over 2,000 aftershocks in the month following the quake. They were already weary and exhausted when a separate series of tremors began in southwest Puerto Rico in late December.

A USGS report expects the region to experience aftershocks for “years to decades.”

The frequency of daily quakes significant enough to be felt by the population has diminished, but bursts of tectonic activity continue to startle the region, home to natural wonders like the Lajas bioluminescent bay and the Guánica dry forest.

For researchers, the tremors have been scientifically astonishing to witness, offering a unique opportunity to understand the hazardous, active fault lines that run across Puerto Rico and the Caribbean.

“Earthquakes in those faults have been observed throughout geological history many times,” said Alberto López Venegas, a researcher at the seismic network and a UPR associate professor of geology. “But we had not been there to observe it.”

Above all, the earthquake trackers acknowledge the devastating human toll of the constant quakes. The southwest includes some of Puerto Rico’s poorest municipalities.

In early 2020, thousands of people slept outside in tent cities, community camps, and makeshift shelters near their houses. At one point, there were as many as 20,000 residents forced from their homes, according to local officials.

Many buildings collapsed as the tremors cracked structures. Others who slept under the stars were simply terrified of being crushed under their roofs.

The quakes have also destroyed historic and public sites, like a 19th-century church in Guayanilla. Some of the region’s residents have left to other towns across the island, or left Puerto Rico altogether.

“I see how it’s impacting my neighbors,” Vanacore said. “I see how it’s impacting neighboring communities.”

Tracking quakes in hope of saving lives

The Puerto Rico Seismic Network has monitored each and every quake.

Twenty-four hours a day, at least two analysts are recording each tremor by hand, sharing reports for social media and fielding calls from the public. Tropical storms and the pandemic have not deterred them from monitoring the endless tectonic activity.

Technicians have gone into the field as the ground trembles below their feet. José Cancel, the head technical coordinator, recalls experiencing a strong earthquake while installing sensors near fault lines last January.

“The structures around us moved as if they were gelatin,” he said.

The network has offered psychological support to help them manage the long hours, additional workload, and constant shaking.

“That amount of work is herculean,” Vanacore said. “When we have a major event, all of our departments —our IT department, our education outreach, our administration department, our technicians—all go above and beyond to make sure that people get the information they need as quickly as possible.”

The scientists’ research runs a gamut of subjects, from engineering seismology to seismic topography. This year, in addition to previous projects, Vanacore has assisted in pinpointing exact quake positions. Her doctoral students incorporate facts and figures from the quakes into their own investigations.

The multitude of earthquakes has helped shed light on how fault lines in the southwest part of the island interact with each other. While the Punta Montalva fault likely initiated the sequence, multiple faults are now causing the activity.

“Like pool balls on a pool table,” she explained. “The cue ball striking that set of pool balls is that 6.4 event. What we’re seeing now is all those interactions and all those ‘pool balls’ bouncing against each other.”

Through research with partners such as the USGS, scientists at the network are finding “blind faults,” those which cannot be seen by the naked eye, clashing below the Caribbean Sea or covered by sediment. The evidence shows there is a fault zone in the southwest made up of a complex cluster of different, small faults.

Vanacore hopes studying this turbulent time will help protect residents from major quakes in the future while also advancing scientific research and public policy in the region.

Her goals: Make all their findings more accessible so that Puerto Ricans and residents of the Caribbean are less afraid and more informed.

“It’s not just imaging the earth and looking at these large-scale processes,” she said. “Understanding these large-scale processes will help us and help people better prepare for earthquakes.”

This story was originally published January 7, 2021 at 4:24 PM.