A sequence of earthquakes in southwest Puerto Rico continues to affect people living there, with the largest recent aftershock a magnitude 5.2 on Jan. 15. U.S. Geological Survey scientists on the island and the mainland are providing up-to-date scientific information to help the Commonwealth government and the Federal Emergency Management Agency make decisions that protect the public.
The earthquake sequence in southwest Puerto Rico began with a magnitude 4.7 earthquake Dec. 28. A magnitude 6.4 mainshock struck on Jan. 7. The sequence has so far produced more than 300 earthquakes greater than magnitude 3, a size that people can feel, and at times the quakes have come in quick succession; there were 18 aftershocks of magnitude 3 or greater between midday Jan. 14 and midday Jan. 15. There have been 10 earthquakes of magnitude 5 or greater in the sequence, a size that can do damage. This sequence of events has been tracked by scientists at the Puerto Rico Seismic Network and the USGS National Earthquake Information Center in Golden, Colorado, which monitors a network of earthquake sensors in the U.S. and overseas, and provides scientifically verified earthquake information worldwide.
More aftershocks are almost a certainty. The latest USGS aftershock forecast, made Jan. 16 and updated every day, estimates more than a 99 percent chance of magnitude 3 or greater aftershocks, a 54 percent chance of a magnitude 5 or greater aftershock, and an 8 percent chance of a magnitude 6 or greater aftershock in the next seven days. The most likely scenario is that the aftershocks will become less frequent over time, with no earthquake larger than a magnitude 6. Less likely is an earthquake of similar size to the magnitude 6.4 quake, which would temporarily increase the rate of aftershocks. The least likely scenario is the occurrence of a significantly larger earthquake.
USGS seismologists describe this amount of seismic activity as more than average – but within the expected range – for an earthquake sequence centered on a magnitude 6.4 quake. And although Puerto Rico lies in a tectonically active region where earthquakes have occurred for centuries, the quakes and aftershocks of the past three weeks, and the resulting damage, have taken many islanders by surprise. About 7,500 people have left their homes for other kinds of shelter, including, in some cases, cars and tents.
To help residents and government deal with the disruption brought on by the quakes, USGS and the PRSN have mounted an extensive response, including fast, state-of-the-science information about events as they happen and frequent briefings in Spanish and English to officials and the news media.
“This is a special situation requiring a special response, and the (USGS) natural hazards mission area has stepped up to address it,” said USGS Emergency Management Coordinator Marie Peppler, who led a team of three USGS emergency managers from the mainland deployed in Puerto Rico this week. “Our goal is the ensure that the Commonwealth leaders and decision makers at all levels are informed by the latest science.”
Field teams from the USGS Geologic Hazards Science Center in Colorado have been on the ground in Puerto Rico this week surveying changes to the island’s landscape, including numerous rock falls and damage to buildings. At Playa de Guayanilla, they found the ground has dropped about 6 inches, while at Cana Gorda beach, trees have slumped into the ocean. The teams have identified areas where ground shaking caused loose, water-logged sediments to become unstable in a process known as liquefaction; sites where saturated soil moved across gentle slopes, called lateral spreading; and landslide sites. The field survey teams’ work, identifying and mapping these sites, will help inform emergency managers and local governments about areas especially prone to instability during earthquakes and aftershocks.
Another group is assessing sites to determine which locations are least vulnerable to ground movements. “Their work is helping emergency responders locate their shelters, operations centers and other needed facilities so they are in places of low risk for landslides,” Peppler said.