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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.”

Facts gathered in earthquake zone shape scientists’ forecasts

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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.

Over the past several days, geophysicists from the USGS and the PRSN worked together to install temporary seismometers – sensitive solar-powered instruments that transmit information about subtle ground movements, to help scientists better understand the geologic structures involved in the quakes, and to improve the information that goes into mathematical models projecting the likelihood of future quakes. 

On the mainland, scientists at the National Earthquake Information Center are using information from both of these teams and existing seismic monitoring networks to assist the PRSN and provide up-to-the-minute, scientifically verified information on the occurrence and magnitude of aftershocks. That information is quickly made public and informs the work of the USGS aftershock forecast team, which is issuing daily estimates of the probability that the region will experience aftershocks as strong as, or stronger than, the Jan. 7 quake, over the next seven days and the next 30 days.

Closing the loop, the scientific information flows to Puerto Rico, where USGS and PRSN experts are passing along the latest findings to officials making public safety decisions; answering emergency managers’ questions about earthquake science; and helping to educate the public about earthquake coping skills, such as “Drop, Cover and Hold On” when sudden earth movements happen.

Caribbean Earthquakes Have Natural Causes

Earthquakes and tsunamis in Puerto Rico and adjacent islands are primarily driven by the convergence of the North American tectonic plate with the Caribbean tectonic plate, the section of the Earth’s crust on which the islands are located. The rate these plates come together is about 20 millimeters a year, about half the rate a human fingernail grows. Puerto Rico’s rocky island crust and its surrounding seafloor are squeezed between these tectonic plates. The rocks are naturally full of fractures and faults. Some of these faults may move abruptly to relieve the stress, causing earthquakes.

In the 20th century alone there have been several very large earthquakes north of Puerto Rico (magnitude 7.3 in 1918; magnitude 7.8 in 1943; magnitude 8.0 in 1946 and four major aftershocks of magnitude 7.6, 7.0, 7.3 and 7.1 between 1946 and 1953). The earthquakes in the recent sequence are occurring offshore of southwest Puerto Rico in a deformation zone, or an area where rocks are under strain, bounded by the Punta Montalva Fault on land and the Guayanilla Canyon offshore. Earthquake locations and other data show that several fault structures have been active in this sequence.

All the evidence indicates that this is a natural series of events. USGS research conducted over many years in other parts of the United States proved that injecting fluid underground, as is sometimes done to dispose of wastewater following oil and gas production, can induce earthquakes. However, the USGS is not aware of oil and gas exploration or other human activities in Puerto Rico that could have caused the recent earthquakes. The Energy Information Administration reports that there are no oil and gas operations in Puerto Rico, and the USGS does not extract oil or gas for production anywhere in the world.

New instruments help forecast aftershocks

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The USGS and fellow scientists have been monitoring seismic activity in and near Puerto Rico through the Puerto Rico Strong Motion Program at University of Puerto Rico Mayaguez and the PRSN, a permanent array of instruments in Puerto Rico and the Virgin Islands that was recently expanded.

“After the devastating Hurricane Maria struck Puerto Rico, the federal government invested in rebuilding damaged seismic stations in the region,” said USGS Director Jim Reilly. “These stations made it possible for the USGS and our partners at the Puerto Rico Seismic Network to provide more accurate and rapid information about the earthquakes and their possible impacts, along with better forecasts of potentially damaging aftershocks.”

The USGS sent six temporary seismometers, solar-powered and streaming data in real-time to the NEIC, to Puerto Rico Jan. 7, the same day the damaging magnitude 6.4 earthquake struck the southwest part of the island. The instruments have two types of sensors, broadband for detecting large earth movements and very sensitive short band sensors that pick up subtle movements. Scientists from the USGS and the Puerto Rico Seismic Network buried the seismometers 1-2 feet underground in locations along the southwest coast. Along with five temporary sensors installed by the Puerto Rico Seismic Network, they began transmitting data immediately and will remain in place as long as the earthquake sequence continues, said USGS geophysicist Thomas Pratt.

The additional seismometers will help researchers pinpoint the location of earthquakes in this sequence, which in turn will help them locate faults in the area and guide research into how the region may behave seismically in the future, Pratt said. By comparing the readings from different seismometers in the network, experts can also assess how far seismic waves travel; this will help estimate the likely damage from any future quakes.

Finally, by picking up signals from very small quakes that might otherwise go undetected, the temporary seismometers add to the information used in USGS aftershock forecasting, and that helps make the forecasts more accurate, said Michael Blanpied, associate coordinator of the USGS earthquake hazards program.

No one can predict the exact time or place of any earthquake, including aftershocks. USGS aftershock forecasts give an understanding of the chances of having more earthquakes within a given time period in the affected area. They use information on the timing, intensity and location of earthquakes that have already taken place to conduct a statistical analysis of the probability of future earthquakes of a given size.

“We do not know when this earthquake sequence will go back to past levels of seismic activity,” Blanpied said. “Sequences can last some time. If we get another large aftershock, this will mean an increase in aftershocks again. We do know that over the long term, these aftershocks will generally decrease in frequency, but over the short term the activity may remain high. The longer the sequence goes on, the more we learn about its behavior. This means we can further refine our aftershock models, getting a better understanding of how this sequence may behave in future.”

To learn more about how the USGS forecasts aftershocks, see the aftershock forecast overview.

Bringing the science back home

All this information, gathered on the ground in Puerto Rico and elsewhere, and analyzed at USGS science centers on the mainland, is brought back to the people of Puerto Rico, their elected officials and emergency managers. USGS emergency management and communications specialists are participating in a Joint Information Center with the Federal Emergency Management Agency. Together, the two agencies are briefing local officials at least daily; meeting frequently with the Commonwealth’s Governor, Wanda Vázquez Garced, and its resident commissioner to the U.S. Congress, Jenniffer González; and providing information to local and national news media.

While the Joint Information Center is a short-term undertaking and most USGS scientists currently on the ground in Puerto Rico will return to the mainland once their work is complete, the USGS has conducted hydrological science on the island for many years, through the San Juan office of the Caribbean-Florida Water Science Center, and the agency will continue to have a presence there after this earthquake sequence has ended.