Quantcast
Environmental News for a Healthier Planet and Life

Help Support EcoWatch

Earthquake Forecast for Puerto Rico: Dozens of Large Aftershocks Likely

Science
A crack caused by an earthquake is seen in La Guancha boardwalk in Ponce, Puerto Rico on Jan. 9, 2020. RICARDO ARDUENGO / AFP via Getty Images

By Richard Aster

Multiple strong and damaging earthquakes in southern Puerto Rico starting around Dec. 28, 2019 have killed at least one person, caused many serious injuries and collapsed numerous buildings, including a multistory school in the town of Guánica that luckily was empty at the time. These quakes are the most damaging to strike Puerto Rico since 1918, and the island has been under a state of emergency since Jan. 6, 2020.



This flurry of quakes includes onshore and offshore events near the town of Indios and along Puerto Rico's southwestern coast. So far it has included 11 foreshocks – smaller earthquakes that preceded the largest event, or mainshock – with magnitudes of 4 and greater. Major quakes occurred on Jan. 6 (magnitude 5.8) and Jan. 7 (magnitude 6.4 mainshock), followed by numerous large aftershocks.

Seismologists like me are constantly working to better understand earthquakes, including advancing ways to help vulnerable communities before, during and after damaging events. The physics of earthquakes are astoundingly complex, but our abilities to forecast future earthquakes during a strong sequence of events in real time is improving.

Forecasting earthquakes is not a strict prediction – it's more like a weather forecast, in which scientists estimate the likelihood of future earthquake activity based on quakes that have already occurred, using established statistical laws that govern earthquake behavior.

An Undersea Fault Zone

Puerto Rico spans a complex boundary between the Caribbean and North American tectonic plates, which are sliding past each other in this region at a relative speed of about 2 centimeters per year. Over geologic time, this motion has created the Muertos Trough, a 15,000-foot depression in the sea floor south of the island.

This plate boundary is riddled with interconnected fault structures. The present activity is occurring on and near at least three interrelated large faults.

Multiple faults crisscross the eastern Caribbean. Those outlined in red have a potential to generate a large earthquake. The arrow at top right shows the direction of the North American plate's motion relative to the Caribbean plate. Red stars denote intensity centers for past earthquakes. USGS

Faults are pre-existing weak zones between stronger rocks. In response to surprisingly small force (stress) changes, they rapidly slip to produce earthquakes. The "hair-trigger" nature of fault slip means that predicting the precise timing, location and size of individual quakes is extremely challenging, if not impossible.

During an earthquake sequence, changing stresses act on nearby fault systems as stress is gradually redistributed within the earth. This process generates thousands of protracted aftershocks.

Many earthquake sequences simply start with the mainshock. But it is not especially rare for scientists to recognize after the fact that foreshocks were occurring before the main event. Improvements in earthquake instrumentation and analysis are helping scientists detect foreshocks more often, although we have not yet figured out how to recognize them in real time.

Will One Shock Lead to Another?

Researchers have known for over a century that the rate of earthquakes following a mainshock declines in a way that we can characterize statistically. There is also a well-established relationship between the magnitude of earthquakes and their relative number during an earthquake sequence. In most seismically active regions, for a decrease of one magnitude unit – say, from 4.0 to 3.0 – people can expect to experience about 10 times as 3s compared to 4s in a given time period.

Using such statistical relationships allows us to forecast the probability and sizes of future earthquakes while an earthquake sequence is underway. Put another way, if we are experiencing an aftershock sequence, we can project the future rate of earthquakes and what magnitudes we expect those quakes to have.

Richard Aster is a professor of geophysics and the department head at Colorado State University.
Disclosure statement: Richard Aster has received funding for earthquake research from the National Science Foundation, Los Alamos National Laboratory, Sandia National Laboratories and the U.S. Geological Survey. He is a past president of the Seismological Society of America (SSA) (2009-2011) and current chair of the board of directors of Incorporated Research Institutions for Seismology. Aster also chairs the U.S. Geological Survey's Advanced National Seismic System Advisory Committee, and is a member of the Southern California Earthquake Center Advisory Council.

Reposted with permission from The Conversation.

EcoWatch Daily Newsletter

Food Tank

By Danielle Nierenberg and Alonso Diaz

With record high unemployment, a reeling global economy, and concerns of food shortages, the world as we know it is changing. But even as these shifts expose inequities in the health and food systems, many experts hope that the current moment offers an opportunity to build a new and more sustainable food system.

Read More Show Less
Pexels

By Brian J. Love and Julie Rieland

The COVID-19 pandemic has disrupted the U.S. recycling industry. Waste sources, quantities and destinations are all in flux, and shutdowns have devastated an industry that was already struggling.

Read More Show Less
Pixabay

By Kris Gunnars, BSc

Unhealthy foods play a primary role in many people gaining weight and developing chronic health conditions, more now than ever before.

Read More Show Less
A man pushes his mother in a wheelchair down Ocean Drive in South Beach, Miami on May 19, 2020, amid the novel coronavirus pandemic. CHANDAN KHANNA / AFP via Getty Images

The U.S. reported more than 55,000 new coronavirus cases on Thursday, in a sign that the outbreak is not letting up as the Fourth of July weekend kicks off.

Read More Show Less
To better understand how people influence the overall health of dolphins, Oklahoma State University's Unmanned Systems Research Institute is developing a drone to collect samples from the spray that comes from their blowholes. Ken Y. / CC by 2.0

By Jason Bruck

Human actions have taken a steep toll on whales and dolphins. Some studies estimate that small whale abundance, which includes dolphins, has fallen 87% since 1980 and thousands of whales die from rope entanglement annually. But humans also cause less obvious harm. Researchers have found changes in the stress levels, reproductive health and respiratory health of these animals, but this valuable data is extremely hard to collect.

Read More Show Less

Sunscreen pollution is accelerating the demise of coral reefs globally by causing permanent DNA damage to coral. gonzalo martinez / iStock / Getty Images Plus

On July 29, Florida Governor Ron DeSantis signed into law a controversial bill prohibiting local governments from banning certain types of sunscreens.

Read More Show Less

Trending

Oat milk is popping up at coffee shops and grocery stores alike, quickly becoming one of the trendiest plant-based milks. jacqueline / CC by 2.0

By Kelli McGrane

Oat milk is popping up at coffee shops and grocery stores alike, quickly becoming one of the trendiest plant-based milks.

Read More Show Less