Nestled between two oceans and several tectonic plates, Costa Rica is known not only for its natural beauty and biodiversity, but also for its hazardous geology. Active volcanoes and steady seismic activity are part of residents’ daily lives, along with the occasional devastating earthquake.
New research suggests that Costa Rica’s complex web of seismic faults has weak, potentially unstable zones that can respond to shaking from major earthquakes far away. This process, called dynamic triggering, often arises within geothermal and volcanic sites, but it is less common along continental faults. In the past decade, two major earthquakes have unsettled faults in Costa Rica by way of dynamic triggering, seismologists reported at AGU’s Annual Meeting 2023 in San Francisco.
Stresses Far Underground
Costa Rica is a seismically active country. Every decade or so, an event greater than magnitude 7 causes violent shaking and extensive damage. In 2012, the second-largest earthquake in the country’s recorded history struck off the Nicoya Peninsula. The magnitude 7.6 quake caused $45 million in damages. The largest event, a magnitude 7.7 quake in 1991, killed 75 people.
Seismologists have debated whether seismic waves from the world’s biggest earthquakes can set off distant faults where stress has been accumulating. Signs of dynamic triggering have surfaced along Southern California’s San Jacinto Fault Zone and Salton Sea Geothermal Field, among other sites, but hadn’t been detected in Costa Rica.
Seismologist Esteban Chaves of the Volcanological and Seismological Observatory of Costa Rica and undergraduate student Sonia Hajaji of the University of Costa Rica set out to look for that evidence.
“For it to be dynamic, we need to see that the rate of the seismicity here increases after the arrival of these waves.”
They combed through data from a network of seismic stations throughout Costa Rica that were recording before, during, and after the 25 largest earthquakes in the Americas since 2010. They also included the magnitude 7.8 earthquake that struck the Türkiye-Syria border on 6 February 2023 because it was the most recent major event at the time of the analysis.
The researchers looked for a sign that seismic activity increased following the distant large earthquakes. “We first have to see the waves of the distant earthquake here in Costa Rica,” Hajaji said. “For it to be dynamic, we need to see that the rate of the seismicity here increases after the arrival of these waves.” Hajaji and Chaves searched for upticks in high-frequency local seismic activity just after the arrival of low-frequency waves from the distant events.
They saw this on two occasions: after a magnitude 7.6 earthquake in 2018 in the Caribbean Sea off the coast of Honduras and after the Türkiye-Syria magnitude 7.8 earthquake.
“Maybe [dynamic triggering] is more common than we think.”
The spike in microseismic activity after the Honduras earthquake was concentrated in Costa Rica’s northern volcanic regions. After the Türkiye-Syria quake, seismicity peaked along Costa Rica’s Pacific coastal subduction zone—where the 2012 earthquake hit—and at shallow faults in the center of the country.
The team’s dissection of local seismic data to reveal high-frequency waves was novel, said seismologist Joan Gomberg of the U.S. Geological Survey. Gomberg studies dynamic triggering but was not affiliated with the study.
“Maybe [dynamic triggering] is more common than we think,” Gomberg said. “If faults are really sensitive, and they’re really sensitive right before they go, it might provide some clue that some areas are ripe for an earthquake.”
Which Waves Are at Fault?
The results left Chaves and Hajaji with a burning question: Why had the disastrous Türkiye earthquake triggered a rash of seismic response in Costa Rica, when other large earthquakes much closer had not?
The 2018 and 2023 events had something in common, the team realized. Both were supershear earthquakes—during which a fault rupture propagates faster than the earthquake’s S wave (shear wave). These notable events are similar to a sonic boom and produce particularly strong shaking. Gomberg agreed that correlation merits further exploration of the underlying mechanism behind triggering.
“No one has looked at the effects of supershear ruptures at a distance,” Chaves said. “Do they have more potential to trigger more earthquakes than other events? We don’t know yet.”
—Gillian Dohrn (@dohrn_gillian), Science Writer