When an earthquake strikes, seismic waves ripple out from the hypocenter, often leaving far-reaching damage in their wake. The energy released can be so enormous that scientists have long wondered if earthquakes could sometimes trigger other destructive events, including volcanic eruptions.
The answer is far from simple, but scientists are beginning to develop a clearer picture. New research out of the University of California, Berkeley, examined the timing of modern eruptions relative to earthquakes. This research showed that contrary to previous research, earthquakes do not appear to trigger volcanic eruptions after just a few days.
“Data from 1800 to 1900 suggests there’s a triggering mechanism going on,” said lead researcher Theresa Sawi, now a geophysics Ph.D. student at Columbia University in New York. “However, when we look at more modern data, that trend totally disappears.”
However, when Sawi and her team examined a longer time frame following an earthquake, they found that the data may tell a different story.
Taking a Closer Look
To learn more, Sawi dug deep into data. She worked with Michael Manga and other Earth and planetary scientist colleagues to recreate a 1998 study published in Nature. This study suggested that earthquakes can trigger eruptions of a specific explosive variety on a very short timescale.
The Nature study defined a triggered eruption as one that occurred within 5 days and 800 kilometers of an earthquake with magnitude M8 or larger. The study concluded that since 1500 CE, these triggered eruptions happened more often than expected by chance—4 times more often, in fact. Sawi and her team recognized that global databases and measurement systems dramatically improved after 1964, so they decided to take a closer look at records after that date.
Sawi expanded the definition of a triggered eruption to include those following earthquakes of magnitude M6 or greater but otherwise used the same parameters to examine the more reliable post-1964 records. She found that contrary to the 1998 study, these so-called triggered eruptions that happened within days of nearby earthquakes occurred infrequently enough to be explained by chance.
Sawi also observed that the so-called triggered eruptions occurred in volcanoes that were highly active and found exclusively in subduction zones, areas where one tectonic plate moves under another. Because of this tectonic activity, subduction zones are known to be places where both eruptions and large earthquakes happen relatively frequently. This observation indicated to Sawi that this supposed triggering could actually just be an artifact of eruptions and earthquakes both happening so often in these tumultuous zones that they may occasionally coincide with each other by chance.
“Short-term triggered volcanism is probably just a coincidence,” Sawi said. “You have a lot of active volcanoes in a place with a lot of earthquakes, some of them are going to happen within days of another.”
In the days immediately following an earthquake, she and her team concluded, volcanic eruptions are triggered less frequently than previous researchers had concluded.
The Long-Term Story
After exploring short-term triggering, Sawi and her colleagues shifted their research to learn if triggered eruptions might still happen over time spans longer than the 5-day window. Beginning 2 months after earthquakes, they observed a 12% increase in the targeted class of explosive eruptions. This elevated number of eruptions slowly decreased over the next 2 years, remaining at a somewhat elevated level until that 2-year mark.
“When we expand the time frame to look at a few months to 2 years, we actually saw a statistical increase in the number of volcanisms following major earthquakes,” Sawi said. “This is consistent with other studies saying there was an increase in volcanism in 1 to 5 years.”
“This is a nice, careful study looking back in detail at previous big earthquakes,” said Jacob Lowenstern, chief of the Volcano Disaster Assistance Program at the U.S. Geological Survey’s Cascades Volcano Observatory, which is based in Vancouver, Wash. He was not involved with this study.
“Earthquakes affect the crustal pathways by which magma moves, so it’s not surprising that [the quakes] would affect volcanism, but it’s very hard to document,” Lowenstern explained. The reverse process can also occur, he noted, describing how magma intruding into Earth’s crust after a volcanic eruption can contribute to earthquakes. “It’s our ongoing challenge to deconvolve all of these complicated processes,” he added.
Sawi agreed, noting that more work is needed to better understand the complex factors driving these hazardous events. She said future studies could expand the scope of this work beyond eruptions to examine the impact of earthquakes on factors like volcanic gas output and temperature increase.
“There’s a lot of evidence that volcanic activity in general can be triggered by large earthquakes,” Sawi said. “A lot of work [still] needs to be done in this domain.”
Correction, 1 February 2019: In the first sentence, “epicenter” was corrected to “hypocenter.”