Powerful earthquakes can cause active volcanoes to exhibit brief periods of unusual activity. This has been documented. Researchers have struggled to ascertain whether a volcano that stirred was actually on the verge of erupting or simply disturbed for a short time.
In 2017, an 8.2 magnitude earthquake struck off the coast of southern Mexico, presenting researchers with a rare opportunity to investigate this very question. Two nearby active volcanoes initially showed increased activity before quieting down again. It was the manner of their subsequent silence that proved to be the more significant discovery.
On September 8, 2017, an 8.2 magnitude earthquake occurred deep within the Gulf of Tehuantepec, off the southern coast of Mexico. This was one of the most significant seismic events recorded in the country in modern times, resulting in approximately 100 fatalities and a minor tsunami reaching the shore.
The region is home to two notable volcanoes. El Chichón famously erupted in 1982, causing around 2,000 deaths and burying nearby villages in hot ash. Tacaná lies on the border with Guatemala, with its last eruption dating back to 1986. Both are still considered active.
The question facing researchers was straightforward, even if the geological underpinnings were less so: Had this powerful earthquake brought either volcano any closer to erupting?
Denis Legrand, a seismologist at the National Autonomous University of Mexico (UNAM), led a study that compared the seismic activity of both volcanoes before and after the massive earthquake. Volcanoes under stress generate tiny earthquakes – faint signals detectable only by sensitive instruments. The findings of this research were published in The Journal of South American Earth Sciences.
These small tremors appear in data as sharp, clustered signals, often occurring in swarms. Counting these tremors and tracking their occurrence provides researchers with an approximate indicator of what is happening deep underground.
If the 2017 earthquake had placed additional strain on the plumbing of either active volcano, these counts would have been expected to increase and continue to rise. A swift return to baseline levels would indicate the opposite.
El Chichón’s response was almost immediate. In the days following the earthquake, small earthquakes beneath the volcano became more frequent. This elevated frequency persisted for about five weeks before subsiding back to normal background levels.
Tacaná, initially, remained quiet. The surge in seismic activity beneath it didn’t begin until 15 weeks after the earthquake, and when it did, it lasted for only about two weeks. The same trigger, but a vastly different timeline.
This discrepancy caught the team’s attention. Two active volcanoes, shaken by the same fault rupture, reacted in entirely different ways. Prior to this study, no one had compared the behavior of both volcanoes following an earthquake without isolating their data.
The difference, they propose, lies in what is happening internally within each volcano. At El Chichón, the team attributes the swarm of small earthquakes to the circulation of hot water and gas through fractured rock – a hydrothermal system briefly pressurized.
Pressurized water moving quickly through fractured rock, the research team believes, explains why the response at El Chichón occurred within days. The swarm itself clustered around areas where hot springs and fumaroles are already evident at the surface today.
At Tacaná, the signal looked different. These small earthquakes occurred in deeper rock layers, and the team associates them with magma. This is their interpretation of the 15-week delay – denser, slower-moving material takes longer to react, if this data is accurate.
The reaction itself was not the headline event – what was striking was how quickly each volcano went back to “sleep.” Had the 2017 earthquake truly altered internal forces deep beneath each mountain, the volcanic swarms would have increased or continued to increase. Instead, both signals subsided within weeks.
Legrand and his colleagues argue that this rapid settling is precisely the key to the mystery researchers needed to solve. The tremor had briefly agitated the internal fluids of each volcano before quieting down, leaving no lasting imprint.
This data aligns clearly with what followed. Neither volcano erupted in 2017, 2018, or the years thereafter. This lack of eruption was registered in the seismic record within mere months of the earthquake.
The location spoke for itself. At El Chichón, the volcanic swarm was situated directly beneath known hot spring zones, corroborating earlier field studies that mapped the volcanic network from the surface.
Tacaná’s deeper magma swarm coincided with what the team interprets as a magma chamber and a narrow conduit rising from it. Two independent methods pointed to the same location. This alignment provides future monitoring instruments with a clearer target.
The conclusion is remarkably understated. A massive earthquake occurred near two known active volcanoes, and neither was destabilized in any significant way.
Before this study, confidence was based on the simple absence of eruptions. Now, it is provided by direct measurement of how the two volcanoes responded and how swiftly they recovered.
The main advantage, however, lies in the method. Wherever seismic networks are situated near major faults, the same approach allows authorities to assess in real-time whether a nearby volcano warrants an elevated alert level.
For those living near an active mountain, this is a tangible advancement in what science can offer. It doesn’t guarantee an eruption won’t occur. But it allows for a clearer understanding of whether the latest tremor was merely a footnote or a warning.
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