Volcano-tectonic Seismicity at Villarrica Volcano: A Decade of Evidence (2014–2023) to Understand Its Internal Dynamics
This study analyzes a decade of volcano-tectonic seismicity at Villarrica Volcano to reveal distinct swarm evolution patterns and spatial stress variations that link shallow magmatic pressurization to delayed explosive activity, thereby providing a multiparametric framework for understanding the volcano's internal dynamics and improving hazard assessment.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Volcanoes are often thought of as sleeping giants that wake up with a single, violent roar, but the reality of how they behave is far more subtle and complex. Beneath the surface of an active volcano, the ground is constantly shifting, cracking, and rearranging itself as magma and hot fluids move through rock. When this movement happens, it generates earthquakes. In volcanic settings, scientists distinguish between two main types of these tremors. One type, known as long-period events, sounds like a deep, rhythmic thumping and usually signals that gas or magma is churning inside the main pipe of the volcano. The other type, called volcano-tectonic earthquakes, sounds more like the sharp crack of a breaking branch. These occur when the solid rock itself fractures under stress, often because pressure from below pushes against the walls of the magma reservoir structure. By listening to these sharp cracks and mapping where they happen, researchers can build a picture of the invisible forces reshaping the volcano from the inside out. Understanding these patterns is critical for communities living in the shadow of active volcanoes, as the way the ground cracks can sometimes, but not always, signal that an eruption is on the way.
For a decade, from 2014 to 2023, a team of researchers closely monitored Villarrica Volcano in Chile, one of the most active and dangerous volcanoes in South America. They focused specifically on the sharp, rock-breaking earthquakes to understand how the volcano's internal engine works. By analyzing 2,364 of these events, the scientists identified nine distinct periods of intense activity, known as swarms, where the number of earthquakes spiked for weeks or months at a time. What they found was a remarkably consistent story of how stress travels through the mountain. In most of these swarms, the activity began at depth beneath the center of the volcanic caldera, where the rock was under the greatest pressure. As the swarm progressed, the earthquakes would migrate outward, moving from the center of the caldera toward the edges and into the surrounding landscape. This outward movement suggests that, once the rock inside the caldera begins to fracture, the stress is released and travels along pre-existing cracks in the ground, spreading the seismic activity to the volcano's periphery.
However, the researchers were careful to note that this pattern is a strong tendency, not a strict rule. While most swarms followed this path from the center outward, two of the nine groups behaved differently. One swarm stayed active in the center even as new earthquakes appeared outside, while another occurred completely outside the caldera, never touching the central zone. This variation tells scientists that while the volcano has a preferred way of releasing stress, it is not a machine that operates on a single, unchanging script. The location and timing of these events depend on the specific conditions of the rock and the fluids moving through it at that moment. The study also looked at the size of the earthquakes within each swarm. They found that the largest, most energetic earthquake in a swarm usually happened in the middle of the event, not at the beginning. This larger-magnitude earthquake marks a turning point: before it, activity accumulates inside the caldera; after it, seismicity spreads outside the caldera and then gradually fades.
The team also examined whether these underground cracks led to visible eruptions. They discovered a delayed connection in some cases. In four of the nine swarms, about one to two months after the largest earthquake occurred, the volcano showed signs of increased surface activity, such as more frequent explosions or hotter temperatures detected by satellites. This delay suggests that the breaking of the rock inside the mountain creates new pathways for heat and gas to travel upward, but it takes time for these fluids to reach the surface. Yet, this link is not guaranteed. In the other five swarms, the ground shook and the stress redistributed just as it did in the active ones, but there was no corresponding increase in explosions or heat. This finding is crucial for hazard assessment: it means that while a swarm of earthquakes is a sign that the volcano is active and stressed, it does not automatically mean an eruption will follow. The internal reorganization of the volcano can happen without ever reaching the surface.
By combining the locations of the earthquakes with measurements of how the rock is breaking, the scientists mapped out different zones within the volcano. The area directly beneath the caldera showed signs of high stress and concentrated rock fracturing, while the areas just outside the caldera were characterized by a different type of fracturing, likely influenced by the presence of hot fluids. Further out, the earthquakes aligned with major fault lines in the region, showing how the volcano's internal pressure interacts with the broader tectonic forces of the Earth. The study concludes that Villarrica Volcano is a dynamic system where shallow magma and fluids pressurize the rock, causing it to fracture in a predictable sequence that usually moves from the center outward. However, because the connection between these deep cracks and surface eruptions is conditional rather than certain, monitoring the volcano requires watching the whole picture—the location of the quakes, the type of rock breaking, and the surface signals—rather than relying on a single warning sign. This detailed understanding helps scientists better interpret the volcano's behavior, offering a clearer, more nuanced view of the risks it poses to the surrounding communities.
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