Q COSTARICA — The Poás volcano registered 24 volcano-tectonic earthquakes between December 3 and 4, a number higher than what is considered normal, as confirmed by the Observatorio Vulcanológico y Sismológico (OVSICORI)—Volcanological and Seismological Observatory.
According to OVSICORI volcanologist Maarten de Moor, although this seismicity is high, there is no evidence of a new eruptive cycle.
“It’s a high number of volcano-tectonic earthquakes, higher than normal, but we don’t see significant changes in other parameters such as gases, deformation, and emissions, which remain stable,” explained De Moor.
According to Moor, the seismic events could be due to two situations: New magma injection into the system and Tectonic faults near the volcano.
“What we look for to say there’s a significant change is an increase in other parameters as well. At the moment, we haven’t seen any major changes,” he emphasized.
He also emphasized that these swarms of earthquakes have been observed at other volcanoes in the country without necessarily leading to major crises, so the interpretation should always be made in conjunction with the other variables.
Eruption
A few hours after this increase in seismic activity, Poás registered a phreatic eruption on December 6th with an ash column of approximately 400 meters and the ejection of ballistic projectiles that lasted for five minutes.
However, the specialist clarified that there is no proven direct relationship between the earthquakes of the preceding days and this specific explosion.
“It is what we call an isolated eruption, which is part of the same eruptive episode that began in January. We have no evidence of a change in behavior or a direct relationship with the earthquakes,” he added.
The last eruption had occurred in October.
Unstable Volcano
Poás continues to show variations characteristic of a system that has not yet fully stabilized after the intense eruptive period recorded between January and May 2025.
“We are still within the same episode of activity. These small eruptions are part of the tail end of what we saw at the beginning of the year,” explained De Moor.
The expert confirms that this has been the most intense year since 2017, with eruptions of comparable size—although less dangerous—and a prolonged duration of the eruptive episode.
De Moor recalled that the 2025 eruptive period began on January 5 and lasted for several months, with episodes of significant explosions, ash emissions, and changes in the crater.
“It was a very intense episode compared to recent years. These large eruptions were comparable to those of 2017,” the specialist noted.
He also highlighted that, unlike other quieter years, this cycle has extended for almost a full year and the system has not yet fully stabilized.
“Poás goes through eruptive episodes that usually last between three months and a year. We are still within that same period,” he indicated.
Surface magma continues
On the other hand, regarding the magma, Poás has maintained an active surface magma system for decades, which explains its recurring eruption cycles.
“At the Poás volcano, we have always seen considerable activity due to the presence of surface magma. It is still there; it hasn’t fully emerged,” De Moor stated.
This characteristic explains the recurrence of phreatic eruptions and incandescence, although the latter has decreased considerably in recent months.
“After Saturday’s eruption, we saw a glowing spot near vent B, but it’s very rare to see it now,” he added.
Regarding the crater lake level, it has remained stable for several weeks.
Although the bottom was practically dry in April and May, there is currently water covering all three vents, which could trigger steam eruptions when heat builds up beneath the surface.
“There is enough water to generate pressure beneath the lake and cause explosions,” De Moor stated.