Costa Rica's National Seismological Network (RSN) published a detailed report about the changes to the country’s active volcanoes induced by the great M7.6 earthquake of September 5 and recommends the access closure to the craters of Rincón de la Vieja and Turrialba volcanoes, which increased activity.
On September 5, 2012 at 8:42 am (local time) there was a great earthquake at 10 km south of Samara on the Nicoya Peninsula with a depth of 20 km, located in the subduction zone between the Cocos and Caribbean plates.
Expert from RSN (University of Costa Rica and the Costa Rican Electricity Institute (ICE)), said that there are reports in the literature that suggest evidence that large earthquakes can trigger eruptions of volcanoes: Some cases include the Aso volcano (Japan) and an earthquake in 1914 (Mw = 7.0), Puyehue volcano (Chile) and the Valdivia earthquake (1960, Mw = 9.6), and Karagentang volcano (Indonesia) with the Tohoku earthquake (2011, Mw = 9.0).
According to RSN, also Costa Rica’s volcanoes have often produced eruptions linked to large earthquakes. Rincon de la Vieja volcano had a series of major eruptions of magma in 1991, two weeks after the earthquake of 7.7 magnitude earthquake of Limón. In 2009, the M6.2 Cinchona earthquake was followed by a small phreatic explosion from Poas Volcano four days later. The volcano has remained in a state of recurring phreatic eruptions.
The study of the RSN, published yesterday, said that for this reason it is very important to increase routine visits to monitor the active volcanoes even when volcanic systems remain calm. Changes in volcanic activity, after an earthquake of this magnitude, can occur months, even years after the earthquake.
The report goes on to present a summary of field observations taken during the month of September 2012, and aerial images of overflights coordinated with the National Emergency Commission (CNE):
Turrialba volcano activity showed an unusual degassing activity during the weeks after the earthquake, although temperatures of the fumaroles are still the same (≈ 800 ° C for the highest in the small crater that opened in January 2012). Some fractures in the active crater indicate that the rock mass stability remains precarious and small landslides may occur within the same.
For Irazu volcano, its crater lake and fumarolic field showed no significant changes, however there have been a series of earthquakes near the mountain.
At Poas volcano there were a number of felt earthquakes and a distinct change in the lake was noted. A small phreatic eruption occurred between the evening of 6 and early morning of 7 September 2012. Measurements with the thermal camera indicate declining temperatures from dome fumaroles (less than 200° C). At Laguna Caliente lake, increased degassing and upwelling of water with large quantities of floating sulfur were seen on 26 September. The temperature at the lake bottom increased to 150°C at a vent which is producing molten sulfur and has also been the site of previous phreatic explosions. Lake water temperatures remained stable at around 50° C.
Arenal Volcano experienced some small landslides on the slopes due to the instability of the rock mass without showing major changes.
Rincon de la Vieja volcano has a very active hot lake with unusual release of gas compared to the month of April 2012. Its color is a milky gray, due to mixing of sediments and an increase in temperature (48° C). Besides, its degassing activity increased significantly.
It is possible that the energy released by the earthquake of Samara on 5 September 2012 disrupted the volcano’s hydrothermal system and at least partly caused the observed changes of increased temperatures of fluids (water and gasses), increased degassing, new fractures, landslides, increased local seismicity and possible phreatic eruptions.
The lake level decreased despite the current rainy season. Around 35,000 cubic meters of water were lost with comparison to the month of April due to the constant evaporation.