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Understanding the cause/effect relationship between tectonic earthquakes and volcanic eruptions is a striking topic in Earth Sciences. Volcanoes may erupt due to the impact of seismic waves (i.e. dynamic stress) and changes in the stress field (i.e. static stress) with variable reaction times. In 2012, three large (Mw≥7.3) subduction earthquakes struck Central America within ten weeks; some volcanoes in the region erupted days after, meanwhile for others it took months to years to erupt. Here we show that the three earthquakes contributed to the increase in the number of volcanic eruptions during the seven years that followed. We found out that only those volcanoes that were already in a critical state of unrest effectively erupted, indicating that the earthquakes only prompted the eruptions. We recommend the permanent monitoring of active volcanoes to reveal which are more susceptible to culminate into eruption when the next large-magnitude earthquake hits a region.

Understanding the relationship cause/effect between tectonic earthquakes and volcanic eruptions is a striking topic in Earth Sciences. Volcanoes erupt with variable reaction times as a consequence of the impact of seismic waves (i.e. dynamic stress) and changes in the stress field (i.e. static stress).In2012, three large(Mw≥7.3)subductionearthquakes struck Central Americawithin a period of 10 weeks;subsequently, some volcanoesin the regionerupted a few days after, whileotherstook months or even years to erupt. Here, we show thatthese threeearthquakes contributed to the increase in the number of volcanic eruptionsduring the 7 years that followed these seismic events. We found that onlythosevolcanoes that were already in a critical state ofunrest eventually erupted, which indicates that the earthquakes only prompted the eruptions. Therefore, we recommend the permanent monitoring of active volcanoes toreveal which are more susceptible to culminate into eruption in the aftermath of the next large-magnitude earthquake hits a region., Funding was provided by Japan International Cooperation Agency (JICA) and Istitute Nazionale di Geofisica e Vulcanologia (INGV).

This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazú volcanoes in central Costa Rica, manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007-2012) coincides with the pre- and early syn-phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140 years of quiescence. Peculiarly, the generally stable Irazú crater lake dropped its level during this reawakening of Turrialba. The isotopic composition of discharged fluids reveals the Caribbean meteoric origin; a contribution of “andesitic water” for Turrialba fumaroles up to ~50% is suggested. Four groups of thermal springs drain the northern flanks of Turrialba and Irazú volcanoes into two main rivers. Río Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazú, mainly fed by the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazú. All thermal spring waters are of SO4-type (i.e. steam heated waters), although none of the springs has a common hydrothermal end-member. A water mass budget for thermal springs results in an estimated total output flux of 187 ± 37 L/s, with 100 ± 20 L/s accounted for by the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~3,000 m3/y (~2,400 m3/y by San Cayetano-Río Sucio). Despite Irazú being the currently less active volcano, it is a highly efficient rock remover, which, on the long term can have effects on the stability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~260 L/s above pre-eruptive background fumarolic vapor fluxes. This extra vapor loss implies that the drying of the summit hydrothermal system of Turrialba could tap deeper than previously thought, and could explain the coincidental disappearance of Irazú’s crater lake in April 2010.

The critical role of rare earth elements (Lanthanides plus Yttrium; hereafter REE) in high-tech technologies and consequently their increasing demand from the industry, in addition to the capability of REE to trace water–rock interaction processes, boosted the study of REE in unconventional extreme environments. This study is focused on the geochemical behaviour of REE in the hyperacid sulphate-rich brine of the crater lake of Poas volcano (Costa Rica), where the precipitation of gypsum occurs. This system can hence be considered as a natural laboratory to evaluate the fractionation of REE between the lake water (mother brine) and the precipitating gypsum mineral. Total REE concentrations dissolved in waters range from 1.14 to 2.18 mg kg−1. Calculated distribution coefficients (KD) for REE between the gypsum and the mother brine indicate a preferential removal of the light REE (LREE) with respect to the heavy REE (HREE), with KD values mainly decreasing from La to Lu. During the observation period (2007–2009), the distributions of REE concentrations dissolved in lake water normalized to the average local volcanic rock show two different trends: i) LREE depleted patterns, and ii) flat patterns. The identification of the LREE depleted pattern is justified by the KD calculated in this study. We demonstrate that the precipitation of gypsum is able to strongly fractionate the REE in hyperacid sulphate-rich brine, inducing changes in REE concentrations and distributions over time. X-ray computed tomography imaging was performed on gypsum crystal (precipitated from the lake waters) to gain insights on crystal-scale processes possibly controlling the REE geochemistry, i.e. surface processes vs. structural substitution. Accordingly, the heavy metals and possibly the REE seem to be mainly located on the crystal surface rather than inside the crystal, suggesting that a surface process could be the major process controlling REE removal from the water to the crystal.

Since 1978 water chemistry of the Laguna Caliente crater lake has been used to monitor volcanic activity at Poas, Costa Rica, making it arguably the best studied hyper-acidic crater lake on Earth. During these 39 years, three phases of unrest occurred, manifested through frequent phreatic eruptions, with each a duration of several years to over a decade (1978–1980, 1986–1996, 2006–2016). We here present a novel technique to deal with the long time series of the chemical composition of water of Laguna Caliente, independent on previous deterministic research and resulting conceptual models. Common patterns of chemical parameters in relation with phreatic eruptive activity for the period 1978–September 2014 are sought, applying the objective statistical method of Pattern Recognition. This resulted in the definition of the strongest precursory signals and their respective thresholds. Numerical outcomes often confirm findings based on geochemical models (e.g. SO4, SO4/Cl and pH are strong monitoring parameters). However, some surprising parameters (opposite behavior of Mg/Cl ratios, decreases in Ca and Mg concentrations, increasing Al/Mg ratios) still need a geochemical explanation and should be a focus for future research strategies. The obtained parameters and thresholds were retrospectively applied for the “test period” of the Pattern Recognition method (November 2014–February 2016). This test provided hints that suggested that eruptive activity at Poas was not yet over, despite apparent quiescence in early 2016. Indeed, after new phreatic eruptions since May 2016, the 2006–2016 phreatic eruptive cycle culminated in phreatomagmatic activity in April 2017. We conclude that evaluating time series of chemical composition of crater lakes framed in the Pattern Recognition method can be a useful monitoring approach. Moreover, increased sampling frequency can provide more details and more adequate prediction of phreatic activity at Poas. Comparing Laguna Caliente with other two well monitored acidic crater lakes (Ruapehu Crater Lake, New Zealand and Yugama, Kusatsu-Shirane, Japan) Poas results unique in many ways and undoubtedly the most active crater lake of the three during the past four decades.

Poas is a complex stratovolcano with an altitude of 2,708 m a.s.l., located in the Cordillera Volcanica Central of Costa Rica. Prior to 2017, the last three historical eruptions occurred on 7th of February 1834, between January and May 1910 and during the period 1953–1955. Very few information exists on the 1834 eruption. The only references state that: it was an important event; ash reached >53 km W–SW of Poas, and it harmed the grasslands around the volcano. Related deposits of this eruption suggest phreatic activity, which launched bombs and blocks. Moreover, there is evidence of pyroclastic flow deposits near the crater. The 1910 eruption is better described. Despite the fact that ash fall is only reported near the volcano, a volume of the deposit of 1.6 × 107 m3 was estimated. Deposits of the eruption are white in color with many hydrothermally altered, and minor presence of juvenile fragments (vesicular lapilli). The eruption is classified as vulcanian, with deposits of ash fall and pyroclastic flows close to the crater. A Volcano Explosivity Index 3 (VEI 3) is estimated. The eruption affected agriculture. The 1953–1955 eruptions had a longer duration. Various ash fall deposits at several sites were reported. Deposits of this eruption, easily distinguished in the field, are black scoria lapilli, bombs with, sometimes fusiform, bread crust textures. In the eastern sector of the crater bombs can reach meters in size; such large bombs near the eruption centre at one side suggest the inclination of the eruptive conduct, or an asymmetrical vent-crater system. Inside the crater a 40 m-high dome and a lava flow were extruded during the eruption. Towards the eastern side of the current Laguna Caliente crater lake, relicts of a 8.5 m thick lava pool are found. During the entire eruptive episode, the acid lake presumably lacked. The eruption is described to be of a mixed type: strombolian, phreatomagmatic, vulcanian and dome extrusion eruptions. Considering the characteristics of this eruption, the height of the eruption column, ejected volume (2.1 × 107 m3), and its presumed duration, a VEI 3 is estimated. The eruptions damaged agricultural activity (including cattle), and forced the spontaneous evacuation of some people. In April 2017 magmatic eruptions followed a decade-long period of intense phreatic activity. These eruptions destroyed the 1953–1955 Dome and led to the complete dry out of Laguna Caliente. Pyroclastic cones and sulfur volcanism manifested at the bottom of the former crater lake bottom. The 2017 eruption severely affected touristic activities at and near Poas, with an estimated economic loss of 20 million dollars. By May–August 2018 Laguna Caliente reappeared. The volcanic hazards related to the three studied historical eruptions are: pyroclastic flows (at least 1 km from the eruptive centre, including reaching the current mirador sector), ballistics (bomb ejections up to 2 km from the emission centre), dispersion and fall of pyroclasts (tens of kms), gas emission and acid rain, dispersed by WSW dominant winds, and lahars in most of the river canyons SW of the volcano.

© 2019 The Authors, We used the 16S rRNA gene pyrosequencing approach to investigate the microbial diversity and community composition in several Costa Rican hot springs alongside the latitudinal axis of the country, with a range of temperatures (37–63°C), pH (6–7.5) and other geochemical conditions. A principal component analyses of the physicochemical parameters showed the samples were separated into three geochemically distinct habitats associated with the location (North, Central, and South). Cyanobacteria and Chloroflexi comprised 93% of the classified community, the former being the most abundant phylum in all samples except for Rocas Calientes 1, (63°C, pH 6), where Chloroflexi and Deinococcus-Thermus represented 84% of the OTUs. Chloroflexi were more abundant as temperature increased. Proteobacteria, Bacteriodetes and Deinococcus-Thermus comprised 5% of the OTUs represented. Other Phyla were present in very small percentages ( 6.6 from samples with pH < 6.4. Thus, both pH and temperature were relevant for community composition even within the moderate ranges of variables studied. These results provide a basis for an understanding of the physicochemical influences in moderately thermophilic microbial mats., Fundación Costa Rica Estados Unidos para la Cooperación, Grant/Award Number: 6/1-(1E)-10; Vicerrectoría de Investigación, Universidad de Costa Rica, Grant/Award Number: VI-801-B1-542; CRUSA – CSIC, Grant/Award Number: 2010CROO08; Fondo Nacional de Desarrollo Científico y Tecnológico, Grant/Award Number: 1110696 and 1150171; Consejo Superior de Investigaciones Científicas, Grant/ Award Number: COOPB20096; Fondo de Financiamiento de Centros de Investigación en Áreas Prioritarias (Fondap), Grant/Award Number: 15110009 ; Spanish Ministry of Science and Innovation, Grant/Award Number: CTM2016-80095-C2-1-R

The heat and gas released by a degassing magma affects the overlying predominantly meteoric aquifers to form magmatic-hydrothermal systems inside the solid body of a volcano. This chapter reviews how fluid geochemical signals help to track the evolution throughout the various stages of volcanic unrest. A direct view into a degassing magma is possible at open-conduit degassing volcanoes. Nevertheless, in most cases gas is trapped (i.e. scrubbed) by abundant water, leading to the loss of the pure signal the magma ideally provides. Deciphering how magmatic gas rises through, reacts, and re-equilibrates with the liquids in the magmatic-hydrothermal system in time and space is the only way to trace back to the pure signal. The most indicative magmatic gas species (CO2, SO2–H2S, HCl and HF) are released as a function of their solubility in magma. The less soluble gas species are released early from a magma at higher pressure conditions (CO2) (deeper), whereas the more soluble species are released later, at lower pressures (SO2, HCl and HF) (shallower depth). When these gases hit the water during their rise towards the surface, they will be more or less scrubbed. Depending on the chemical equilibria inside the magmatic-hydrothermal system (e.g. SO2–H2S conversion, acidity), the gas that eventually reaches the surface will carry the history of its rise from bottom to top. Tracking volcanic unrest implies a time frame; the kinetics of magma degassing throughout the liquid cocktail inside the volcano impose the maximum resolution the volcano provides and hence the monitoring time window to be adopted for each volcano. Gas-dominated systems are “faster” and require a higher monitoring frequency, water-dominated systems are slower and require a lower monitoring frequency.

As part of the tasks of Red Sismologica Nacional of Costa Rica (RSN, UCR-ICE), volcanic monitoring is a topic that need all our attention. Likewise, the record of the results from this monitoring (field and laboratory work), is a useful tool for the research in short, middle and long term. This tecnic note comprises a synthesis of the results and observations of the last two years of volcanic surveillance at active volcanoes in Costa Rica (Rincon de la Vieja, Arenal, Poas, Barva, Irazu and Turrialba). These results enrich the RSN data base, covering four decades of experience in the implementation of advanced technologies in the area. In the beginning of 2012 all volcanoes manifested background behavior, representing low levels of phreatic and seismic activity. In September 2012, an increase in volcanic activity occurred following the Samara earthquake (Mw=7,6). This increased unrest was manifested mainly through the increase in temperature of fumaroles, some changes in crater lake dynamics and chemistry, increased seismic and phreatic activity. These higher activity conditions were maintained during the rest of the period.