The effect of three large Mw¿7.3 subduction earth-quakes (August-November 2012) on volcanic unrest in Central America

¿Was the volcanic eruption triggered by the earthquake?¿ The answer to this question usually is ¿maybe¿ or ¿a coincidence¿. A region like Central America, is an adequate area to find hints to answer this question because have the necessary ingredients: the frequent occurrence of large earthquakes (M5+) and dozens of active volcanoes. This research focuses on whether the uncommon occurrence of three large earthquakes in the subduction zone of Central America, within a time span of ten weeks in 2012, promoted enhanced volcanic activity. The time window analyzed is from 2000 to 2019, which includes a total of 50 volcanic eruptions with a VEI¿2. Before the 2012 earthquakes, 22 eruptions occurred. The Monte Carlo statistical simulation method allowed to demonstrate that this increase in the number of volcanic eruptions after the three large earthquakes of 2012 it is not a temporal coincidence. We analyzed the characteristics of each earthquake and described how they could disturb the volcanic systems. Although Central America hosts 24 volcanoes with historical eruptions, only 11 of them erupted after the 2012 earthquakes. Why did only these volcanoes erupt? To answer this question, we calculated the dynamic and static stress in each volcano and the level of volcanic unrest (the change in volcanic activity beyond background behavior to worrisome levels) prior to the earthquakes. We found that volcanoes in a unrest stage before the earthquakes but, without experiencing explosive eruptions before, erupted after receiving the seismic shocks. This fact suggests that the earthquakes by themselves did not transfer enough energy to generate the volcanic eruptions when volcanoes were not ready to erupt. However, earthquakes could promote volcanic eruptions when volcanoes were already at unrest. This research offers a tool for forecasting volcanic activity when a large earthquake hits a region, if the volcanic activity is previously monitored

Ground Penetrating Radar Imaging of Tephra Stratigraphy on Poás and Irazú Volcanoes, Costa Rica

Ground penetrating radar (GPR) has been shown to be a useful tool for mapping geometry and thicknesses for volcanic fall, surge, granular flow, and lahar deposits. However, the success of GPR surveys is highly dependent on soil properties and the nature of stratigraphic layering. The efficacy of the method at a given site can be difficult to predict. Small-scale (10s to 100s of meters) test surveys with ground penetrating radar show that geologic features of interest can be resolved to depths up to 20 m on both Poás and Irazú volcanoes in Costa Rica. The antenna frequencies used in this pilot study, 50 MHz, 100 MHz, and 200 MHz, produce wavelengths too long to resolve most individual layers (mm’s to cm’s thick) in the near-surface tephra fall and surge deposits. However, these GPR profiles clearly show the attitude of beds and resolve some distinct contacts at depth, particularly the base of the 1963-1965 intracrater deposits on Irazú. On Irazú GPR profiles also confirm field observations that 1963 surge deposits thin consistently with distance from the crater rim, while packages from 1723 and older show uniform thicknesses or increasing thickness with distance from crater rim, suggesting reworking or reverse flow of surges returning from the Playa Hermosa caldera wall. On Poás, bright reflectors are present at depths below 2 m on near-vent profiles, but the lack of nearby stratigraphic observations precludes geological interpretation. The test profiles on both volcanoes also clearly show diffractions produced by blocks on the order to 5 cm or more in diameter embedded in the surficial deposits, as well as evidence of sag beneath blocks. Resolution of blocks decreases with depth, presumably due to both the inherent loss in lateral resolution with wave travel distance and to clearly observed dispersion (loss of high frequencies). Future studies, particularly with higher frequency antennas on Poás, could be useful for tracking depositional units between exposures, and for resolving the distribution of blocks or bombs in deposits in the uppermost few meters

Past, present and future of volcanic lake monitoring

International audienceVolcanic lake research boosted after lethal gas burst occurred at Lake Nyos (Cameroon) in 1986, a limnic rather than a volcanic event. This led to the foundation of the IAVCEI-Commission on Volcanic Lakes, which grew out into a multi-disciplinary scientific community since the 1990s. We here introduce the first data base of volcanic lakes VOLADA, containing 474 lakes, a number that, in our opinion, is surprisingly high. VOLADA could become an interactive, open-access working tool where our community can rely on in the future. Many of the compiled lakes were almost unknown, or at least unstudied to date, whereas there are acidic crater lakes topping active magmatic–hydrothermal systems that are continuously or discontinuously monitored, providing useful information for volcanic surveillance (e.g., Ruapehu, Yugama, Poás). Nyos-type lakes, i.e. those hosted in quiescent volcanoes and characterized by significant gas accumulation in bottom waters, are potentially hazardous. These lakes tend to remain stably stratified in tropical and sub-tropical climates (meromictic), leading to long-term build-up of gas, which can be released after a trigger. Some of the unstudied lakes are possibly in the latter situation. Acidic crater lakes are easily recognized as active, whereas Nyos-type lakes can only be recognized as potentially hazardous if bottom waters are investigated, a less obvious operation. In this review, research strategies are lined out, especially for the “active crater lakes”. We make suggestions for monitoring frequency based on the principle of the “residence time dependent monitoring time window”. A complementary, multi-disciplinary (geochemistry, geophysics, limnology, statistics) approach is considered to provide new ideas, which can be the bases for future volcanic lake monitoring. More profound deterministic knowledge (e.g., precursory signals for phreatic eruptions, or lake roll-over events) should not only serve to enhance conceptual models of single lakes, but also serve as input parameters in probabilistic approaches. After more than 25 years of pioneering studies on rather few lakes (~ 20% of all), the scientific community should be challenged to study the many poorly studied volcanic lakes, in order to better constrain the related hazards

REE fractionation during the gypsum crystallization in hyperacid sulphate-rich brine: The Poás Volcano crater lake (Costa Rica) exploited as laboratory

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 Poás 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.Published87-963V. Proprietà dei magmi e dei prodotti vulcanici2IT. Laboratori sperimentali e analiticiJCR Journa

Behaviour of polythionates in the acid lake of poás volcano: Insights into changes in the magmatic-hydrothermal regime and subaqueous input of volatiles

In this chapter, we document an extensive record of concentrations and speciation of polythionates (PTs: S4O6 2−, S5O6 2−, and S6O6 2−), which form in the warm (21–60 °C) and hyper-acidic (pH < 1.8) waters of the crater lake of Poás volcano (Costa Rica) through interaction with gaseous SO2 and H2S of magmatic origin. Our data set, together with earlier published results, covers the period 1980–2006 during which lake properties and behavior were marked by significant variations. Distinct stages of activity can be defined when combining PT distributions with geochemical, geophysical and field observations. Between 1985 and mid-1987, when fumarolic outgassing was centered on-shore, the total concentration of PTs in the lake was consistently high (up to 4,200 mg/kg). Mid-1987 was the start of a 7-year period of vigorous fumarolic activity with intermittent phreatic eruptions from the lake, which then dried out. Concentrations of PTs remained below or close to detection limits throughout this period. After mid-1994, when a new lake formed and fumarolic outgassing shifted to the dome, the total PT concentrations returned to relatively stable intermediate levels (up to 2,800 mg/kg) marking more quiescent conditions. Since early 1995, numerous weak fumarole vents started, opening up at several other locations in the crater area. During short intervals (November 2001–May 2002 and October 2003–March 2005), PTs virtually disappeared. After April 2005, PTs re-appeared in large amounts (up to more than 3,000 mg/kg) until February 2006, one month before the onset of the March 2006–2017 cycle of phreatic eruptions, when concentrations dropped and remained below 100 mg/kg. The observed behavior of PTs records changes in the input and SO2/H2S ratios of subaqueous fumaroles. The prevailing distribution of PTs is S4O6 2− > S5O6 2− > S6O6 2−, which is common for periods when total PT concentrations and SO2/H2S ratios of the gas influx into the lake are relatively high. PTs are virtually absent as a consequence of thermal or sulphitolytic breakdown during periods of strong fumarolic outgassing in response to shallow intrusion of fresh magma or fracturing of the solid envelope around a pre-existing body of cooling magma. They are also low in abundance or undetected during quiescent periods when subaqueous fumarolic output is weak and has low SO2/H2S ratios, resulting in a concentration sequence S5O6 2− > S4O6 2− > S6O6 2−. The onset of phreatic eruptions are preceded by an increase in PT concentrations, accompanied by a change in the dominance from penta- to tetrathionate, and followed by a sharp drop in total PT content, up to several months before. Periods of phreatic eruptive activity that started in 1987 and 2006 followed these PT signals of increased input of sulfur-rich gas, in both cases possibly in response to shallow emplacement of fresh magma or hydrofracturing

Vertical profiles (in mg L⁻¹) of HCO₃⁻, NO₃⁻, SO₄²⁻, NH₄⁺, Fe_tot and Mn in Lake Hule (a) and Lake Río Cuarto (b).

<p>Vertical profiles (in mg L⁻¹) of HCO₃⁻, NO₃⁻, SO₄²⁻, NH₄⁺, Fe_tot and Mn in Lake Hule (a) and Lake Río Cuarto (b).</p

Trace elements composition of water samples collected.

<p>Chemical concentrations are in µg L⁻¹. n.a.: not analyzed.</p

Library coverage estimations and sequence diversity of 16S rRNA.

<p>*Library coverage was calculated as C = 1-n/N, where n is the number of OTU₉₇ without a replicate, and N is the total number of sequences.</p><p>**Shannon diversity index calculated using PAST.</p

Chemical composition (µmol L⁻¹) and total pressure (pTOT; in atm) of dissolved gases (CO₂, N₂, CH₄, Ar, O₂, Ne, H₂ and He) and δ¹³C-CO₂ (expressed as ‰ V-PDB), δ¹³C-CH₄ (expressed as ‰ V-PDB), δD-CH₄ (expressed as ‰ V-SMOW) and R/Ra values of gas samples collected.

<p>Dissolved gas concentrations are in µmol L⁻¹. n.a.: not analyzed; n.d.: not detected.</p