Sierra Negra Volcano Research Articles

Seismo‐Acoustic Characterization of the 2018 Sierra Negra Caldera Resurgence and Fissure Eruption in the Galápagos Islands

AbstractThe 2018 eruption of Sierra Negra volcano, Galápagos, Ecuador has provided new insights into the mechanisms of caldera resurgence, subsidence, and fissuring at basaltic shield volcanoes. Here, we integrate local (∼0.4 km) seismo‐acoustic records and regional (∼85 km) infrasound array data to present new observations of the 2018 Sierra Negra eruption with improved time and spatial resolutions. These observations include: air‐to‐ground coupling ∼2 hr before the time of the eruption onset, migration of the infrasound tremor from 22:54 June 26 to 12:31 June 27 UT (all times in UT), and persistent infrasound detections during the weeks between 5 July and 18 August from an area that does not coincide with the previously documented eruptive fissures. We interpret air‐to‐ground coupling as infrasound tremor generated in the nearby fissures before the main eruptive phase started, although ambiguity remains in interpreting a single seismic‐infrasonic sensor pair. The progressive location change of the infrasound tremor agrees with the migration of the eruption down the north flank of Sierra Negra at a rate of ∼0.15 ± 0.04 m/s. The weeks‐long persistent detections coincide with a region that has thermal anomalies, co‐eruptive deformation, lava fields, and geological features that could be interpreted as multiple lava tube skylights. Our observations and interpretations provide constraints on the mechanisms underlying fissure formation and magma emplacement at Sierra Negra.

Performance of the HAWC Observatory and TeV Gamma-Ray Measurements of the Crab Nebula with Improved Extensive Air Shower Reconstruction Algorithms

The High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, located on the side of the Sierra Negra volcano in Mexico, has been fully operational since 2015. The HAWC collaboration has recently significantly improved their extensive air shower reconstruction algorithms, which has notably advanced the observatory performance. The energy resolution for primary gamma rays with energies below 1 TeV was improved by including a noise-suppression algorithm. Corrections have also been made to systematic errors in direction fitting related to the detector and shower plane inclinations, O(0.°1) biases in highly inclined showers, and enhancements to the core reconstruction. The angular resolution for gamma rays approaching the HAWC array from large zenith angles (>37°) has improved by a factor of 4 at the highest energies (>70 TeV) as compared to previous reconstructions. The inclusion of a lateral distribution function fit to the extensive air shower footprint on the array to separate gamma-ray primaries from cosmic-ray ones based on the resulting χ 2 values improved the background rejection performance at all inclinations. At large zenith angles, the improvement in significance is a factor of 4 compared to previous HAWC publications. These enhancements have been verified by observing the Crab Nebula, which is an overhead source for the HAWC Observatory. We show that the sensitivity to Crab-like point sources (E −2.63) with locations overhead to 30° zenith is comparable to or less than 10% of the Crab Nebula’s flux between 2 and 50 TeV. Thanks to these improvements, HAWC can now detect more sources, including the Galactic center.

Intrusive mechanism of the 2018 Sierra Negra Galápagos eruption, constrained by 4D tomographic images

Understanding the mechanisms of magma intrusion underpins our ability to interpret geophysical monitoring signals at volcanoes and thus issue reliable forecasts of future activity. The basaltic caldera volcanoes of the western Galápagos Islands, Ecuador, exhibit exceptionally high rates of deformation driven by shallow magma accumulation and migration. However, the nature and evolution of magma storage at these volcanoes is poorly constrained by earthquake hypocentre locations or geodetic inversions. Here we show that transient variations in seismic velocity before, during, and after the 2018 eruption of the Sierra Negra volcano track the accumulation of magma in a shallow sill complex and the emplacement of a lateral flank intrusion. A four-dimensional tomographic technique applied to the P-wave arrivals of local earthquakes provides high spatiotemporal resolution of changes in the physical properties of the shallow volcanic system. In the month before the eruption, the expansion of a low-velocity zone above the sub-caldera sill complex coincides with caldera uplift and near-surface fracturing, driven by persistent shallow magma accumulation. A new low-velocity anomaly appeared progressively in the western flank of the volcano in the days after the onset of the eruption, coinciding with the opening of a curved sill that supplied magma to secondary eruptive fissures. The anomaly disappeared as the curved sill deflated after the initial opening, despite it remaining the conduit for magma from the caldera complex to the flank fissures. Low velocities across the shallow caldera after the end of the eruption likely result from rapid inflation due to recharge with fresh magma from depth. These results indicate a previously unknown complexity to the magmatic plumbing system at Sierra Negra and suggest velocity changes resulting from an interplay of thermal and stress perturbations.

The High-Altitude Water Cherenkov (HAWC) observatory in México: The primary detector

The High-Altitude Water Cherenkov (HAWC) observatory is a second-generation continuously operated, wide field-of-view, TeV gamma-ray observatory. The HAWC observatory and its analysis techniques build on experience of the Milagro experiment in using ground-based water Cherenkov detectors for gamma-ray astronomy. HAWC is located on the Sierra Negra volcano in México at an elevation of 4100 meters above sea level. The completed HAWC observatory principal detector (HAWC) consists of 300 closely spaced water Cherenkov detectors, each equipped with four photomultiplier tubes to provide timing and charge information to reconstruct the extensive air shower energy and arrival direction. The HAWC observatory has been optimized to observe transient and steady emission from sources of gamma rays within an energy range from several hundred GeV to several hundred TeV. However, most of the air showers detected are initiated by cosmic rays, allowing studies of cosmic rays also to be performed. This paper describes the characteristics of the HAWC main array and its hardware.

Current motion and deformation of the Nazca Plate: new constraints from GPS measurements

SUMMARYWe use new GPS data to determine an updated Euler pole describing the present-day motion of the oceanic Nazca Plate. Our solution includes continuous GPS (cGPS) measurements at Malpelo Island offshore Colombia, two sites in the Galapagos archipelago, Easter Island and Salas y Gomez Island in the western part of the plate and Robinson Crusoe Island offshore Chile. A careful analysis of geodetic time-series reveals that (1) previous estimates using former cGPS site EISL are biased by several millimetres per year eastward due to station malfunctioning (2) north velocity component of cGPS site GLPS at Santa Cruz Island in the Galapagos is impacted by volcanic deformation at the 1–2 mm yr–1 level, probably caused by the recurrent volcanic activity of the Sierra Negra volcano. In addition, we find shortening at ∼1 mm yr–1 between Easter Island (cGPS ISPA) and Salas y Gomez Island (cGPS ILSG), consistent with the elastic deformation induced by rapid opening at the East Pacific rise. cGPS site at Robinson Crusoe Island shows ∼4–5 mm yr–1 abnormally fast East velocity induced by the visco-elastic relaxation following the Maule Mw 8.8 2010 earthquake. Using this information, we determine a new Euler pole (longitude: –90.93°E, latitude 56.19°N, 0.588 deg Myr–1) describing the present-day Nazca–South America Plate motion, using five sites (Malpelo Island, two sites in the Galapagos archipelago, Easter Island and Salas y Gomez Island). The proposed Euler pole provides a weighted root mean square (wrms) of residual velocities of 0.6 mm yr–1, slightly higher than usually observed for other major tectonic plates and accounting for the uncertainty of potential volcanic–tectonic deformation. Our model predicts a maximum convergence rate at 65.5 ± 0.8 mm yr–1 at latitude ∼30°S along the Chile trench, decreasing to 50.8 ± 0.7 mm yr–1 in northern Colombia and 64.5 ± 0.9 mm yr–1 in southern Chile (1σ confidence level). Comparison with the geological models NUVEL1A and MORVEL indicates constant decrease since 3.16 Ma of opening rate along the Nazca–Antarctic Plate boundary spreading centres at ∼1 cm yr–1 per Myr. Combined with the ITRF2014 pole for the Pacific and Antarctic plates, our derived Euler pole predicts closure at the ∼1 mm yr–1 level for Pacific–Antarctic–Nazca Plate circuit. However, combining our results with MORVEL estimates for the Cocos Plate, the non-closure of the Pacific–Cocos–Nazca Plate circuit is 9.7 ± 1.6 mm yr–1, 30 per cent lower than the 14 ± 5 mm yr–1 reported in MORVEL model, but still significant. A small (∼1.5 mm yr–1) velocity residual at Malpelo Island neither supports the hypothesis of an independent Malpelo microplate offshore Colombia nor large scale internal deformation induced by thermal contraction. Our solution rather suggests that non-closure of the Pacific–Cocos–Nazca Plate circuit arises from the determination of the Cocos Plate motion in MORVEL, an hypothesis further supported by the large discrepancy between MORVEL’s prediction and the observed GPS velocity observed at Cocos Islands (cGPS ISCO).

Forecasting mechanical failure and the 26 June 2018 eruption of Sierra Negra Volcano, Galápagos, Ecuador.

Using recent advancements in high-performance computing data assimilation to combine satellite InSAR data with numerical models, the prolonged unrest of the Sierra Negra volcano in the Galápagos was tracked to provide a fortuitous, but successful, forecast 5 months in advance of the 26 June 2018 eruption. Subsequent numerical simulations reveal that the evolution of the stress state in the host rock surrounding the Sierra Negra magma system likely controlled eruption timing. While changes in magma reservoir pressure remained modest (<15 MPa), modeled widespread Mohr-Coulomb failure is coincident with the timing of the 26 June 2018 moment magnitude 5.4 earthquake and subsequent eruption. Coulomb stress transfer models suggest that the faulting event triggered the 2018 eruption by encouraging tensile failure along the northern portion of the caldera. These findings provide a critical framework for understanding Sierra Negra’s eruption cycles and evaluating the potential and timing of future eruptions.

Seismic tremor reveals slow fracture propagation prior to the 2018 eruption at Sierra Negra volcano, Galápagos

Seismic tremor observed near active volcanoes is an important tool for volcano monitoring as it often appears shortly before eruptions. Although tremor can be generated by a variety of physical processes it is usually interpreted as direct evidence for flowing magma in the sub-surface. These interpretations typically feed into risk assessments for potential eruptions. Using the temporal evolution of tremor amplitude and spectral data from a distributed seismic network that captured the 2018 eruption at Sierra Negra in Galápagos, we determine that tremor is not directly related to sub-surface fluid movement. Instead at Sierra Negra tremor likely indicates a slowly propagating fracture, which is later exploited as a pathway for silent magma flow. Distinct differences in the source migration and the spectral character of pre-eruptive and co-eruptive tremor allow both a location estimate of the future eruption site and a precise timing of the eruption onset.

Constraints on Absolute Magma Chamber Volume From Geodetic Measurements of Trapdoor Faulting at Sierra Negra Volcano, Galapagos

AbstractMagma chamber volume is critical for volcano hazards assessment and forecasting. Standard geodetic methods constrain volume change, not the total volume. Here, we show that the deformation response of the magma chamber to trapdoor faulting events at Sierra Negra volcano, Galapagos, depends on the product of the absolute chamber volume and the magma compressibility. Bubble‐free magma provides the lower limit on compressibility, thus an upper bound on the chamber volume of 13.6–20.6 km3, depending on fault dip. We estimate an upper limit on compressibility using a conduit model relating volatile content to lava fountain height, compared with observations from the 2005 eruption, constrained by volatile content of olivine melt inclusions. This yields a lower bound on chamber volume roughly half the upper bound. We find that the best fitting trapdoor fault is near‐vertical; reverse dips are slightly favored (88°).

Gas Leakage From Shallow Ponding Magma and Trapdoor Faulting at Sierra Negra Volcano (Isabela Island, Galápagos)

AbstractWe report on new volcanic gas composition results acquired in October 2017 at Minas de Azufre, a persistent fumarolic field topping the resurgent Sierra Negra caldera, in the Galápagos archipelago. Our results indicate that the Minas de Azufre fumaroles are moderately hydrous (52–64 mol.% H2O) and rich in CO2 (35–46 mol.%), with total sulfur (ST) being 21–35 times less abundant than CO2. SO2, the most abundant S species, is released at an average rate of 19 ± 9 tons/day. Using a volatile saturation model that provides the composition of magmatic gases at equilibrium with western Galápagos basaltic melt (48 wt. % SiO2) in the 400–0.1 MPa pressure range, we infer that Minas de Azufre fumarolic emissions consist of a mixture of (a) magma‐derived gases coexisting with a melt at ∼50–60 MPa and (b) shallow meteoric water. We thus propose that the fumaroles are supplied by outgassing of magma stored in a ∼2 km deep sill‐like reservoir underneath the caldera floor, and that the trapdoor fault system at the western margin of the resurgent caldera block acts as a preferential pathway for magmatic gas ascent and surface discharge. Our results thus suggest that, in contrast to the majority of the volcano‐hosted hydrothermal systems worldwide, Minas de Azufre releases a relatively pristine magmatic gas.

Dynamic earthquake triggering response tracks evolving unrest at Sierra Negra volcano, Galápagos Islands.

The propensity for dynamic earthquake triggering is thought to depend on the local stress state and amplitude of the stress perturbation. However, the nature of this dependency has not been confirmed within a single crustal volume. Here, we show that at Sierra Negra volcano, Galápagos Islands, the intensity of dynamically triggered earthquakes increased as inflation of a magma reservoir elevated the stress state. The perturbation of short-term seismicity within teleseismic surface waves also increased with peak dynamic strain. Following rapid coeruptive subsidence and reduction in stress and background seismicity rates, equivalent dynamic strains no longer triggered detectable seismicity. These findings offer direct constraints on the primary controls on dynamic triggering and suggest that the response to dynamic stresses may help constrain the evolution of volcanic unrest.

Mapping and classification of volcanic deposits using multi-sensor unoccupied aerial systems

The deposits from volcanic eruptions represent the record of activity at a volcano. Identification, classification, and interpretation of these deposits are crucial to the understanding of volcanic processes and assessing hazards. However, deposits often cover large areas and can be difficult or dangerous to access, making field mapping hazardous and time-consuming. Remote sensing techniques are often used to map and identify the deposits of volcanic eruptions, though these techniques present their own trade-offs in terms of image resolution, wavelength, and observation frequency. Here, we present a new approach for mapping and classifying volcanic deposits using a multi-sensor unoccupied aerial system (UAS) and demonstrate its application on lava and tephra deposits associated with the 2018 eruption of Sierra Negra volcano (Galápagos Archipelago, Ecuador). We surveyed the study area and collected visible and thermal infrared (TIR) images. We used structure-from-motion photogrammetry to create a digital elevation model (DEM) from the visual images and calculated the solar heating rate of the surface from temperature maps based on the TIR images. We find that the solar heating rate is highest for tephra deposits and lowest for ʻaʻā lava, with pāhoehoe lava having intermediate values. This is consistent with the solar heating rate correlating to the density and particle size of the surface. The solar heating rate for the lava flow also decreases with increasing distance from the vent, consistent with an increase in density as the lava degasses. We combined the surface roughness (calculated from the DEM) and the solar heating rate of the surface to remotely classify tephra deposits and different lava morphologies. We applied both supervised and unsupervised machine learning algorithms. A supervised classification method can replicate the manual classification while the unsupervised method can identify major surface units with no ground truth information. These methods allow for remote mapping and classification at high spatial resolution (< 1 m) of a variety of volcanic deposits, with potential for application to deposits from other processes (e.g., fluvial, glacial) and deposits on other planetary bodies.

Origin of color variations of thin, nano-sized layers of volcanic cinder from the Sierra Negra Volcano of the Galapagos Islands

Volcanic cinder, also known as scoria, is an extrusive igneous rock that forms when gas-rich magmas of basaltic or andesitic composition cool quickly. It is typically dark in color, ranging from black to red depending on its chemical composition. Sometimes fresh cinder samples show a variety of shiny metallic colors on its surface ranging from blue to gold to silver. The origin of these colors has remained unknown up to now. Cinder samples from an eruptive event occurred in October 2005 have been collected in the surroundings of the Sierra Negra volcano in the Galápagos Islands. The samples’ crystallographic structure, chemical composition, and surface morphology have been analyzed using X-Ray diffractometry (XRD), energy dispersive X-Ray spectroscopy (EDS) and a field gun emission scanning electron microscopy (SEM), respectively. Based on an extensive physical and chemical analysis, we were able to demonstrate that these colors are due to a light interference phenomenon. These results have a great potential to be used for a wide variety of purposes such as determining the temperature and composition of magma and evaluating volcanic samples for planetary studies.

Uplift and Seismicity Driven by Magmatic Inflation at Sierra Negra Volcano, Galápagos Islands

AbstractAlthough episodes of surface uplift and elevated seismicity precede many volcanic eruptions, their temporal evolution is often complex, and apparently in contradiction to simple trends predicted by mechanical deformation models. Here, we use continuous global positioning system and seismic data recorded at Sierra Negra volcano, Galápagos Islands, to show how the edifice responded to stress changes driven by magma accumulation in a shallow sill. The rate of uplift varied during the 13 years and 6.5 m of inflation before the 2018 eruption. The number of earthquakes per unit of uplift increased exponentially with total uplift as the differential stress increased. Accordingly, the temporal seismicity rate varied in time as a function of both the total uplift and the uplift rate. The Gutenberg‐Richter b‐value decreased as a function of total uplift. In the final six months before the eruption, a sequence of large (M &gt; 4) earthquakes regulated the state of stress on the fault, each being followed by 2–3 days of postseismic quiescence, and retarding the increase in seismicity rate. These earthquakes did not affect the overall uplift rate. Subsidence of 8.5 m accompanied the 2‐month eruption. On resumption of uplift, the number of earthquakes per unit of uplift was very low, and the b‐value high, reflecting the relaxed stress state of the fault system. These observations show that crustal deformation becomes increasingly brittle at higher stress states, and supports theoretical models based on elastic‐brittle mechanics. They suggest that joint interpretation of deformation and seismicity is key for forecasting future eruptions in similar volcanic settings.

Local dew-point temperature, water vapor pressure, and millimeter-wavelength opacity at the Sierra Negra volcano

Aims. Some astronomical facilities are in operation at the Sierra Negra volcano (SNV), at ∼4.5 km over the sea level (o.s.l.) in Mexico. We asses whether it is possible to estimate the opacity for millimeter-wavelength observations based on the meteorological parameters at the site. A criterion for allowing astronomical observations at SNV depends on the atmospheric opacity at 225 GHz, which has to be τ225 ≤ 0.30 Nepers. The correlation of the opacity at SNV, measured with a radiometer at 225 GHz, τ225, with the local dew point temperature, TDP, the water vapor pressure, PH2O and the water vapor content (WVC) at SNV is studied with the aim to determine whether these parameters can be used to estimate the opacity at similar high-altitude locations for astronomical observations at millimeter wavelengths. Methods. We used radiosonde data taken in various decades in Mexico City (MX) and Veracruz City (VR) to compute the WVC in 0.5 km altitude (h) intervals from 0 km for VR and from 2.0 km for MX to 9.5 km o.s.l. to study the altitude profile WVC(h) at SNV by interpolating data of MX and VR. We also fit exponential functions to observed WVC (WVCobs(h)), obtaining a fit WVC (WVCftd(h)). The WVCobs(h) and WVCftd(h) were integrated, from lower limits of hlow = 2.5–5.5 km to the upper limit of 9.5 km as a measure of the input of WVCobs(h ≥ hlow) to the precipitable water vapor. Results. The largest differences between WVCobs and WVCftd values occur at low altitudes. The input of WVCobs(h) to the precitpitable water vapor for h ≥ 4.5 km ranges from 15% to 29%. At 4.5–5.0 km, the input is between 4% and 8%. This means that it is about a third of the WVC (h ≥ 4.5 km). The input above our limit (from 9.5–30.0 km) is estimated with WVCftd(h) and is found to be lower than 1%. The correlation of τ225 with TDP, PH2O, and WVCSNV takes values between 0.6 and 0.8. A functional relation is proposed based on simultaneous data taken in 2013–2015, according to which it is possible to estimate the opacity with the TDP, PH2O, or WVCSNV at the site. Conclusions. With local meteorological parameters, it is possible to know whether the opacity meets the condition τ225 ≤ 0.30 Nepers, with an uncertainty of ±0.16 Nepers. The uncertainty is low for low opacities and increases with increasing opacity.

New insights into the genesis of pyroducts of The Galápagos islands, Ecuador

There has been little research on the genesis and development of pyroducts (or lava tubes) originating from Gala´pagos volcanoes. Pyroducts are responsible for the lateral, post-eruptive transport of lava because they are highly effective as thermal insulators. After eruptions terminate, these conduits often become accessible as caves. In March 2014 the 16th International Symposium on Vulcanospeleology brought a large group of vulcanospeleological specialists to the Islands. During the meeting a number of pyroducts were visited and studied in context on the island of Santa Cruz and around Isabella’s Sierra Negra volcano in the western, most active, part of the Gala´pagos. The longest of the caves, Cueva del Cascajo, about 3 km in length, was partly surveyed and nine other caves were visited. Structural features such as thickness of roof, evidence of downcutting, presence of oxbows, secondary ceilings, lavafalls, collapses and pukas were particularly studied for evidence they reveal about developmental stages of pyroducts. The resulting data show that the pyroducts were formed by "inflation" with the primary roof consisting of uninterrupted paŻhoehoe sheets. No pyroducts were identified that developed by the crusting-over of channels. The studies strongly confirm inferences drawn from other hot-spot related islands, such as Hawai’i.

Caldera resurgence during the 2018 eruption of Sierra Negra volcano, Gal\xe1pagos Islands

Recent large basaltic eruptions began after only minor surface uplift and seismicity, and resulted in caldera subsidence. In contrast, some eruptions at Galápagos Island volcanoes are preceded by prolonged, large amplitude uplift and elevated seismicity. These systems also display long-term intra-caldera uplift, or resurgence. However, a scarcity of observations has obscured the mechanisms underpinning such behaviour. Here we combine a unique multiparametric dataset to show how the 2018 eruption of Sierra Negra contributed to caldera resurgence. Magma supply to a shallow reservoir drove 6.5 m of pre-eruptive uplift and seismicity over thirteen years, including an Mw5.4 earthquake that triggered the eruption. Although co-eruptive magma withdrawal resulted in 8.5 m of subsidence, net uplift of the inner-caldera on a trapdoor fault resulted in 1.5 m of permanent resurgence. These observations reveal the importance of intra-caldera faulting in affecting resurgence, and the mechanisms of eruption in the absence of well-developed rift systems.

Simulation and experimental validation of optimum read-out electronics design for scintillator bar cosmic ray telescope

This paper presents a simulation/experimental technique used in the design of optimum performance read-out electronics for a new cosmic ray telescope composed of scintillator bars. This new detector, called SciBar Cosmic Ray Telescope (SciCRT), is installed on top of Sierra Negra volcano in Mexico, operating at an atmospheric depth of 575gcm−2. The severe atmospheric conditions and high background rate of the place currently limit the performance of the detector, therefore the requirements of our design are low power consumption and high throughput rate. The simulation developed considers the interaction of high energy cosmic ray muons with the scintillator bar, generation and propagation of photons inside a wavelength shifting fiber (WLS) fiber and the detection by a multi-anode photomultiplier (MAPMT). To produce realistic signals coming from the interaction of particles, our method relies on the extraction of different parameters throughout experiment to include them in the simulation. In addition, the paper presents a muon cosmic ray experiment designed to validate the results of the simulation and test the performance of the new electronics.

Probabilistic retrieval of volcanic SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; layer height and partial column density using the Cross-track Infrared Sounder (CrIS)

Abstract. During most volcanic eruptions and many periods of volcanic unrest, detectable quantities of sulfur dioxide (SO2) are injected into the atmosphere at a wide range of altitudes, from ground level to the lower stratosphere. Because the fine ash fraction of a volcanic plume is, at times, colocated with SO2 emissions, global tracking of volcanic SO2 is useful in tracking the hazard long after ash detection becomes dominated by noise. Typically, retrievals of SO2 vertical column density (VCD) have relied heavily on hyperspectral ultraviolet measurements. More recently, infrared sounders have provided additional VCD measurements and estimates of the SO2 layer altitude, adding significant value to real-time monitoring of volcanic emissions and climatological analyses. These methods can provide fast and accurate physics-based retrievals of VCD and altitude without regard to solar irradiance, meaning that they are effective day and night and can observe high-latitude SO2 even in the winter. In this study, we detail a probabilistic enhancement of an infrared SO2 retrieval method, based on a modified trace gas retrieval, to estimate SO2 VCD and altitude probabilistically using the Cross-track Infrared Sounder (CrIS) on the Joint Polar Satellite System (JPSS) series of satellites. The methodology requires the characterization of real SO2-free spectra aggregated seasonally and spatially. The probabilistic approach replaces altitude and VCD estimates with probability density functions for the layer height and the partial VCD at multiple heights, fully quantifying the retrieval uncertainty and allowing the estimation of SO2 partitioning by layer. This framework adds significant value over basic VCD and altitude retrieval because it can be used to assign probabilities of SO2 occurrence to different atmospheric intervals. We highlight analyses of several recent significant eruptions, including the 22 June 2019 eruption of Raikoke volcano, in the Kuril Islands; the mid-December 2016 eruption of Bogoslof volcano, in the Aleutian Islands; and the 26 June 2018 eruption of Sierra Negra volcano, in the Galapagos Islands. This retrieval method is currently being implemented in the VOLcanic Cloud Analysis Toolkit (VOLCAT), where it will be used to generate additional cloud object properties for real-time detection, probabilistic characterization, and tracking of volcanic clouds in support of aviation safety.

The conservation status of the Galapagos MartinProgne modesta: Assessment of historical records and results of recent surveys

SummaryThe endemic Galapagos MartinProgne modestais a rare species. Population sizes in the past are unknown, but the few reports available suggest that it has become rarer in some sites. To obtain a better understanding of its population status, a simultaneous survey was conducted in 2017 around the coastline of 14 islands and 23 islets resulting in sightings of only 26 individuals. A simultaneous survey on Sierra Negra volcano on Isabela Island added five more individuals. However, observations from early expeditions and expeditions since 2015 have shown that the volcanoes of Isabela Island are important feeding areas and probable breeding areas for the Galapagos Martin, but these were largely excluded from the survey in 2017 due to logistical reasons. Historical and recent nesting sites include Tagus Cove on Isabela Island and Daphne Major Island. During our 2017 count, three possible new nesting sites were discovered: Daphne Minor, Santiago and a small crater on Punta Cristóbal on southwestern Isabela Island. From 2015 to 2018, 15 nests were also observed on the cliff in Tagus Cove. Two accessible nests were collected and revealed the first records ofPhilornis downsiin Galapagos Martin nests. The impact of parasitism byP. downsiis unknown but potentially severe. Given the lack of general knowledge about the biology of this species and its apparent low population sizes, further studies are urgently needed.

Neutron and gamma-ray fluxes measured by SciCRT prototype at the top of Sierra Negra volcano, Mexico

The mini-SciCR is a cosmic ray detector, it is made of scintillator bars with a total volume of 20 x 20 x 20.8 $cm^{3}$. The array of scintillator bars act both as a target and as a tracker of the incident radiation. In this paper we describe the method developed with the help of a Monte Carlo simulation to distiguish the neutron signals from gamma ray signals, it is based on the different maximum energy deposited at a scintillator bar by neutrons and gamma rays. To distiguish the neutral emission signals (neutrons and gamma rays) from charged particles signals, we implemented via software a system of anti-coincidence between edge bars and internal bars of the detector. We also report the flux of neutrons and gamma rays measured by the mini-SciCR at the top of the Sierra Negra volcano at 4,600 m.a.s.l., in Eastern Mexico. The mini-SciCR was operating from October 2010 to July 2012. We also present the Forbush decrease registered by the mini-SciCR on march, 2012. The mini-SciCR is a prototype of a new cosmic ray detector called SciBar Cosmic Ray Telescope installed in the same place, which is in the process of operation parameter tuning and calibration. The SciCRT will work mainly as a Solar Neutron and Muon Telescope.