Fig 6 - uploaded by José Madeira
Content may be subject to copyright.
14. Synthetic and antithetic faults at Falca Fault zone, exposed on the footwall block at Santa Iria zone: (a) site 152 and (b) site 188.
Source publication
The Azores archipelago is located at the triple junction between the Eurasia, Nubia and North America lithospheric plates. São Miguel Island, situated at the southeastern part of the western segment of the Azores–Gibraltar Fracture Zone, presents an east–west elongated shape, comprising three quiescent central volcanoes with summit calderas linked...
Citations
... In the case of the Fogo volcanic system, the fumarolic manifestations are all located on the northern flank of the volcano (Pereira et al., 2022;Viveiros et al., 2023a) associated with the NW-SE Ribeira Grande graben (Carmo et al., 2015): Caldeiras da Ribeira Grande, Caldeira Velha, and Pico Vermelho fumaroles. The first two fumarolic emissions show the general above-mentioned composition, while gas samples from Pico Vermelho fumaroles show a composition dominated by water vapour with minor amounts of CO 2 and high concentrations of N 2 and O 2 that reflect air contamination at subsurface levels. ...
The Azores archipelago is located in the North Atlantic Ocean and is formed by nine volcanic islands. Present-day volcanic activity in the archipelago comprises not only seismic swarms and episodes of ground deformation in some volcanoes, but also hydrothermal gas manifestations. The main fumaroles are associated with central quiescent volcanoes and have been sampled in the past decade through the Giggenbach methodology. Analysis of the fumarolic fluids from four volcanic systems in São Miguel, Terceira, and Graciosa islands are displayed for the period between July 2015 and February 2023. No significant changes have been observed both on the selected gas ratios (CO2/CH4, H2/CH4, He/CH4, He/CO2, and H2/Ar), and on the equilibrium temperatures estimated for the reservoirs feeding the fumaroles using the H2/Ar gas geothermometer, what is in agreement with the dormant state of activity of the studied volcanic systems. Equilibrium temperatures ranging between 223°C and 262°C are estimated for Fogo and Furnas fumarolic fields (São Miguel Island), and an average equilibrium temperature of 254°C is inferred for Terceira fumaroles. This study also suggests a procedure based on established guidelines to evaluate the analytical uncertainties of the methodologies associated with the Giggenbach sampling strategy. Comparison of the analytical uncertainties with the overall data variation (including additional natural variations and sampling uncertainty), shows that the analytical uncertainties are not the limiting factor for the interpretation of the survey results.
... A value around 954 t d −1 was estimated for the hydrothermal CO 2 diffusely released by the soils at Furnas Volcano (Viveiros et al., 2010;Viveiros et al., 2012). The diffuse degassing studies showed that anomalous zones are essentially associated with the WNW-ESE tectonic structures found out at Furnas Volcano (Viveiros et al., 2010;Carmo et al., 2015). Silva et al. (2015a), Silva et al. (2015b) mapped soil radon ( 222 Rn) anomalies in both villages showing also a good correlation between the anomalous CO 2 and 222 Rn areas. ...
Carbon dioxide released permanently from soils in diffuse degassing areas may constitute a permanent hazard for the population. Several villages in the Azores archipelago (Portugal) are placed in areas with anomalous soil CO2 degassing and lethal indoor CO2 concentration (>10 vol%) has been already recorded in some buildings. The 2021-2022 dislodgements of population at Vulcano (Italy) and La Palma (Spain) volcanic islands due to high soil CO2 degassing highlight the importance of defining criteria to produce human CO2 exposure risk maps, which are useful to mitigate the risk and should constitute valuable tools for land-use planners. Risk is assessed in the current study by combining susceptibility, exposure, and vulnerability maps. The defined criteria were applied to two villages in Furnas Volcano (São Miguel Island, Azores), showing that 58% and 98% of the buildings, respectively, at Furnas and Ribeira Quente villages are at high risk of CO2 exposure.
... The caldera is characterised by fault systems trending WNW-ESE, NE-SW, N-S and NNW-SSE. A minor WNW-ESE-oriented fault system is observed in Lagoa das Furnas margins (Carmo et al., 2015). The lake has a surface area of 1.87 km 2 and is elongated in an NNE-SSW direction (maximum length and width of 2025 m ✕ 1600 m). ...
... Kinematic measurements suggest a second stress field in the eastern São Miguel and Graciosa islands that may alternate with the first one in time. 40,41 The interplay between volcanic activity, surface faulting, and subaerial geomorphological processes of denudation are reflected in the morphology of the islands. Middle Pleistocene to Holocene era formations are affected by these morphologies and structures. ...
Azores Islands are seismically active due to the tectonic structure of the region. Since the 15th century, they have been periodically shaken by approximately 33 moderate to strong earthquakes, with the most recent one in 1998 ( M w = 6.2). Nonetheless, due to insufficient instrumental seismic data, the region lacks a uniform database of past real records. Ground motion simulation techniques provide alternative region‐specific time series of prospective events for locations with limited seismic networks or regions with a seismic gap of catastrophic earthquake events. This research establishes a local ground motion model (GMM) for the Azores plateau (Portugal) by simulating region‐specific records for constructing a homogeneous dataset. Simulations are accomplished in bedrock using the stochastic finite‐fault approach by employing validated input‐model parameters. The simulation results undergo validation against the 1998 Faial event and comparison with empirical models for volcanic and Pan‐European datasets. A probabilistic numerical technique, namely the Monte‐Carlo simulation, is employed to estimate the outcome of uncertainty associated with these parameters. The results of the simulations are post‐processed to predict the peak ground motion parameters in addition to spectral ordinates. This study uses XGBoost to circumvent the difficulties inherent to linear regression‐based models in establishing the form of equations and coefficients. The input parameters for prediction are moment magnitude ( M w ), Joyner and Boore distance ( R JB ), and focal depth ( FD ). The quantification of GMM uncertainty is accomplished by analyzing the residuals, providing insight into inter‐ and intra‐event uncertainties. The outcomes demonstrate the effectiveness of the suggested model in simulating physical phenomena.
... The Terceira Rift is marked by an alignment of basins and volcanic highs that runs from the Western Graciosa Basin to the Formigas Islets, located at the eastern edge of the Gloria Fault. The Terceira Rift currently concentrates most of the seismic activity of the Azores, accommodating slow transtensional deformation at a rate of approximately 5 mm/yr (Miranda et al., 2014;Miranda et al., 2015;Carmo et al., 2015;Madeira et al., 2015). Focal mechanisms of earthquakes along the Terceira Rift show a mix of normal and strikeslip earthquakes, with the extensional T axes consistently oriented NE-SW, perpendicular to the ridge (Ekström et al., 2012, Custódio et al., 2016. ...
... S. Miguel Digital Elevation Model (DEM) was generated from a 1:5000 scale digital altimetric database from the Secretaria Regional do Turismo e Transportes of the Azores Government. It is also shown the locations of the volcano-tectonic features identified in Carmo et al. (2015) In this work, we tackle two main objectives: 1) To validate a highly automated strategy based on the analysis of continuous waveform data to quickly characterize seismo-volcanic crises; and 2) To use this strategy to study the spatio-temporal evolution of seismicity during the recent February 2018 S. Miguel crisis. We start by analyzing the local earthquake catalogs to establish the February 2018 S. Miguel crisis as significantly above the background seismicity level. ...
The island of São Miguel is among the most seismically active areas of the Azores archipelago. This work focuses on the most significant recent swarm, which occurred on February 2018. We set up an automated procedure to process continuous full seismic waveform data from local stations to generate high-quality earthquake information on the volcano unrest episode. First, we applied an automated detector software, next we located the detected events and then classified the earthquakes based on their waveform similarity, identifying three families of seismic events. We then extended the catalog by template matching. Finally, we computed moment tensors to investigate the source mechanisms of the largest earthquakes. Our results image the ∼2-week swarm evolution. The activity started with a precursory phase with low rate and low magnitude (ML < 2.0) seismicity and the activation of a deeper structure (∼10-15 km). After ∼1 week, a new earthquake family emerged at shallower depths (∼8–12 km) reaching magnitudes up to ML 3.4. Finally, a third slightly shallower family was activated. Moment tensors show mostly normal faulting mechanisms, striking ∼NW-SE, compatible with the orientation of the regional stress field. A surface deformation transient was recorded by geodetic stations, starting with the swarm, and continuing over the following ∼17 months, corresponding to either inflation or extension around the swarm region. The prolonged surface deformation implies a process that was initiated during the swarm and subsequently accommodated mostly aseismically. We interpret the seismicity observed at the early stage of deformation as indicating episodic fluid injection through the crust, related to the local hydrothermal or magmatic systems. We conclude that the Fogo-Congro region continues to be seismo-volcanically active, with both seismic and aseismic deformation observed and requiring close multidisciplinary monitoring. The proposed methology based on the automated analysis of continuous waveform data provides high-quality imaging of the spatio-temporal evolution of seismicity, which can be used elsewhere in the operational monitoring of seismo-volcanic crises to gain insight into the ongoing deformation processes, improve hazard assessment and help in the development of effective mitigation strategies.
... The island has an east-west elongated shape and is formed by three quiescent polygenetic central volcanoes with summit calderas (Sete Cidades,Fogo,Furnas;Fig. 1b), linked by two active fissure zones (Gaspar et al., 2015). São Miguel is shaped by several tectonic structures with a dominant NW-SE trend, including the Ribeira Grande graben, and WNW-ESE, with two groups of faults dipping 60º-90º to NE and SW (Carmo et al., 2015). Fogo Volcano is in the central part of the island and started to form nearly 200 ka BP (Wallenstein et al., 2015). ...
... As known, CO 2 degassing occurs as permanent manifestations or as episodic phenomena and may be affected by local topography, meteorology, surface roughness and/or atmospheric stability (Oliveira et al., 2018;Rinaldi et al., 2012;Viveiros et al., 2009;2015a). The DDS defined in the sampled area (Fig. 4a) show a general NW-SE direction, similar to the alignments previously inferred for the area by Carmo et al. (2015). Tectonic structures were not previously mapped at Caldeiras da Ribeira Grande, probably due to the existing vegetation together with the thick pumice deposits that hide eventual structures (Carmo et al., 2015). ...
... The DDS defined in the sampled area (Fig. 4a) show a general NW-SE direction, similar to the alignments previously inferred for the area by Carmo et al. (2015). Tectonic structures were not previously mapped at Caldeiras da Ribeira Grande, probably due to the existing vegetation together with the thick pumice deposits that hide eventual structures (Carmo et al., 2015). However, the consistent lineaments of gas anomalies observed in the current study suggest a deep structural control for the gas emission and the existence of hidden tectonic structures in the study area. ...
Caldeiras da Ribeira Grande is one of the degassing areas of Fogo, a trachytic central volcano located at São Miguel Island (Azores archipelago). Recently, new steam emissions, soil CO2 and temperature anomalies developed towards the inhabited area, causing high indoor CO2 values and affecting the vegetation and several small animals that were found dead in depressions and low-ventilated zones. During July–August 2021, a soil CO2 flux survey was carried out on the north flank of the volcano, estimating a soil gas release of at least 40 t d⁻¹ (excluding the contribution of the fumaroles) over an area of ∼0.27 km². Two populations for the CO2 released were found, highlighting the biogenic and volcanic-hydrothermal origins. General NW-SE diffuse degassing structures (DDS) were identified, in agreement with the tectonic lineaments previously recognized in the area. In this regard, we investigated the passive gas dispersion in the atmosphere at Caldeiras da Ribeira Grande performing a model validation aimed to estimate the fumarolic gas flux at source and the potential hazard for human and animal lives posed by CO2. Numerical simulations were carried out with the DISGAS-2.3, a 3D Eulerian advection-diffusion model, and the relative outputs processed through the VIGIL-1.3 workflow able to provide probabilistic long-term CO2 concentration maps, considering a meteorological variability over the last 30 years (1991–2020) taken from the ECMWF ERA5 reanalysis dataset. A best-fit between observed and simulated CO2 concentrations allowed us to estimate the total gas flux of the area (∼209 t d⁻¹) obtained by scaling the soil CO2 gas flux by a factor 30. Such an estimate is composed of ∼174 t d⁻¹ as unknown fumarolic and ∼ 35 td⁻¹ as diffuse contribution, in a good agreement with measurements. Although the present-day CO2 concentration at 0.3 m height cannot be considered to raise serious concerns for human health, we reasonably infer that the death of small animals may be due to local conditions of CO2 accumulation or to the presence of H2S. The current study highlights the relevance of coupling gas flux maps, concentration data, and gas dispersion modeling to obtain robust estimation of gas fluxes, including the fumarolic contribution, and identify zones potentially impacted by dangerous concentrations of volcanic gases, which are relevant for land-use planning and hazard assessment in case of renewed escalations of volcanic activity.
... Furnas volcano is a quiescent volcano located in the Eastern side of São Miguel Island, in the Azores volcanic archipelago (Searle, 1980;Carmo et al., 2015;Madeira et al., 2015). It comprises two nested calderas with a total size of 5 × 8 km 2 (Moore, 1990;Guest et al., 1999Guest et al., , 2015. ...
... Furnas volcano is a quiescent volcano located in the Eastern side of São Miguel Island, in the Azores volcanic archipelago (Searle, 1980;Carmo et al., 2015;Madeira et al., 2015). It comprises two nested calderas with a total size of 5 × 8 km 2 (Moore, 1990;Guest et al., 1999Guest et al., , 2015. ...
Induced polarization is used to image the feeder dike of a monogetic dome thanks to the signature of pronounced alteration around these conduits. We performed a 3D tomography of the electrical conductivity and normalized chargeability of the 1630-monogenetic dome located inside the caldera of Furnas, a quiescent stratovolcano in the Eastern side of São Miguel Island (Azores volcanic archipelago, Atlantic Ocean). A total of 2634 apparent resistivity and chargeability data were collected and inverted using a classical regularized least-squares inversion (based on a quasi-Gauss-Newton approach). The 3D tomograms display an area of both high electrical conductivity and normalized chargeability. This area corresponds likely to the altered subvolume associated with the magmatic feeding conduit of the monogenetic dome. The same conduit can be also observed on a 2D magnetotelluric (MT) section, at least for the first 200 m below the ground surface. Assuming that the in-situ pore water conductivity is the same as in Furnas Lake (0.016 S m−1, 25 °C), the electrical conductivity and normalized chargeability are combined to obtain tomograms of the water content and cation exchange capacity of the monogenetic dome. The cation exchange capacity provides a proxy to image alteration associated with the formation of clay minerals and zeolites. This study highlights the usefulness of these imaging techniques to image the structure of monogenetic domes.
... The landslide scar lies on the edge of the Pleistocene Pico do Vento dome, formed from trachyte, an igneous rock extruded by the volcano. The crown (top) of the landslide is within a cinder cone (mapping by Carmo et al., 2015). A unit of igneous rock, formed as a flow, runs parallel with the scar, and there are outflow deposits (i.e. ...
The multidisciplinary research described here shows how archaeologists can help reconstruct past seismic episodes and understand the subsequent relief operation, rehabilitation, and reconstruction processes. In October 1522, a major earthquake and landslide struck the then capital of the Azores, Vila Franca do Campo, 1500 km from the European mainland. Damage was extensive, destroying key monuments, affecting most of the inhabited area, and leaving few survivors among the early colonists. The results from twenty-six archaeological trenches, geological and geoarchaeological investigations, and documentary analysis are reviewed here. Distinctive archaeological deposits are identified and explained, using the high density of artefacts and the erosional contact between the landslide and the pre-1522 palaeosol to reconstruct the episode in detail.
... The Azores archipelago is volcanic in origin and is located on the triple junction between the Eurasian, African, and North American plates (Carmo et al., 2015 and references therein) (Fig. 1A). Numerous seismic and volcanic events have been registered since human settlement in the archipelago in the 15th century (Pacheco et al., 2013;Gaspar et al., 2015b). ...
... The tectonics in the area of the Sete Cidades Volcano are dominated by a prominent NWeSE general trend (Carmo et al., 2015), which are morphologically expressed by a set of fault scarps on the NW flank of the volcano from the coastal area to the caldera rim. This is referred to as the Mosteiros Graben, which cuts the caldera rim and extends towards the SE to the Picos Fissural Volcanic System. ...
... The NWeSE fault system plays an important role in the volcanic evolution, as it displaces older and recent deposits. The NNWeSSE to NeS trending structures are less obvious (Carmo et al., 2015). ...
Sete Cidades Lake (São Miguel Island, Portugal) is subdivided into two interconnected branches: the Green Lake and Blue Lake. The lake has an area and maximum depth of 4.39 km² and 29.5 m (Blue Lake), respectively, with evidence of eutrophication, particularly in the northern area of the Green Lake. In this study, we conducted a sampling survey during January 2017 to measure CO2 fluxes from the lake using a floating accumulation chamber. We also produced two hydrogeochemical profiles for each of the lake’s branches. A total of 1760 CO2 flux measurements were taken along the lake’s surface. The lake water was relatively cold (14.0 °C on average) and weakly mineralised (average electrical conductivity of 116 μS cm⁻¹) with a neutral pH (7.7 on average). The relative composition of major ions occurred in the following decreasing order: Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ for cations and Cl⁻ > HCO3⁻ > SO4²⁻ for anions. The lake water was mainly the Na–Cl type due to sea salt input from seawater spraying. CO2 fluxes ranged from 0.3 to 17.2 g m⁻² d⁻¹ and from 2.1 to 17.9 g m⁻² d⁻¹ for the Blue and Green Lakes, respectively. Highest CO2 degassing occurred in areas dominated by macrophytes and algal blooms. The measured values suggest that the CO2 was predominantly biogenically sourced, which was further supported by the δ¹³C isotopic data. The estimated total CO2 emissions varied between 5.8 t d⁻¹ (Green Lake; area = 0.81 km²) and 24.9 t d⁻¹ (Blue Lake; area = 3.58 km²). This study further elucidates the lake’s trophic and chemical pollution status and has major implications for lacustrine CO2 emissions to the atmosphere. Our study also provides a reference for understanding potential future variations in volcanic activity.
... The volcano is affected by fault systems trending west-northwest to east-southeast, northeast to southwest, north to south, and north-northwest to south-southeast. Some west-northwest to east-southeast tectonic structures were mapped in Furnas Lake margins (Carmo et al., 2015). Today, the main volcanic manifestations at the Furnas volcano are characterized by CO 2 and 222 Rn diffuse degassing areas, thermal and cold CO 2 -rich springs, and hydrothermal fumaroles (Cruz et al., 1999;Ferreira et al., 2005;Viveiros et al., 2010;Silva et al., 2015). ...
... Recent studies carried out in the lake showed that the most significant anomalies on the diffuse CO 2 emitted from Furnas Lake are located near the northern margin of the lake and therewith close to the surface fumarolic fields, suggesting a hydrothermal signature (Andrade et al., 2016). These anomalies are interpreted to be associated with tectonic structures and faults previously mapped (Carmo et al., 2015). In the northern margin, the high CO 2 fluxes might be correlated with a west-northwest to east-southeast-trending fault that also crosses the surface fumarolic fields (Andrade et al., 2016). ...
... The northern lake area close to the fumarole fields is outlined by a white rectangle. (b) A CO 2 degassing map modified after Andrade et al. (2016) with known fault M1 running through the northern bay and possible faults marked M2 and M3 (Carmo et al., 2015). The areas of highest degassing are labeled F1A, F1B, and F1C. ...
Water-covered areas may lead to gaps in surface electromagnetic surveys, causing reduced resolution and, in consequence, increased uncertainty in derived subsurface models. We describe a boat-towed floating central loop time-domain electromagnetic technique which allows mitigating this problem. It facilitates obtaining data with a spatial sampling density, which is rarely possible with standard instrumentation on land, and only requires moderate logistical effort. A unique field study on a shallow volcanic lake demonstrated that this method is feasible with only minor loss of accuracy when compared to anchored and land soundings. We show that the noise sources arising from the moving instrument and the boat engine can be neglected. The field survey was performed on the Lagoa das Furnas (São Miguel, Azores Islands, Portugal), which is located within an active volcanic area and characterized by fumarolic fields and CO2 degassing. The associated hydrothermal system is thus expected to extend below the lake. However, the character, geometry and extent of this system are unknown because of the lack of boreholes, and geophysical studies. A total of 600 soundings, combining towed profiles with anchored and land-based soundings, were acquired with an aim of imaging the hydrothermal system beneath the lake down to 200 m. The results from all three types of measurements compare well, and thus led to consistent one-dimensional inversion models. The inversion results delineate a highly conductive, smectite-rich cap layer dipping below the lake away from the main fumarole zone. Near this zone, the extent of the conductor agrees well with an area of intense dispersed CO2 degassing, which appears to be controlled by at least two, electrically distinctive fault zones where the conductor is found at greater depths.
