Unrest Episodes Research Articles

The 2008 Eruptive Unrest at Cerro Azul Volcano (Galápagos) Revealed by InSAR Data and a Novel Method for Geodetic Modelling

AbstractCerro Azul is one of the most active volcanoes in the western Galápagos Islands, but its unrest episodes are poorly studied. Unrest, which started in 2007, culminated in two eruptive phases from 29 May to 11 June 2008. We investigate this unrest and the associated eruptions using interferometric synthetic aperture radar (InSAR) data and geodetic modelling. To overcome the unwrapping errors affecting some of our InSAR data, we invert the wrapped phase directly by estimating the integer ambiguities simultaneously with the geophysical parameters. Our results highlight how the eruption was preceded by long‐term pre‐eruptive inflation (October 2007–April 2008). During the first eruptive phase, most of the magma responsible for the inflation fed the lateral propagation of a radial dike, which caused a first deflation of the magmatic reservoir. During the second eruptive phase, the further lateral propagation of the dike fed a radial eruptive fissure at the base of the edifice, causing further deflation of the magmatic reservoir. From the first to the second eruptive phase, the radial dike changed its strike propagating toward a topographic low between Cerro Azul and Sierra Negra.

Rapid localized flank inflation and implications for potential slope instability at Tungurahua volcano, Ecuador

High rates of volcano surface deformation can be indicative of a forthcoming eruption, but can also relate to slope instability and possible flank collapse. Tungurahua volcano, Ecuador, has been persistently active since 1999 and has previously experienced catastrophic flank failures. During the ongoing eruptive activity, significant surface deformation has been observed, with the highest rates contained within the amphitheatre-shaped scar from the 3000-year-old failure on the west flank. However, the cause of this asymmetric deformation and how it might relate to slope stability has not been assessed. Here, for the first time, we present a range of models to test physical processes that might produce asymmetric deformation, which are then applied to slope stability. Our models are informed by InSAR measurements of a deformation episode in November 2015, which show a maximum displacement of ∼3.5 cm over a period of ∼3 weeks, during which time the volcano also experienced multiple explosions and heightened seismicity. Asymmetric flank material properties, from the rebuilding of the cone, cannot explain the full magnitude and spatial footprint of the observed west flank deformation. The inflation is inferred to be primarily caused by shallow, short-term, pre-eruptive magma storage that preferentially exploits the 3 ka flank collapse surface. Shallow and rapid pressurization from this inclined deformation source can generate shear stress along the collapse surface, which increases with greater volumes of magma. This may contribute to slope instability during future unrest episodes and promote flank failure, with general application to other volcanoes worldwide displaying asymmetric deformation patterns.

Distribution of Partial Melt Beneath Changbaishan/Paektu Volcano, China/Democratic People's Republic of Korea

AbstractChangbaishan/Paektu volcano straddles the border between the Democratic People's Republic of Korea and China. It was responsible for one of the largest eruptions in history, the “Millennium Eruption' of 946 CE. An episode of unrest between 2002 and 2005, characterized by inflation and seismicity, refocused attention on this volcano. While satellite remote sensing has provided synoptic observations, ground‐based surveillance has hitherto supported only disparate analyses and geophysical interpretations on either side of the border. Here, we derive receiver functions using seismic records from both Democratic People's Republic of Korea and China. H‐κ stacking indicates thick crust (up to 40 km) and high average crustal VP/VS (up to 1.93) beneath the volcano. Grid search inversions constrain a significant velocity reduction at 4–8 km depth (below sea level), and harmonic analysis suggests this dips away from the volcano, with shallowest depths centered beneath the volcano. Common conversion point migrations show that this anomaly extends ∼30 km from the volcano summit and possibly as far as neighboring volcanoes. The colocation of the velocity reduction with a zone of high‐conductivity, low‐velocity, and low‐density material at the depth of the inflation source implicated in the 2002–2005 unrest indicates that partial melt is present directly beneath Changbaishan/Paektu, likely recharged during the episode of unrest. Our study highlights the importance of continued surveillance of the volcano and the need for further geophysical studies to constrain more fully the triggers for unrest and controls on its evolution.

Introduction to Special Issue: Critical Interventions on the Spaces and Practices of State Power

Many analyses of the 2011 uprisings and their aftermath 2011 uprisings have been built explicitly on social movements theory with a focus on specific episodes of social unrest. Others have employed procedural approaches from theories of democratization. The nation-level unit of analysis dominant in these approaches reproduces classic regime-character typologies and invokes questions about the relationship between regime type and successful social mobilization. This special issue explores the spaces and practices of state power beyond the conventional framework, giving special attention to space, time, and the multiple scales of relations of power. The contributors explore how these alternative approaches open up different questions into the operation of state power—some in the specific context of the uprisings and some in the course of more quotidian administration.

WOVOdat – the global volcano unrest database aimed at improving eruption forecasts

PurposeThe purpose of this paper is to highlight the importance of a comprehensive global database on volcanic unrest (WOVOdat) as a resource to improve eruption forecasts, hazard evaluation and mitigation actions.Design/methodology/approachWOVOdat is a centralized database that hosts multi-parameter monitoring data sets from unrest and eruption episodes of volcanoes worldwide. Its online interface (https://wovodat.org/) allows interactive data analysis and comparison between volcanoes and eruption styles, which is needed during volcanic crises, as well as to perform basic research on pre-eruption processes, teaching and outreach.FindingsWOVOdat aims to standardize and organize the myriad of monitoring data types at the global scale. Users can compare changes during a crisis to past unrest episodes, and estimate probabilities of outcomes using evidence-based statistics. WOVOdat will be to volcanology as an epidemiological database is to medicine.Research limitations/implicationsThe success of eruption forecast relies on data completeness, and thus requires the willingness of observatories, governments and researchers to share data across the volcano community.Practical implicationsWOVOdat is a unique resource that can be studied to understand the causes of volcanic unrest and to improve eruption forecasting.Originality/valueWOVOdat is the only compilation of standardized and multi-parameter volcano unrest data from around the world, and it is freely and easily accessible through an online interface.

Volcanic structures investigation through SAR and seismic interferometric methods: The 2011–2013 Campi Flegrei unrest episode

Observations from satellites provide high-resolution images of ground deformation allowing to infer deformation sources by developing advanced modeling of magma ascent and intrusion processes. Nevertheless, such models can be strongly biased without a precise model of the internal structure of the volcano. In this study, we jointly exploited two interferometric techniques to interpret the 2011–2013 unrest at Campi Flegrei caldera (CFc). The first is the Interferometric Synthetic Aperture Radar (InSAR) technique, which provides highly-resolved spatial and temporal images of ground deformation. The second is the Ambient Noise Tomography (ANT), which images subsurface structures, providing the constraints necessary to infer the depth of the shallow source at CFc (between 0.8 and 1.2 km). We applied for the first time a tool to delineate the deformation source boundaries from the observed deformation maps: the Total Horizontal Derivative (THD) technique. The THD processes the vertical component of the ground deformation field detected through InSAR applied to COSMO-SkyMed data. The patterns retrieved by applying the THD technique show consistent spatial correlations with (1) the seismic group-velocity maps achieved through the ANT and (2) the distribution of the earthquakes nucleated during the unrest at ~1 km. High-velocity anomalies, the retrieved geometrical features of the deformation field, and the spatial distribution of seismicity coincide with extinct volcanic vents in the eastern part of the caldera (Solfatara/Pisciarelli and Astroni). Such a coincidence hints at a significant role of the extinct plumbing system in either constraining or channeling the eastward propagation of magmatic fluids. Here, we demonstrated that a joint analysis of the InSAR patterns, seismic structures, and seismicity allows us to model in space and time the characteristics and nature of the shallow deformation source at CFc. Using published literature, we show that the effects of structural heterogeneities at shallow depths may have a more significant early-stage impact on the evolution of the surface displacement signals than deeper magmatic sources: these secondary structural effects may produce local amplification in the deformation records which can be mistakenly interpreted as early signals of impending eruptions. The achieved results are particularly relevant for the understanding of the origin of deformation signal at volcanoes where magma propagation within sills is expected, as at CFc.

Unrest at Domuyo Volcano, Argentina, Detected by Geophysical and Geodetic Data and Morphometric Analysis

New volcanic unrest has been detected in the Domuyo Volcanic Center (DVC), to the east of the Andes Southern Volcanic Zone in Argentina. To better understand this activity, we investigated new seismic monitoring data, gravimetric and magnetic campaign data, and interferometric synthetic aperture radar (InSAR) deformation maps, and we derived an image of the magma plumbing system and the likely source of the unrest episode. Seismic events recorded during 2017–2018 nucleate beneath the southwestern flank of the DVC. Ground deformation maps derived from InSAR processing of Sentinel-1 data exhibit an inflation area exceeding 300 km2, from 2014 to at least March 2018, which can be explained by an inflating sill model located 7 km deep. The Bouguer anomaly reveals a negative density contrast of ~35 km wavelength, which is spatially coincident with the InSAR pattern. Our 3D density modeling suggests a body approximately 4–6 km deep with a density contrast of –550 kg/m3. Therefore, the geophysical and geodetic data allow identification of the plumbing system that is subject to inflation at these shallow crustal depths. We compared the presence and dimensions of the inferred doming area to the drainage patterns of the area, which support long-established incremental uplift according to morphometric analysis. Future studies will allow us to investigate further whether the new unrest is hydrothermal or magmatic in origin.

Stress heterogeneities and failure mechanisms induced by temperature and pore-pressure increase in volcanic regions

We study the strain and stress fields produced by temperature and pore pressure increases within and outside a Thermo-Poro-Elastic (TPE) inclusion (the source region), embedded within a medium (the matrix) in isothermal drained conditions. This model is suitable to describe a crustal region in a volcanic environment pervaded by hot pressurized fluids released by an underlying magma chamber. After introducing the pertinent constitutive relations, a formal solution for the displacement field is provided in terms of the Green's function for an elastic medium with drained isothermal elastic moduli, employing a generalization of Eshelby (1961) procedure. If an unbounded medium is considered, a displacement potential can be introduced, obeying the Laplace equation within the source region and the Poisson equation within the matrix. If a spherically symmetric source region is considered, simple analytical solutions are obtained for the displacement, the strain and the stress fields, showing that thrust faulting mechanisms are promoted within the source region while normal faulting mechanisms prevail in the embedding matrix. Employing reasonable numerical values for the thermo-poro-elastic parameters, suitable to describe highly porous sedimentary rock, strain and stress variations are found to be significant even for moderate changes of temperature and pore pressure. Variations of the Coulomb failure function are high in the TPE region and are strongly dependent on the friction coefficient and pore pressure. Application of these results to the 1982-84 and 2011-13 unrest episodes at Campi Flegrei caldera (Italy) suggests that an oblique dike intrusion across a previously unfaulted TPE region took place with a mixed tensile-thrust dislocation mechanism in both events, as previously inferred from accurate inversion of geodetic data.

Seismic activity during the 2013–2015 intereruptive phase at Lascar volcano, Chile

SUMMARYIn addition to enabling the physical processes of volcanic systems to be better understood, seismology has been also used to infer the complexity of magma pathways and plumbing systems in steep-sided andesitic and stratovolcanoes. However, in these volcanic environments, the application of seismic location methods is particularly challenging and systematic comparisons of common methods are lacking. Furthermore, little is known about the characteristic seismicity and deep structure of Lascar volcano, one of the most historically active volcanoes in northern Chile known to produce VEI-4 eruptions. To better understand the inner processes and deep structure of Lascar, the local broad-band seismic monitoring network was densified during a temporal installation in 2014–2015. Herein, we focus on the local seismicity during the 2014–2015 unrest episode, during which we recorded numerous seismic events mainly classified as long-period (LP) type, but also denote volcano-tectonic (VT) activity. Specifically, a long-lasting phase of LP activity is observed over a period of ∼14 months that starts in tandem with a pulse of VT activity. The LP rate and amplitude are modulated over time; they are lower in the initial phase, rise during the intermediate period from October 2014 to July 2015, and finally slowly decay while approaching the eruption time. The location of LPs is challenging due to the typical lack of clear seismic onsets. We thus encompass this problem by comparing a broad range of different standard and novel location techniques to map the source region of LPs by fitting the amplitude decay, polarization patterns, coherence of characteristic functions and cross-correlation differential times. As a result, we principally constrain LP locations within the first 5 km depth below the summit extending downward along a narrow, conduit-like path. We identify different regions of complexity: VTs dominate at depth, both VTs and LPs cluster in an intermediate depth region (down to 1.5 km), suggesting a change in the plumbing system geometry, and LPs dominate the shallowest region. Based on these results, we infer the presence of a subvertical conduit extending down to a depth of ∼5 km, and a region of path divergence, possibly accommodating a magma plumbing system, at a depth of ∼3 km beneath the volcano summit. Identifying the locations of complexities in the magma pathways at Lascar may help identify future unrest. The results are compared with independent observations, demonstrating the strength of the location method used herein that will be tested at volcanoes elsewhere.

Time-Varying Skewness and Real Business Cycles

In the context of a quantitative real business cycle (RBC) model, we document that shocks to the higher-order moments, especially the skewness, of productivity can have large first-order effects on business cycles. We augment a standard small open economy RBC model with a new feature: a discrete regime switching between higher-order moments of total factor productivity shocks between an unrest state and a quiet state. To map the theory to data, we exploit an extensive database of mass political unrest around the world. We calibrate the model to the observed increases in the volatility and skewness of the growth rates of key economic variables during episodes of unrest. The calibrated model shows that increases in negative skewness play an important role in explaining the observed first-order decline in economic activities.

Modulation of seismic activity in Kīlauea’s upper East Rift Zone (Hawaiʻi) by summit pressurization

AbstractKīlauea Volcano (Hawaiʻi, USA) is underlain by a complex, laterally extensive magmatic plumbing system. Although in recent decades it has mainly erupted through vents along the middle East Rift Zone and summit caldera, eruptions can occur anywhere along its laterally extensive rift zones, as demonstrated by the dramatic eruptive activity of 2018. Forecasting eruptive activity requires an understanding of whether an episode of volcano-seismic unrest at Kīlauea and similar volcanoes is caused directly at the edges of an active intrusion or reservoir, or in a volume of wall rock at a distance from the intrusion. Seismic unrest in Kīlauea’s upper East Rift Zone (UERZ) has to date been interpreted as the result either of magma intrusion in this region of the volcano or of stresses due to seaward flank migration. However, recent observations suggest that UERZ seismicity may result from variable pressurization of Kīlauea’s summit magma system. We analyze seismic and deformation (multi-temporal interferometric synthetic aperture radar [InSAR] and GPS) data during a period of variable summit deformation and UERZ seismicity in mid- to late 2007 and calculate Coulomb stress changes on UERZ faults due to modeled summit inflation or deflation. UERZ seismicity during our study period can be explained entirely by stresses arising from pressure changes within Kīlauea’s summit reservoirs. Furthermore, a comparison of UERZ fault plane solutions (FPSs) calculated for this study to published UERZ FPSs for previous periods suggests that the UERZ has undergone a transition from a mechanically strong, discontinuous, and immature magma transport system to a mature, mechanically weak, and fully connected transport system over the course of the 1983–2018 eruption.

Reservoir Structure and Hydraulic Properties of the Campi Flegrei Geothermal System Inferred by Audiomagnetotelluric, Geochemical, and Seismicity Study

AbstractThe Campi Flegrei caldera is a large volcanic complex lying in the Campanian Plain, Southern Italy. During its history the caldera experienced episodes of bradyseism and intense swarm seismicity. The mechanism leading to unrest episodes is still debated, and great efforts are ongoing to improve the knowledge of this structure and its evolution due to the high volcanic risk in such a densely populated area. Here we present a resistivity model from a two‐dimensional inversion of audiomagnetotelluric data acquired along an approximately 5.6‐km long profile crosscutting the Solfatara‐Pisciarelli district and the Agnano plain. The resistivity model shows (1) very low resistivity values confined in the first 500 m of depth both in correspondence of the Solfatara‐Pisciarelli districts and the Agnano depression; (2) a resistive plume that extends underneath the Solfatara crater down to 2,000‐ to 3,000‐m depth, and (3) an adjoining relative conductive unit eastward. We discuss the resistivity structures in a multidisciplinary framework integrating inedited geochemical and seismological observations with existing surface geology and subsurface information. The Solfatara‐Pisciarelli district and the Agnano plain, both being expression of intense hydrothermal activity, show different characteristics. Below the Solfatara‐Pisciarelli area, the shallow conductive zone is interpreted as a faulted clay cap that overlies a highly active vapor‐dominated reservoir characterized by a convective mechanism. Conversely, below the Agnano plain, a liquid phase seems to prevail in the reservoir. The spatiotemporal variations of seismicity imply a combined action of preexisting tectonic lineaments and fluid interaction between the gas/steam reservoir and the outflow zone.

Seismically Induced Soft‐Sediment Deformation Phenomena During the Volcano‐Tectonic Activity of Campi Flegrei Caldera (Southern Italy) in the Last 15 kyr

AbstractWe report, for the first time, evidence of seismically induced soft‐sediment deformations in the central area of the active Campi Flegrei caldera (southern Italy). We analyzed the marine‐transitional and continental sequences located along the coastal La Starza cliffs and several stratigraphic logs exposed during the excavation of a 1‐km‐long tunnel in the Pozzuoli area. The successions host several soft‐sediment structures including sand dikes and sand volcanoes, which are largely dated within the 4.55‐ to 4.28‐kyr BP interval. The volcano‐sedimentary sequence, deposited within the Campi Flegrei caldera in the last 15 kyr, is schematically formed by the superposition of three layers with different rheological behaviors; from the base progressing upward we recognize (1) a massive tuff, (2) marine‐transitional sands of the La Starza unit, and (3) a dominance of continental volcanoclastics. We envisage that during unrest episodes of the volcano, which included ground deformation and seismic activity, the whole volcano‐sedimentary pile was deformed through brittle mechanisms with the formation of normal faults. However, the intermediate layer, when subject to seismic shaking, behaved locally as a viscous material facilitating liquefaction processes and lateral spreading deformation. Furthermore, new geophysical, stratigraphic, and structural surveys allowed us to model the deformation evolution of this area over the last 15 kyr. The evidence of seismically induced soft‐sediment deformation within the volcano‐sedimentary record suggests that moderate earthquakes could occur during future volcano‐seismic unrests. Consequently, liquefaction and related gravitational mass movements must be considered as a hazard during these unrest and volcanic crises.

Change in seismic attenuation as a long-term precursor of gas-driven eruptions

Abstract A large fraction of volcanic eruptions do not expel magma at the Earth’s surface. Although less known than magmatic eruptions, gas-driven eruptions expel fragments of preexisting rocks, volcanic gases, and steam, causing substantial casualties. The destructive potential of these eruptions lies in the difficulty in identifying clear warning signals. Some gas-driven eruptions have been preceded by some physicochemical changes, but these were extremely short-term (from minutes to hours), and no long-term trends have been clearly evidenced so far. Here, we show that unheralded gas-driven eruptions can be forecast in the long term using seismic signals recorded at nearby active craters. In particular, we have found that the most recent gas-driven eruptions at Kawah Ijen (Indonesia) and Ruapehu and Tongariro (New Zealand) volcanoes were all preceded by a systematic relative increase in lower-frequency (4.5–8 Hz) seismic amplitude compared to higher frequencies (8–16 Hz) over time scales of months to years. We show that this precursory activity reflects significant increases in seismic attenuation affecting preferentially high-frequency travelling waves; this probably results from the accumulation of volatiles in the shallow crust, which increases pore pressure in small-scale rock heterogeneities and eventually leads to gas-driven eruptions. Our results highlight the feasibility of better constraining the onset and the end of an unrest episode, which is of paramount importance for agencies in charge of volcano monitoring.

Noneruptive Unrest at the Caldera of Alcedo Volcano (Galápagos Islands) Revealed by InSAR Data and Geodetic Modeling

AbstractUnderstanding volcanic unrest is crucial to forecasting eruptions. At active mafic calderas unrest culminates in eruption more frequently than at felsic calderas. However, the mafic caldera of Alcedo Volcano (Ecuador) has experienced repeated episodes of unrest without erupting, since at least 1992, when geodetic monitoring began. Here we investigate the unrest that occurred between 2007 and 2011 using interferometric synthetic aperture radar (InSAR) data and geodetic modeling. We observe an initial asymmetric uplift of the southern caldera floor (~30 cm of vertical motion) from 2007 to 2009, followed by subsidence of the uplifted area and contemporary uplift of the northwestern caldera rim between January and June 2010. Finally, from June 2010 through March 2011, caldera uplift resumed. The first uplift episode is best explained by inflation of a sill and the activation of an inner ring fault. Successive caldera subsidence and rim uplift are compatible with the withdrawal of magma from the previously inflated sill and its northwestern migration. The resumption of uplift is consistent with the repressurization of the sill. This evolution suggests episodic magma emplacement in a shallow reservoir beneath the caldera, with aborted lateral magma migration, probably due to the discontinuous supply from depth. This short‐term deformation pattern matches well geological observations showing a longer‐term (hundreds of years at least) asymmetric uplift of the caldera floor, culminating in a weak resurgence of ~30 m. We propose that the monitored episodes of uplift represent short‐term stages of the rarely observed incremental growth of a resurgent basaltic caldera.

Temporal variations of repeating low frequency volcanic earthquakes at Ngauruhoe Volcano, New Zealand

A persistent swarm of micro-earthquakes was observed at Ngauruhoe volcano, New Zealand from 2005 to 2010, marking the first unrest episode at the volcano since 1975. Detailed waveform and spectral analysis of the continuous seismic record revealed eight earthquake families (F1–F8) within a sequence of 8561 low-frequency earthquakes. An analysis of stacked waveforms of each family allowed the families to be linked systematically into four super-families that may indicate four different mechanisms, and/or source locations or geometries.The first super-family mostly occurred in 2005 and showed stable waveforms and no significant evolution. The second super-family was dominant over a two-year period starting in mid-2006 and displayed a slow evolution with progressively higher frequency. The third super-family emerged from early 2008 with intermittent occurrence and high seismicity, and progressed to higher peak frequency in 2010. The fourth super-family was mainly observed from late 2008 to mid-2009 and had the highest peak frequency in 2009.The observed evolutionary pattern may be driven by a modest multi-year input of gas and heat from a deeper relict magmatic zone into the shallow volcano hydrothermal system. The interplay of gas from below and seasonal meltwater from above may have modulated the seismic activity and may relate to repeated over-pressurization and failure of constricted evolving pathways within a pervasive crack system.

Deciphering the evolution of Deception Island\u2019s magmatic system

Deception Island (South Shetland Islands) is one of the most active volcanoes in Antarctica, with more than 20 explosive eruptive events registered over the past two centuries. Recent eruptions (1967, 1969, and 1970) and the volcanic unrest episodes that happened in 1992, 1999, and 2014–2015 demonstrate that the occurrence of future volcanic activity is a valid and pressing concern for scientists, technical and logistic personnel, and tourists, that are visiting or working on or near the island. We present a unifying evolutionary model of the magmatic system beneath Deception Island by integrating new petrologic and geochemical results with an exhaustive database of previous studies in the region. Our results reveal the existence of a complex plumbing system composed of several shallow magma chambers (≤10 km depth) fed by magmas raised directly from the mantle, or from a magma accumulation zone located at the crust-mantle boundary (15–20 km depth). Understanding the current state of the island’s magmatic system, and its potential evolution in the future, is fundamental to increase the effectiveness of interpreting monitoring data during volcanic unrest periods and hence, for future eruption forecasting.

Thoughts on the criteria to determine the origin of volcanic unrest as magmatic or non-magmatic.

As our ability to detect volcanic unrest improves, we are increasingly confronted with the question of whether the unrest has a magmatic origin (magma on the move) or a non-magmatic origin from a change in the hydrothermal system (fluids that are not magma on the move) or tectonic processes. The cause of unrest has critical implications for the potential eruptive hazard (e.g. used in constructing Bayesian Event Trees), but is frequently the subject of debate, even at well-studied systems. Here, we propose a set of multi-disciplinary observations and numerical models that could be used to evaluate conceptual models about the cause of unrest. These include measurements of gas fluxes and compositions and the isotopic signature of some components (e.g. H2, He, C, SO2, H2 O, CH4 and CO2), the spatial and temporal characteristics of ground deformation, thermal output, seismicity, changes in gravity, and whether there is topographic uplift or subsidence spanning hundreds to thousands of years. In several volcanic systems, both magmatic and non-magmatic unrest is occurring at the same time. While none of these observations or models is diagnostic on its own, we illustrate several examples where they have been used together to make a plausible conceptual model of one or more episodes of unrest and whether eruptions did or did not follow the unrest. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Eruption frequency patterns through time for the current (1999–2018) activity cycle at Volcán de Fuego derived from remote sensing data: Evidence for an accelerating cycle of explosive paroxysms and potential implications of eruptive activity

Volcán de Fuego is a stratovolcano in Guatemala that has produced over 50 VEI ≥ 2 eruptions since 1524. After two decades of quiescence, in 1999 Fuego entered a new period of eruptive activity that continues until the present day, characterized by persistent Strombolian activity interspersed with occasional “paroxysmal” eruptions of greater magnitude, the most recent of which occurred in 2018. The land surrounding Fuego accommodates tens of thousands of people, so greater understanding of its eruptive behaviour has important implications for hazard assessment. Nevertheless, there is relatively little literature that studies recent (since 1999) activity of Fuego in detail.Using time-series analysis of remote sensing thermal data during the period 2000–2018 combined with recent bulletin reports, we present evidence for a new eruptive regime beginning in 2015. We find that this regime is defined by a greater frequency of paroxysmal eruptions than in previous years and is characterized by the following sequence of events: (i) effusion of lava flows and increase in summit explosive activity, followed by (ii) an intense eruptive phase lasting 24–48 h, producing a sustained eruptive column, continuous explosions, and occasional pyroclastic flows, followed by (iii) decrease in explosive activity. We discuss various models that explain this increase in paroxysmal frequency, and consider its implications for hazard assessment at Fuego. We advocate the pairing of remote sensing data with monitoring reports for understanding long-term changes in behaviour of poorly-instrumented volcanoes. The results that we present here provide a standard for informed assessment of future episodes of unrest and paroxysmal eruptions of Fuego.

Anatomy of the Campi Flegrei caldera using Enhanced Seismic Tomography Models

Campi Flegrei caldera (Southern Italy) is a densely inhabited area and suffered several unrest episodes in the last centuries. The dynamic of the caldera is highly debated because of conflicting interpretations. Here we present a detailed reconstruction of the Campi Flegrei structure obtained using the microseismicity recorded during the 1984 unrest. Enhanced Seismic Tomography models obtained with these data allow us describing seismic velocities, attenuation, and scattering patterns. Results show: (1) a plumbing system with a diameter of 1 km located between 2.3 km and 4 km depth (2) a 0.5 km thick caprock located at 2 km depth interpreted as the main structure regulating the fluid interchange between deep and shallow sectors of the caldera, (3) the shape and volume of a shallow reservoir beneath the city of Pozzuoli; this reservoir played a key role during the 1982–1984 unrest, (4) several small reservoirs beneath the main craters of the caldera. All these features fit into the debated question on magmatic or hydrothermal mechanism driving the caldera deformation resulting of crucial importance to allow a better assessment of the hazard.