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Stress transfer between magma bodies: Influence of intrusions prior to 2010 eruptions at Eyjafjallajökull volcano, Iceland
Abstract
Stress transfer between separate magma bodies is evaluated by considering how pressure changes related to magma accumulation/propagation influence the stability of a separate nearby magma body. Three-dimensional numerical models are used to evaluate the stability evolution of a magma body through the calculation of two variables: (i) the variation of the threshold pressure needed to cause failure around the magma body and (ii) the magma pressure change. A parametric study indicates that stress interactions are strongly dependent on the distance between magma bodies as well as the body's shape. Such models are then applied to evaluate stress influence of intrusive activity in 1994, 1999 and 2010 at Eyjafjallajökull volcano, that preceded two eruptions there in 2010. Two cases are considered: influence of these intrusions on i) a magma reservoir at 20 km distance under the Katla volcano, and ii) a silicic magma body under Eyjafjallajökull. The distance between the Eyjafjallajökull intrusions and the Katla reservoir is sufficiently long to reduce the stress interaction to insignificant levels, with an amplitude of the same order as Earth tides (few kPa). However, cumulative stress transfer due to the intrusions to a remnant silicic shallow body situated below the Eyjafjallajökull is much larger (0.5-2.5 MPa). This mechanical transfer could have contributed to the failure of the silicic body and promoted the chemical mixing/mingling between different magma types, which is commonly interpreted as the main cause of the 2010 explosive eruption of Eyjafjallajökull.
... The variability in the source depths suggests that this actually occurs at Yellowstone, even on short time scales of less than one year, like during the NGB uplift in early 2014 (Figure 1). However, the exact pattern of fluid migration, potential magma mixing and mingling and stress interaction (e.g., Albino & Sigmundsson, 2014) are yet to be unraveled. ...
... We speculate that the reversal from uplift to subsidence at NGB in March 2014 resulted from fluid migration into the shallow hydrothermal system following the breaching of the self-sealed layer that separates the BDT. Since the caldera source did not change its behavior when NGB uplifted in early 2014 (Figure 8), we speculate that the BDT breaching might have changed the stress field in the deeper source (e.g., Albino & Sigmundsson, 2014), potentially allowing for magma to be injected from a mantle source. The exact mechanism is beyond the scope of this study. ...
The 2004–2009 caldera uplift is the largest instrumentally recorded episode of unrest at Yellowstone caldera. We use GPS and Interferometric Synthetic Aperture Radar (InSAR) time series spanning 2004–2015, with a focus in the aforementioned event to understand the mechanisms of unrest. InSAR data recorded ∼25 and ∼20 cm of uplift at the Sour Creek (SCD) and Mallard Lake (MLD) resurgent domes during 2004–2009, and ∼8 cm of subsidence at the Norris Geyser Basin (NGB) during 2004–2008. The SCD/MLD uplift was followed by subsidence across the caldera floor with a maximum at MLD of ∼1.5–2.5 cm/yr, and no deformation at NGB. The best-fit source models for the 2004–2009 period are two horizontal sills at depths of ∼8.7 and 10.6 km for the caldera source and NGB, respectively, with volume changes of 0.354 and −0.121 km³, and an overpressure of ∼0.1 MPa. The InSAR and GPS time series record exponentially increasing followed by exponentially decreasing uplift between 2004 and 2009, which is indicative of magma injection into the caldera reservoir, with no need for other mechanisms of unrest. However, magma extraction from NGB to the caldera is unable to explain the subsidence coeval with the caldera uplift. Models of magma injection can also explain other episodes of caldera uplift like that in 2014–2015. Distributed sill opening models show that magma is stored across the caldera source with no clear boundary between MLD and SCD. Since the magma overpressure is orders of magnitude below the tensile strength of the encasing rock, historical episodes of unrest like these are very unlikely to trigger an eruption.
... Freed, 2005;Árnadóttir et al., 2003;Stein, 1999 and references therein) along with earthquake-volcano interactions (e.g., Walter and Amelung, 2007;Parsons et al., 2006;Díez et al., 2005;Toda et al., 2005;La Femina et al., 2004;Hill et al., 2002;Nostro et al., 1998). However, fewer studies have investigated volcanic stress triggering at neighbouring volcanoes (e.g., Albino and Sigmundsson, 2014;Gonnermann et al., 2012). ...
... These reside at a similar distance from each other as Bárdarbunga and Tungnafellsjökull. In that case, in-trusive and eruptive activity at Eyjafjallajökull is inferred to have had minor influence on the magmatic system of Katla (Albino and Sigmundsson, 2014). Magma moving in the roots of the Eyjafjallajökull volcano was an order of magnitude less than at Bárdarbunga, explaining the lack of triggering effects. ...
Stress transfer associated with an earthquake, which may result in the seismic triggering of aftershocks (earthquake–earthquake interactions) and/or increased volcanic activity (earthquake–volcano interactions), is a well-documented phenomenon. However limited studies have been undertaken concerning volcanic triggering of activity at neighbouring volcanoes (volcano–volcano interactions). Here we present new deformation and stress modelling results utilising a wealth of diverse geodetic observations acquired during the 2014–2015 unrest and eruption within the Bárdarbunga volcanic system. These comprise a combination of InSAR, GPS, LiDAR, radar profiling and optical satellite measurements. We find a strong correlation between the locations of increased seismicity at nearby Tungnafellsjökull volcano and regions of increased tensile and Coulomb stress changes. Our results suggest that stress transfer during this major event has resulted in earthquake triggering at the neighbouring Tungnafellsjökull volcano by unclamping faults within the associated fissure swarm. This work has immediate application to volcano monitoring; to distinguish the difference between stress transfer and new intrusive activity.
... Jellinek 22 & DePaolo, 2003; Caricchi et al., 2014) or magma mixing driving catastrophic destabilisation 23 of the magmatic system (e.g. Saunders et al., 2012; Albino and Sigmundsson, 2014; Till et 24 al., 2015). External triggers, outside of the magmatic system include tectonic activity (e.g. 25 Allan et al., 2012) or changing stress-state (e.g. ...
Ascension Island, in the south Atlantic is a composite ocean island volcano with a wide variety of eruptive styles and magmatic compositions evident in its ~ 1 million year subaerial history. In this paper, new observations of a unique zoned fall deposit on the island are presented; the deposit gradationally changes from trachytic pumice at the base, through to trachy-basaltic andesite scoria at the top of the deposit. The key features of the eruptive deposits are described and are coupled with whole rock XRF data, major and trace element analyses of phenocrysts, groundmass glass and melt inclusions from samples of the compositionally-zoned fall deposit to analyse the processes leading up to and driving the explosive eruption. Closed system crystal fractionation is the dominant control on compositional zonation, with the fractionating assemblage dominated by plagioclase feldspar and olivine. This fractionation from the trachy-basaltic andesite magma occurred at pressures of ~ 250 MPa. There is no evidence for multiple stages of evolution involving changing magmatic conditions or the addition of new magmatic pulses preserved within the crystal cargo. Volatile concentrations range from 0.5 to 4.0 wt.% H2O and progressively increase in the more-evolved units, suggesting crystal fractionation concentrated volatiles into the melt phase, eventually causing internal overpressure of the system and eruption of the single compositionally-zoned magma body. Melt inclusion data combined with Fe–Ti oxide modelling suggests that the oxygen fugacity of Ascension Island magmas is not affected by degree of evolution, which concentrates H2O into the liquid phase, and thus the two systems are decoupled on Ascension, similar to that observed in Iceland. This detailed study of the zoned fall deposit on Ascension Island highlights the relatively closed-system evolution of felsic magmas at Ascension Island, in contrast to many other ocean islands, such as Tenerife and Iceland.
... Walter et al. 2014) or potentially interconnection between magma reservoirs (e.g. Albino and Sigmundsson 2014). The former explanation seems more likely based on the observation that post-eruption seismicity (31 August-7 October 2011) beneath Mallahle had significantly lower b values than that associated with Nabro (Hamlyn et al. 2014). ...
We present a synthesis of diverse observations of the first recorded eruption of Nabro volcano, Eritrea, which began on 12 June 2011. While no monitoring of the volcano was in effect at the time, it has been possible to reconstruct the nature and evolution of the eruption through analysis of re- gional seismological and infrasound data and satellite remote sensing data, supplemented by petrological analysis of erupted products and brief field surveys. The event is notable for the comparative rarity of recorded historical eruptions in the region and of caldera systems in general, for the prodi- gious quantity of SO2 emitted into the atmosphere and the significant human impacts that ensued notwithstanding the low population density of the Afar region. It is also relevant in understanding the broader magmatic and tectonic signifi- cance of the volcanic massif of which Nabro forms a part and which strikes obliquely to the principal rifting directions in the Red Sea and northern Afar. The whole-rock compositions of
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the erupted lavas and tephra range from trachybasaltic to trachybasaltic andesite, and crystal-hosted melt inclusions contain up to 3,000 ppm of sulphur by weight. The eruption was preceded by significant seismicity, detected by regional networks of sensors and accompanied by sustained tremor. Substantial infrasound was recorded at distances of hundreds to thousands of kilometres from the vent, beginning at the onset of the eruption and continuing for weeks. Analysis of ground deformation suggests the eruption was fed by a shal- low, NW–SE-trending dike, which is consistent with field and satellite observations of vent distributions. Despite lack of prior planning and preparedness for volcanic events in the country, rapid coordination of the emergency response miti- gated the human costs of the eruption.
... These models are mostly used to explain the general behaviour of various geological and physical processes. For instance, detailed studies on problems such as the relation between the ratio of horizontal to vertical displacements and the aspect ratios of magma reservoir (Dieterich and Decker, 1975), the effect of rheological layering on the surface deformations (Trasatti et al., 2003;Manconi et al., 2007;Long and Grosfils, 2009), the initiation and evolution of ring-faults and collapse calderas (Komuro et al., 1984;Saunders, 2001;Walter, 2008;Holohan et al., 2011;Grosfils et al., 2015), propagation of dikes and sills (Dahm, 2000;Mériaux and Lister, 2002;Kühn and Dahm, 2004;Maccaferri et al., 2011), emplacement of radial and circumferential dikes (Chestler and Grosfils, 2013;Grosfils et al., 2015), magma chamber instability and failure (Sartoris et al., 1990;Grosfils, 2007;Hurwitz et al., 2009;Long and Grosfils, 2009;Grosfils et al., 2015), stress transfer and interaction of multiple magma bodies (Albino and Sigmundsson, 2014;Pascal et al., 2014), the link between eruptive activity and edifice growth (Pinel et al., 2010), stress trajectories due to edifice loading and formation and propagation of fractures (Chevallier and Verwoerd, 1988;Muller et al., 2001;Grosfils, 2007;Hurwitz et al., 2009), flank instability (Martel and Muller, 2000), have significantly contributed to the knowledge on these volcanic processes. The wide variety of these examples and also the detailed information that they provide reaffirms that the numerical models are indeed superior to the analytical models for forward modelling applications. ...
The advances in modern geodetic techniques such as the global navigation satellite system (GNSS) and synthetic aperture radar (SAR) provide surface deformation measurements with an unprecedented accuracy and temporal and spatial resolutions even at most remote volcanoes on Earth. Modelling of the high-quality geodetic data is crucial for understanding the underlying physics of volcano deformation processes. Among various approaches, mathematical models are the most effective for establishing a quantitative link between the surface displacements and the shape and strength of deformation sources. Advancing the geodetic data analyses and hence, the knowledge on the Earth’s interior processes, demands sophisticated and efficient deformation modelling approaches. Yet the majority of these models rely on simplistic assumptions for deformation source geometries and ignore complexities such as the Earth’s surface topography and interactions between multiple sources. This thesis addresses this problem in the context of analytical and numerical volcano deformation modelling. In the first part, new analytical solutions for triangular dislocations (TDs) in uniform infinite and semi-infinite elastic media have been developed. Through a comprehensive investigation, the locations and causes of artefact singularities and numerical instabilities associated with TDs have been determined and these long-standing drawbacks have been addressed thoroughly. This approach has then been extended to rectangular dislocations (RDs) with full rotational degrees of freedom. Using this solution in a configuration of three orthogonal RDs a compound dislocation model (CDM) has been developed. The CDM can represent generalized volumetric and planar deformation sources efficiently. Thus, the CDM is relevant for rapid inversions in early warning systems and can also be used for detailed deformation analyses. In order to account for complex source geometries and realistic topography in the deformation models, in this thesis the boundary element method (BEM) has been applied to the new solutions for TDs. In this scheme, complex surfaces are simulated as a continuous mesh of TDs that may possess any displacement or stress boundary conditions in the BEM calculations. In the second part of this thesis, the developed modelling techniques have been applied to five different real-world deformation scenarios. As the first and second case studies the deformation sources associated with the 2015 Calbuco eruption and 2013–2016 Copahue inflation period have been constrained by using the CDM. The highly anisotropic source geometries in these two cases highlight the importance of using generalized deformation models such as the CDM, for geodetic data inversions. The other three case studies in this thesis involve high-resolution dislocation models and BEM calculations. As the third case, the 2013 pre-explosive inflation of Volcán de Colima has been simulated by using two ellipsoidal cavities, which locate zones of pressurization in the volcano’s lava dome. The fourth case study, which serves as an example for volcanotectonics interactions, the 3-D kinematics of an active ring-fault at Tendürek volcano has been investigated through modelling displacement time series over the 2003–2010 time period. As the fifth example, the deformation sources associated with North Korea’s underground nuclear test in September 2017 have been constrained. These examples demonstrate the advancement and increasing level of complexity and the general applicability of the developed dislocation modelling techniques. This thesis establishes a unified framework for rapid and high-resolution dislocation modelling, which in addition to volcano deformations can also be applied to tectonic and humanmade deformations.
... To account for source interaction effects [44][45][46] , we use a finite element method as implemented by the COMSOL ® multiphysics software. We consider large box dimensions (90 km × 110 km × 40 km) with fixed surface at the bottom, free on the top and roller on the side. ...
It is not known whether clustered or aligned volcanic edifices at the Earth surface have connected magmatic systems at depth. Previously suggested by geological records of paired eruptions, volcano interconnectivity still lacks proper geodetic evidence. Here we use GPS time-series and deformation modeling to show how Aira caldera and Kirishima, two adjacent volcanic centers in Kagoshima graben (southern Japan), interacted during Kirishima unrest in 2011. Whereas Aira caldera had been inflating steadily for two decades, it deflated during the eruption of Kirishima which started with a large-volume lava extrusion. This deflation, which cannot be explained by stress changes, is interpreted as the result of magma withdrawal from the Aira system during the Kirishima replenishment phase. This study highlights the behavior of connected neighboring volcanic systems before and after a large eruption, and the importance of taking into account volcano interactions in eruption probability models.
... However, the internal structure of Eyjafjallajökull has thus far not been investigated with seismic tomography although various seismic studies have been carried out (Dahm and Brandsdóttir, 1997;Jónsdóttir et al., 2007;Tarasewicz et al., 2011Tarasewicz et al., , 2012aTarasewicz et al., , 2014Hjaltadóttir et al., 2015) and it has been investigated by electro-magnetic methods (Miensopust et al., 2014) revealing a low resistivity anomaly underneath Fimmvörduháls one year after the flank eruption, interpreted as the hot intrusive material of a flank intrusion. Until present, the internal material properties of Eyjafjallajökull have been approximated as a homogeneous and isotropic half-space Albino and Sigmundsson, 2014;Hjaltadóttir et al., 2015) in all numerical deformation models. ...
We present a shear-velocity model for the Eyjafjallajökull stratovolcano, based on ambient seismic noise tomography applied to seven months of data from six permanent stations and 10 temporary seismic stations, deployed during and after the 2010 volcanic unrest. Vertical components of noise were cross correlated resulting in 30 robust phase-velocity dispersion curves between 1.6 and 6.5. s in period, displaying a ± 20% variation in phase velocity beneath the volcano. The uneven distribution of noise sources, evaluated using signal-to-noise ratios, was estimated to cause less than 2% error in most curves. Sensitivity kernels showed resolution down to 10. km and the lateral resolution of the resulting phase-velocity maps was about 5. km. The model reveals east-west oriented high-velocity anomalies due east and west of the caldera. Between these a zone of lower velocity is identified, coinciding with the location of earthquakes that occurred during the summit eruption in April 2010. A shallow, southwest elongated low-velocity anomaly is located 5. km southwest of the caldera. The limited depth resolution of the shear-velocity model precludes detection of melt within the volcano.
... At the part of the fault plane closest to the magmatic intrusion, slip is close to 277pure right-lateral, so we estimate Coulomb stress change for right-lateral motion and find that Δσ C is 278 positive for early October pressurisation sources located at the northern side of the range required by 279 GPS measurements (10-30 kPa on southwestern corner of the fault, e.g.,Figure 5B-C). A more 280 complex source geometry (e.g. an opening sill or pressurising prolate ellipsoid) could potentially 281 generate quite different stress fields (seeAlbino & Sigmundsson, 2014), but these would also be 282 sensitive to position and trade-offs between depth and volume change or opening. It is therefore 283 possible, but not certain, that the static stress changes contributed to triggering the M w 5.6 earthquake. ...
Magma movement or reservoir pressurisation can drive swarms of low-magnitude volcano-tectonic earthquakes, as well as occasional larger earthquakes (> M 5) on local tectonic faults. Earthquakes > M 5 near volcanoes are challenging to interpret in terms of evolving volcanic hazard, but are often associated with eruptions, and in some cases enhance the ascent of magma. We present geodetic observations from the first episode of unrest known to have occurred near Chiles and Cerro Negro de Mayasquer volcanoes on the Ecuador-Colombian border. A swarm of volcano-tectonic seismicity in October 2014 culminated in a Mw 5.6 earthquake south of the volcanoes. Satellite radar data spanning this earthquake detects displacements that are consistent with dextral oblique slip on a reverse fault at depths of 1.4—3.4 km within a SSW-NNE trending fault zone that last ruptured in 1886. GPS station measurements capture ~ 20 days of uplift before the earthquake, probably originating from a pressure source ~10-15km south of Volcán Chiles, at depths exceeding 13 km. After the Mw 5.6 earthquake, uplift ceased and the rate of seismicity began to decrease. Potential mechanisms for this decline in activity include a decrease in the rate of movement of magma into the shallow crust, possibly caused by the restriction of fluid pathways. Our observations demonstrate that an earthquake triggered during volcanic unrest can inhibit magmatic processes, and have implications for the hazard interpretation of the interactions between earthquakes and volcanoes.
... Walter et al. 2014) or potentially interconnection between magma reservoirs (e.g. Albino and Sigmundsson 2014). The former explanation seems more likely based on the observation that post-eruption seismicity (31 August-7 October 2011) beneath Mallahle had significantly lower b values than that associated with Nabro (Hamlyn et al. 2014). ...
We present a synthesis of diverse observations of the first recorded eruption of Nabro volcano, Eritrea, which began on 12 June 2011. While no monitoring of the volcano was in effect at the time, it has been possible to reconstruct the nature and evolution of the eruption through analysis of regional seismological and infrasound data and satellite remote sensing data, supplemented by petrological analysis of erupted products and brief field surveys. The event is notable for the comparative rarity of recorded historical eruptions in the region, and of caldera systems in general; for the prodigious quantity of SO2 emitted into the atmosphere; and the significant human impacts that ensued notwithstanding the low population density of the Afar region. It is also relevant in understanding the broader magmatic and tectonic significance of the volcanic massif of which Nabro forms a part and which strikes obliquely to the principal rifting directions in the Red Sea and northern Afar. The whole-rock compositions of the erupted lavas and tephra range from trachybasaltic to trachybasaltic andesite, and crystal-hosted melt inclusions contain up to 3000 ppm of sulphur by weight. The eruption was preceded by significant seismicity, detected by regional networks of sensors, and accompanied by sustained tremor. Substantial infrasound was recorded at distances of hundreds to thousands of kilometres from the vent, beginning at the onset of the eruption and continuing for weeks. Analysis of ground deformation suggests the eruption was fed by a shallow, NW-SE-trending dike, which is consistent with field and satellite observations of vent distributions. Despite lack of prior planning and preparedness for volcanic events in the country, rapid coordination of the emergency response mitigated the human costs of the eruption.
Nevado del Ruiz is part of a large volcano complex in the northern Andes of Colombia. Interferometric synthetic aperture radar observations from the RADARSAT-2 satellite since 2011 show steady inflation of the volcano since 2012 at 3–4 cm/yr. The broad (>20 km) deformation pattern from both ascending and descending track data constrain source models for either point or spheroidal sources, both located at >14 km beneath the surface (mean elevation 4.2 km) and 10 km SW of Nevado del Ruiz, below nearby Santa Isabel Volcano. Stress change computations for both sources in the context of a compressive regional stress indicate that dikes propagating from the source should become trapped in sills, possibly leading to a more complex pathway to the surface and explaining the significant lateral separation of the source and Nevado del Ruiz Volcano.
Excess gas phase in magmas erupting explosively is well known worldwide. However, the origin of this gas phase, in excess of what can be dissolved in the erupting magma at depth, and the rate of gas accumulation is less well defined. The 2010 mildly explosive eruption at Eyjafjallajökull, Iceland, produced mingled tephra of benmoreitic and trachytic composition whereas alkali basalt was emitted during preceding flank eruption. Tephra of the first explosive phase are composed of three glass types, alkaline rhyolite, mixed benmoreite, and basalt, which suggests that the basaltic magma intruded a pre-existing rhyolitic magma chamber, and ultimately triggered the eruption. The mixed benmoreitic tephra (erupted on 15 and 17 April 2010) had large 210Po in excess of 210Pb [(210Po /210Pb) 0 = 1.88 ] at the time of eruption, and possibly a small 210Pb excess over its parent 226Ra. In contrast, the preceding flank eruption produced basalt with (210Po) 0 = 0, upon eruption, and the final trachyte had lost most of its 210Po during open-system degassing. The 210Po excess in the first erupted benmoreites is interpreted to result from 210Po degassing of basaltic magma and the accumulation of 210Po-enriched gas, either in the upper part of the basaltic intrusion, below the rhyolite-basalt interface, or in the pre-existing residual rhyolitic magma chamber. From a simple model of radon and polonium accumulation in the rhyolitic reservoir, the ratio of the mass of basalt magma degassing over the mass of magma accumulating the excess gas decreased from 20 to 15 over 2 days, implying zoned magma reservoir, with the uppermost and gas-richest part erupting first. The duration of pre-eruptive gas accumulation in this model is approximately one year. This corresponds closely to the initiation of a seismic swarm beneath Eyjafjallajökull, early June 2009, which was the first pre-eruptive signal detected. The coincidence between initiation of gas accumulation at relatively shallow depth and deeper seismicity strongly suggests that the excess gas phase originated from a basalt magma batch intruded at depth, and that this gas phase reached the surface approximately a year later.
Magma buoyancy is a mechanism usually neglected with respect to magma overpressure in eruption models constrained with geophysical data. In this study we provide evidence for a buoyancy-driven mechanism of magma ascent during the effusive phase of the 2008-2009 Chaitén rhyolitic eruption. We also constrain the volcano plumbing system with source models from InSAR and teleseismic data. InSAR data did not detect unambiguous evidence of pre-eruptive ground deformation up to 1 year before the eruption. Teleseismic data indicate that a dike opened twice during the first two days of the eruption, while InSAR data show that three sources deflated during the eruption. These include one dike and a sill during the onset of the eruption, and a spherical source halfway through it, all located at depths between 3 and 12 km. 75% of the total ground deformation of ∼0.5 m occurred during the first three weeks of the eruption. A Pléiades DEM show that the dome has a bulk volume of 1 km³, and was extruded with an exponential trend, but the lack of deformation during most of the effusive phase of the eruption implies that the dome extrusion did not result in depressurization of a magma reservoir. Instead, we show that the time series of extruded volume can be explained by magma ascending due its buoyancy instead of its overpressure. Further, the end of the first pulse of dome effusion in September 2008 can be explained by an increase in the dome surface load that equals the effect of the magma buoyancy. These results imply that in some cases ground deformation data alone cannot be used to forecast the temporal evolution of an eruption. They also call for the acquisition of denser time series of extruded volume, as a way to better constrain their evolution through time.
In this manuscript we present a new interpretation of the seismic series that accompanied eruptive activity off the coast of El Hierro, Canary Islands, during 2011–2013. We estimated temporal variations of the Gutenberg-Richter b-value throughout the period of analysis, and performed high-precision relocations of the pre- and syn-eruptive seismicity using a realistic 3D velocity model. Our results suggest that eruptive activity and the accompanying seismicity were caused by repeated injections of magma from the mantle into the lower crust. These magma pulses occurred within a small and well defined volume resulting in the emplacement of fresh magma along the crust-mantle boundary underneath El Hierro. We analyzed the distribution of earthquake hypocenters in time and space in order to assess seismic diffusivity in the lower crust. Our results suggest that very high earthquake rates underneath El Hierro represent the response of a stable lower crust to stress perturbations with pulsatory character, linked to the injection of magma from the mantle. Magma input from depth caused large stress perturbations to propagate into the lower crust generating energetic seismic swarms. The absence of any preferential alignment in the spatial pattern of seismicity reinforces our hypothesis that stress perturbation and related seismicity, had diffusive character. We conclude that the temporal and spatial evolution of seismicity was neither tracking the path of magma migration nor it defines the boundaries of magma storage volumes such as a mid-crustal sill. Our conceptual model considers pulsatory magma injection from the upper mantle and its propagation along the Moho, We suggest, within this framework, that the spatial and temporal distributions of earthquake hypocenters reflect hydraulic fracturing processes associated with stress propagation due to magma movement.
Volcanic eruptions often occur simultaneously or tap multiple magma reservoirs. Such lateral interactions between magmatic systems are attributed to stress changes or hydraulic connections but the precise conditions under which coupled eruptions occur have yet to be quantified. Here we use interferometric synthetic aperture radar satellite data to analyse the surface deformation generated by volcanic unrest in the Kenyan Rift. We identify several magma sources located at depths of 2-5 km; importantly, sources that are spaced less than about 10 km apart interact, whereas those spaced more than about 25 km apart do not. However, volcanoes up to 25 km apart have interacted in the geologic past. Thus, volcanic coupling is not simply controlled by the distance between the magma reservoirs. We then consider different tectonic settings globally, including intraplate volcanoes such as Hawaii and Yellowstone, arc volcanism in Alaska and Chile, and other rift settings, such as New Zealand, Iceland and Afar. We find that the most closely spaced magmatic interactions are controlled by the extent of a shallow crystal mush layer, stress changes can couple large eruptions over distances of about 20-40 km, and only large dyke intrusions or subduction earthquakes could generate coupled eruptions over distances of about 50-100 km.
Applying seismic interferometry and network covariance matrix-based analyses to detect and locate the source of volcanic tremor during the 2010 Eyjafjallajökull effusive flank and explosive-effusive summit eruptions has provided new insights into this iconic event. The tremor source locations derived from the network covariance matrix approach were spatially distinct during the two eruptions. The tremor was radiated between the surface and 5–6 km depth during the effusive flank eruption, including an apparently progressive upward migration in early April 2010, but was strictly confined to the surface during the summit eruption. Each phase of the summit eruption left a distinct fingerprint in the seismic records. Effusive phases radiated continuous tremor between 0.6 and 5 Hz, whereas explosive phases produced tremor in a more pulsating fashion over a wider frequency band (0.2–10 Hz). A period of intermittent tremor bursts (called banded tremor) on 15 April, associated with formation of a new vent at the summit, was most likely generated by magma-gas-meltwater interaction within a subglacial enclosure. The banded tremor ceased following an abrupt draining of the newly formed subglacial cauldron, resulting in a large slurry glacial meltwater flood (jökulhlaup). This study highlights the importance of new data processing methodologies for future monitoring of volcanic tremor in real-time.
In this article I present a review of InSAR observations of ground deformation at Cordón Caulle volcano, whose 2011–2012 VEI 4-5 eruption is the best scientifically observed and instrumentally recorded rhyolitic eruption to date. I document a complete cycle of pre-eruptive uplift, co-eruptive subsidence and post-eruptive uplift with InSAR data between March 2003 and May 2020, and produced by a complex interplay of magmatic processes. Pre-eruptive data show ~0.5 m of ground uplift in three distinct episodes between 2003 and 2011, with uplift rates between ~3 and ~30 cm/yr. The uplift was likely caused by magma injection resulting in pressurization of the magmatic system at depths of 4–9 km. Data spanning the first 3 days of the eruption show ~1.5 m of deflation produced by two distinct sources at 4–6 km depth located 18 km from each other and up to 10 km from the eruptive vent -- suggesting hydraulic connectivity of a large magma mush zone. A third source of deformation was recorded during the rest of the eruption at a depth of ~5 km, resulting in a total subsidence of ~4.2 m during the whole eruption. On a much smaller spatial scale (~25 km²), InSAR-derived digital elevation models recorded ~250 m of uplift in the area of the eruptive vent interpreted as the intrusion of a shallow laccolith during the first 2.5 months of the eruption and time averaged lava discharge rates up to ~150 m³/s. The co-eruptive time series of reservoir pressure drop and extruded volume follow exponential trends that can be explained by a model of magma reservoir depressurization and conduit flow. Since the end of the eruption, the surface of the volcano was uplifted ~1 m in a sequence of three transient episodes of unrest during 2012 and 2019, with uplift rates between 6 and 45 cm/yr and lasting between 0.5 and 3.2 years. These pulses can be modeled by the same source, a sub-horizontal sill at a depth of ~6 km. Viscoelastic relaxation is not significant on these time scales, hence I interpret these uplift signals as being produced by episodic pulses of magma injection in the crystal mush that likely underlies the volcano. The episodic and abrupt changes of the ground deformation suggest a restless trans-lateral magmatic system at depths of 4–9 km, and active across multiple spatial and temporal scales. Finally, I also discuss challenges of the InSAR technology that should be addressed to detect ground deformation on short time scales, particularly under the low coherence conditions of Cordón Caulle.
The Eyjafjallajokull volcano, one of the oldest active volcanoes in Iceland, is located in the volcanic flank zone of South Iceland, a few tens of kilometers south of the nearest branch of the mid-Atlantic plate boundary. It is an elongated, broad cone of about 1650 m height. A 100-200 m thick glacier covers the upper part of the volcano and its elliptical 2.5 km-wide summit crater or caldera. An E-W trending rift zone transects the volcano, but a few radial fissures are observed around the summit area. Eruptive fissures on the west flank are curved and tend to be aligned along the maximum gradient of the topography. The E-W orientation of the rift zone and the apparent correlation with the topography suggests strong influence of gravity. Dikes in the older parts of the volcano strike north-easterly and indicate a change in the stress orientation during the last 0.78 My. This change may be related to a southward propagation of the Eastern Volcanic Rift Zone of Iceland and the transfer of spreading from the Western to the Eastern Volcanic Rift Zone. We suggest that the anomalous orientation of the Eyjafjallajokull volcanic system is the result of preexisting topography and gravitational stresses when the volcanic edifice was built up unconformably on old oceanic crust. All known episodes of activity in Eyjafjallajokull have been accompanied by activity in the neighbouring volcano Katla. The most recent examples are the two thermal events, possibly subglacial eruptions, of 1999 and 2011 at Katla following the 1999 sill intrusion and 2010 eruption of Eyjafjallajokull. The coupling mechanism between the volcanoes remains enigmatic. One volcano may be triggered by the other by direct dike or sill injection. Furthermore, pressure perturbation in the mantle may affect the magma sources of both volcanoes.
Global warming causes retreat of ice caps and ice sheets. Can melting glaciers trigger increased volcanic activity? Since 1890 the largest ice cap of Iceland, Vatnajokull, with an area of similar to 8000 km(2), has been continuously retreating losing about 10% of its mass during last century. Present-day uplift around the ice cap is as high as 25 mm/yr. We evaluate interactions between ongoing glacio-isostasy and current changes to mantle melting and crustal stresses at volcanoes underneath Vatnajokull. The modeling indicates that a substantial volume of new magma, similar to 0.014 km(3)/yr, is produced under Vatnajokull in response to current ice thinning. Ice retreat also induces significant stress changes in the elastic crust that may contribute to high seismicity, unusual focal mechanisms, and unusual magma movements in NW-Vatnajokull.
New measurements, statistical analyses, and models support the
conjecture that a large earthquake can trigger subsequent volcanic
eruptions over surprisingly long distance and time scales.
The reported times of major eruptions since 1900 from the world's
nonsubmarine volcanoes have been compared at each location with the
phase of the various components of the solid earth tide. A correlation,
significant to the 5% level, was found between eruption times and the
fortnightly component of the tide for the total data set of 680
eruptions. The probability of eruption is greatest at times of maximum
tidal amplitude. For individual volcanoes significant peaks in eruption
probability occur also at phases other than at the fortnightly tidal
maximum. The volcanoes could be subgrouped in terms of petrology,
geographic location, local crustal deformation rates, and other
geophysical parameters. Subpopulations of andesitic and basaltic
eruptions each showed significant concentrations of events at the tidal
maximum. In addition, basalt eruptions had an equally well developed
concentration at the tidal minimum. In a detailed study of the Japanese
region, each volcano that characteristically erupted at or near the
fortnightly tidal maximum is located in an area having a negative
Bouguer anomaly, a large crustal thickness, and a small rate of
horizontal crustal deformation. Conversely, volcanoes in areas
characterized by thinner crusts and crustal deformation rates greater
than 3.0 cm/yr generally erupt at or near the fortnightly tidal maximum.
Thirteen dike intrusions in the Manda Hararo rift, Afar (Ethiopia), from September 2005 to June 2009, studied using an extensive interferometric synthetic aperture radar (InSAR) data set, provide insight into the mechanics of a major active rift. Kinematic inversions of InSAR data reveal that dikes opened by 0.8–3.5 m at an average 5 km depth, with volumes of 0.04–0.2 km3 (with up to 12 m opening and a volume greater than 1 km3 for the September 2005 megadike). Dikes have their source in a midsegment magma reservoir, which induces a local shallowing of the brittle-ductile boundary, presumably due to thermal weakening of the lithosphere. The smaller dikes in 2006–2009 were emplaced in regions of minimum opening of the September 2005 megadike, above the central magma reservoir. In contrast, the most voluminous dike intrusions in 2006–2009 occurred near the locus of the peak of maximum opening of the September 2005 megadike, ∼10 km north of the magma source. This may suggest that tension on the plate boundary was highest there, both prior to 2005 and possibly also after 2005. Evolution and distribution of normal stress on the plate boundary throughout the rifting episode may indicate that tension near to the magma reservoir is lower than toward segment ends. Average relief of normal stresses of tectonic origin coeval to dike intrusions is comparable with shear stress drops for earthquakes, presumably because dikes in the Manda Hararo rift are intruded at low magma pressure and high tectonic stress.
The activity at Mauna Loa volcano, Hawaii, is characterized by eruptive fissures that propagate into the Southwest Rift Zone (SWRZ) or into the Northeast Rift Zone (NERZ) and by large earthquakes at the basal decollement fault. In this paper we examine the historic eruption and earthquake catalogues, and we test the hypothesis that the events are interconnected in time and space. Earthquakes in the Kaoiki area occur in sequence with eruptions from the NERZ, and earthquakes in the Kona and Hilea areas occur in sequence with eruptions from the SWRZ. Using three-dimensional numerical models, we demonstrate that elastic stress transfer can explain the observed volcano-earthquake interaction. We examine stress changes due to typical intrusions and earthquakes. We find that intrusions change the Coulomb failure stress along the decollement fault so that NERZ intrusions encourage Kaoiki earthquakes and SWRZ intrusions encourage Kona and Hilea earthquakes. On the other hand, earthquakes decompress the magma chamber and unclamp part of the Mauna Loa rift zone, i.e., Kaoiki earthquakes encourage NERZ intrusions, whereas Kona and Hilea earthquakes encourage SWRZ intrusions. We discuss how changes of the static stress field affect the occurrence of earthquakes as well as the occurrence, location, and volume of dikes and of associated eruptions and also the lava composition and fumarolic activity.
The Kobe earthquake struck at the edge of the densely populated Osaka-Kyoto corridor in southwest Japan. We investigate how the earthquake transferred stress to nearby faults, altering their proximity to failure and thus changing earthquake probabilities. We find that relative to the pre-Kobe seismicity, Kobe aftershocks were concentrated in regions of calculated Coulomb stress increase and less common in regions of stress decrease. We quantify this relationship by forming the spatial correlation between the seismicity rate change and the Coulomb stress change. The correlation is significant for stress changes greater than 0.2-1.0 bars (0.02-0.1 MPa), and the nonlinear dependence of seismicity rate change on stress change is compatible with a state- and rate-dependent formulation for earthquake occurrence. We extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of Kobe and calculating the change in probability caused by these stress changes. Transient effects of the stress changes are incorporated by the state-dependent constitutive relation, which amplifies the permanent stress changes during the aftershock period. Earthquake probability framed in this manner is highly time- dependent, much more so than is assumed in current practice. Because the probabilities depend on several poorly known parameters of the major faults, we estimate uncertainties of the probabilities by Monte Carlo simulation. This enables us to include uncertainties on the elapsed time since the last earthquake, the repeat time and its variability, and the period of aftershock decay. We estimate that a calculated 3-bar (0.3-MPa) stress increase on the eastern section of the Arima-Takatsuki Tectonic Line (ATTL) near Kyoto causes fivefold increase in the 30-year probability of a subsequent large earthquake near Kyoto; a 2-bar (0.2-MPa) stress decrease on the western section of the ATTL results in a reduction in probability by a factor of 140 to 2000. The probability of a M,, = 6.9 earthquake within 50 km of Osaka during 1997-2007 is estimated to have risen from 5-6% before the Kobe earthquake to 7-11% afterward; during 1997-2027, it is estimated to have risen from 14-16% before Kobe to 16-22%.
The construction of a large volcanic edifice at Earth's surface generates stresses in the upper crust whose magnitude is comparable to those of tectonic stresses and overpressures within a magma chamber. We study how this affects eruption behavior. Analytical calculations are carried out in two dimensions for a cylindrical reservoir with an internal overpressure in an elastic half-space with an edifice at the surface. Different edifice shapes are considered, from shield volcanoes with gentle slopes to stratovolcanoes with steeper flanks. Without an edifice at the top, the hoop stress at the cavity walls reaches a maximum at two symmetrical points at some distance from the axis, away from the top of the chamber. With an edifice at the top, the maximum is reached at the top of the chamber, just beneath the edifice summit. This implies preferential failure of chamber walls at the axis and hence the focussing of volcanic activity through a central vent system. Tensile failure of the cavity walls occurs for a critical value of magma overpressure which depends on the dimensions of the edifice and on the depth and size of the cavity. For a small magma chamber beneath a large stratovolcano, the magmatic overpressure at the onset of eruption increases as the edifice grows and decreases following edifice destruction. These effects may explain why pressures recorded in phenocryst assemblages at Mount St. Helens, have varied over the past 4000 years as the edifice went through successive phases of growth and destruction.
A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. These expressions are particularly compact and systematically composed of terms representing deformations in an infinite medium, a term related to surface deformation and that is multiplied by the depth of observation point. Several practical suggestions to avoid mathematical singularities and computational instabilities are also presented. The expressions derived here represent power- ful tools both for the observational and theoretical analyses of static field changes associated with earthquake and volcanic phenomena.
We present InSAR observations of deformation due to an intrusion in the Eyjafjallajökull volcano, Southern Iceland, in 1994. More than 15 cm of deformation in the line of sight (LOS) direction is detected in a series of interferograms spanning a micro-earthquake swarm occurring in June 1994. The location of the seismicity is more than 6 km offset compared to the area of inferred maximum surface uplift. Through an inversion scheme we find that a horizontal sill intrusion experiencing variable opening of up to 0.36 m agrees well with the deformation data. The total intrusion volume is 0.017 km3. The northern periphery of the modeled intrusion fits well with the area of recorded seismicity, indicating a close connection. Several processes may be responsible. Our preferred explanation is that the earthquakes are caused by opening of a narrow magma channel from depth, feeding the sill.
The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite. The andesite phenocrysts have a range of textures and zonation patterns which suggest that non-uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.
We examine the potential triggering relationship between large earthquakes and methane mud volcano eruptions. Our data set consists of a 191-year catalog (1810–2001) of eruptions from 77 volcanoes in Azerbaijan, central Asia, supplemented with reports from mud volcano eruptions in Japan, Romania, Pakistan, and the Andaman Islands. We compare the occurrence of historical regional earthquakes (M > 5) with the occurrence of Azerbaijan mud volcano eruptions and find that the number of same-day earthquake/eruption pairs is significantly higher than expected if the eruptions and earthquakes are independent Poisson processes. The temporal correlation between earthquakes and eruptions is most pronounced for nearby earthquakes (within ∼100 km) that produce seismic intensities of Mercalli 6 or greater at the location of the mud volcano. This assumed magnitude/distance relationship for triggering observed in the Azerbaijan data is consistent with documented earthquake-induced mud volcano eruptions elsewhere. We also find a weak correlation that heightened numbers of mud volcano eruptions occur within 1 year after large earthquakes. The distribution of yearly eruptions roughly approximates a Poisson process, although the repose times somewhat favor a nonhomogenous failure rate, which implies that the volcanoes require some time after eruption to recharge. The volcanic triggering likely results from some aspect of the seismic wave's passage, but the precise mechanism remains unclear.
Arc volcanoes often erupt andesite that appears to have been stored in
reservoirs at shallow depth for protracted periods. As crystal-rich
andesite is close in density to upper crust, such storage may be quite
stable. Petrological evidence, and occasionally geological and
geophysical evidence as well, suggests that the immediate trigger for
eruption of the stored magma is injection of new magma into the
reservoir, presumably through dykes rising from depth. When the dyke
magma is more mafic than the stored andesite, effusive eruption
typically results. When the dyke magma is voluminous and more silicic,
the results are catastrophic, with production of discontinuously zoned
tephra deposits and caldera collapse. Contrasting end-members are
illustrated by the eruptions of Karymsky Volcano in 1996 and of Mt
Katmai in 1912.
Tectonic activity in the Hengill volcanic area in southwestern Iceland accelerated in July 1994 when an unusually persistent swarm of moderate earthquakes began. Although the two largest events in this sequence had magnitudes of 5.2 and 5.0, all the other events are smaller than magnitude 4.0. Yet crustal deformation at the rate of approximately 2 cm/yr since at least 1992 indicates that most of the activity is related to inflation of a magma chamber at depth, rather than earthquake faulting. To monitor this activity, we analyze synthetic aperture radar (SAR) images acquired by the ERS-1 and ERS-2 satellites between 1992 and 1998. By combining these phase images in interferometric pairs, we calculate interferograms which record the change in satellite- to-ground range along the line of sight between the acquisition epochs of the two images. Images acquired during the snow-free summer months remain coherent on Holocene lava flows, even after four or five years. Some of the interferograms show a discontinuity in the fringe pattern, which we interpret as 8 mm of (aseismic) slip on a 3-km-long segment of a N5 °E -striking normal fault, part of which had been mapped previously. This slip must have occurred between July 31 and September 3, 1995 (inclusive) and has been confirmed by observations in the field. The predominant signature in all the interferograms spanning at least one year, however, is concentric fringes centered just south of the Hromundartindur volcanic center. These we interpret as mostly vertical uplift caused by increasing pressure in an underlying magma source. The number of fringes is roughly proportional to the time interval spanned by the interferograms, suggesting that the uplift rate is relatively constant at approximately 2 cm/year. This result confirms that the increased magmatic and seismic activity observed since July 1994 continues through at least the summer of 1998. We model this signal as the deformation caused by the inflationary volume increase of a source buried in an elastic half-space. The volcanic source model which best fits the observed interferograms lies at 7 ± 1 km depth and remains in the same horizontal position at between 1992 and 1998 to within about 2 km. It yields 19 ± 2 mm/year of uplift between June 1992 and September, 1998, which is equivalent to increasing the volume of the source by ~ 10 6 m3/year.
We test the possibility of mechanical interaction between eight central volcanoes in the central part of the active Iceland rift zone. The average distance between the volcanoes is 30 km; all are thought to have shallow magma chambers, and many contain collapse calderas. We analyzed many finite-element models with the volcanoes subject to a tensile stress of 5 MPa (equal to the maximum in situ tensile strength of the crust) in a direction parallel to the spreading vector, N105°E. The results show zones between many nearby volcanoes where the tensile stresses exceed the in situ tensile strength of the crust. The results indicate that mechanical interaction between volcanoes in a pair, such as simultaneous dike emplacement, seismogenic faulting, and deformation, may be common in this part of Iceland, in agreement with observations.
The growth of large volcanoes is commonly interrupted by episodes of flank collapse that may be accompanied by catastrophic debris avalanches, explosive eruptions, and tsunamis. El Hierro, the youngest island of the Canary Archipelago, has been repeatedly affected by such mass-wasting events in the last 1 Ma. Our field observations and petrological data suggest that the largest and most recent of these flank collapses-the El Golfo landslide-likely influenced the magma plumbing system of the island, leading to the eruption of higher proportions of denser and less evolved magmas. The results of our numerical simulations indicate that the El Golfo landslide generated pressure changes exceeding 1 MPa down to upper-mantle depths, with local amplification in the surroundings and within the modeled magma plumbing system. Stress perturbations of that order might drastically alter feeding system processes, such as degassing, transport, differentiation, and mixing of magma batches.
Approximately 0.4% of explosive volcanic eruptions occur within a few days of large, distant earthquakes. This many “triggered” eruptions is much greater than expected by chance. Several mechanisms have been proposed to explain triggering through changes in magma overpressure, including the growth of bubbles, the advection of large pressures by rising bubbles, and overturn of magma chambers. Alternatively, triggered eruptions may occur through failure of rocks surrounding stored magma. All these mechanisms require a process that enhances small static stress changes caused by earthquakes or that can convert (the larger) transient, dynamic strains into permanent changes in pressure. All proposed processes, in addition to viscoelastic relaxation of stresses, can result in delayed triggering of eruptions, although quantifying the connection between earthquakes and delayed, triggered eruptions is much more challenging. Mud volcanoes and geysers also respond to distant earthquakes. Mud volcanoes that di...
1] Mt. Merapi is one of the most dangerous volcanoes in Indonesia, located within the tectonically active region of south-central Java. This study investigates how Mt. Merapi affected -and was affected by -nearby tectonic earthquakes. In 2001, a Mw6.3 earthquake occurred in conjunction with an increase in fumarole temperature at Mt. Merapi. In 2006, another Mw6.3 earthquake took place, concomitant with an increase of magma extrusion and pyroclastic flows. Here, we develop theoretical models to study the amount of stress transfer between the earthquakes and the volcano, showing that dynamic, rather than static, stress changes are likely responsible for the temporal and spatial proximity of these events. Our examination of the 2001 and 2006 events implies that volcanic activity at Mt. Merapi is influenced by stress changes related to remote tectonic earthquakes, a finding that is important for volcano hazard assessment in this densely inhabited area.
Pressure variations in a magma reservoir may cause deformation at the surface and a redistribution of the stress in the surrounding rock. In this study, we use two-dimensional numerical models and elaborate how magma chamber inflation and deflation affect the stress field around and surface displacement. We test how a pre-existing normal fault near the magma reservoir may influence the pattern of stress. We demonstrate the possibility of initiating both normal and reverse slip on faults during the inflation of the magma reservoir. The Coulomb failure stress changes are calculated during the periods of pressure variation. An increase of Coulomb failure stress can be predicted above and below the magma chamber during increasing magma chamber pressure that may encourage earthquakes. This process can produce cracks and fault growth encouraging magma propagation along the cracked zone. A different distribution of the stress change is expected in the case of subsequent deflation of the overpressured magma reservoir. In this case, seismicity is expected on a plane at equal depth than the magma chamber, laterally offset from the extent of the magma chamber. Magma could propagate laterally from the magma reservoir into zones where cracks have been generated, but only if the resolved shear stress on the fault is small compared with the excess magma pressure.
Eyjafjallajökull volcano, located in southern Iceland, is characterized by its quiet nature. Only about a handful of earthquakes
associated with the volcanic system had been detected prior to the 1990s. Earthquake swarms did, however, occur in 1994 and
1999. Here we investigate the spatio-temporal evolution of a magmatic intrusion associated with the 1999 earthquake swarm
via analysis of produced surface deformation. A series of interferometric synthetic aperture radar (InSAR) images, spanning
various periods of the intrusion, show that in 1999 surface deformation occurred mainly on the southern flanks of the volcano.
The deformation amounts to more than 20cm of range change. Inverse modeling resolves the deformation source to be a sill
intrusion at 6.3km depth. Sill opening was up to 1m and the total intruded volume amounts to ∼0.03km3. The InSAR data display a migration of the center of deformation through time, enabling us to create time-dependant sill-opening
models. Furthermore, we investigate the spatio-temporal distribution of earthquakes and find that the distribution supports
the InSAR derived model and additionally provides indications for a possible site of a feeder channel. Magmatic flow-rate
estimates indicate an initial intrusion rate of 4–6m3/s, declining over a few weeks.
A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. These expressions are particularly compact and systematically composed of terms representing deformations in an infinite medium, a term related to surface deformation and that is multiplied by the depth of observation point. Several practical suggestions to avoid mathematical singularities and computational instabilities are also presented. The expressions derived here represent powerful tools both for the observational and theoretical analyses of static field changes associated with earthquake and volcanic phenomena.
A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. Several practical suggestions to avoid mathematical singularities and computational instabilities are presented. -from Author
The major element chemistry of nine silicic tephras of historical age from Iceland is assessed as a key step in the development of the recent tephrochronology of the North Atlantic region. The tephras include the largest such layers produced by each of the five central volcanoes Hekla, Ö ræfajökull, Eyjafjallajökull, Torfajökull and Askja since the ninth century ad (H 1104, Ö1362, E 1821, Landnám tephra c. 870, A 1875) and four other tephras (H 1158, H 1510, H 1947, Ö1727). The determination of grain discrete major element chemistry of the glass fraction is a fundamental stage in the identification and correlation of tephra, and allows links to be made between Icelandic source areas (with precise dating evidence) and distal deposits elsewhere in the North Atlantic region. Although major element data can be used to discriminate between tephra layers produced by the different central volcanoes, on its own it cannot be used to identify all the Holocene layers produced by each central volcano. However, integration with other stratigraphic and chronological data can resolve ambiguous cases, permitting the confident identification of precise isochrones.
Reports a study of the distribution of volcanoes in 16 active plate margins, corresponding to a total of 479 volcanic systems. The active volcanic arcs are found to have a ribbon geometry with an average length/width ratio of around 10. The shape of the volcanic arc is compared to the shape of the associated trench by projecting one onto the older. The projection direction agrees with the direction of plate convergence. In each arc, the distribution of volcano spacing is best represented by a Gamma distribution which corresponds to randomly generated points in the same geometrical conditions. In order to explain how such distributions may be generated, the gravitational instability of a layer of buoyant liquid which is fed at a constant rate at the bottom of denser fluid is investigated. -from Authors
The eruptive activity at the neighbouring Hawaiian volcanoes, Kīlauea and Mauna Loa, is thought to be linked, despite both having separate lithospheric magmatic plumbing systems. Over the past century, activity at the two volcanoes has been anti-correlated, which could reflect a competition for the same magma supply. Yet, during the past decade Kīlauea and Mauna Loa have inflated simultaneously. Linked activity between adjacent volcanoes in general remains controversial. Here we present a numerical model for the dynamical interaction between Kīlauea and Mauna Loa, where both volcanoes are coupled by pore-pressure diffusion, occurring within a common, asthenospheric magma supply system. The model is constrained by measurements of gas emission rates, indicative of eruptive activity, and it is calibrated to match geodetic measurements of surface deformation at both volcanoes, inferred to reflect changes in shallow magma storage. Although an increase in the asthenospheric magma supply can cause simultaneous inflation of Kīlauea and Mauna Loa, we find that eruptive activity at one volcano may inhibit eruptions of the adjacent volcano, if there is no concurrent increase in magma supply. We conclude that dynamic stress transfer by asthenospheric pore pressure is a viable mechanism for volcano coupling at Hawai`i, and perhaps for adjacent volcanoes elsewhere.
Loa and Kilauea volcanoes, Hawaii, are thought to be coupled by pore
pressure diffusion through an asthenospheric melt layer. However,
abundant observations of independent activity of these volcanoes suggest
a more complicated relationship. Here we analyze surface deformation
data, deep seismicity and gas measurements, to reveal strong coupling of
these volcanoes between 2003 and 2008. In early 2005, we find a shift
from anticorrelation to correlation of magma-chamber inflation. The
shift is preceded by a seismic swarm in the mantle beneath Mauna Loa and
accompanied by a large silent slip event beneath the south flank of
Kilauea. This suggests that these volcanoes are coupled during
mantle-driven surges and that the 2005 silent slip event was triggered
by accelerated magma supply at Kilauea.
Crustal deformation by the M
w
9.0 megathrust Tohoku earthquake causes the extension over a wide region of the Japanese mainland. In addition, a triggered M
w
5.9 East Shizuoka earthquake on March 15 occurred beneath the south flank, just above the magma system of Mount Fuji. To access whether these earthquakes might trigger the eruption, we calculated the stress and pressure changes below Mount Fuji. Among the three plausible mechanisms of earthquake–volcano interactions, we calculate the static stress change around volcano using finite element method, based on the seismic fault models of Tohoku and East Shizuoka earthquakes. Both Japanese mainland and Mount Fuji region are modeled by seismic tomography result, and the topographic effect is also included. The differential stress given to Mount Fuji magma reservoir, which is assumed to be located to be in the hypocentral area of deep long period earthquakes at the depth of 15 km, is estimated to be the order of about 0.001–0.01 and 0.1–1 MPa at the boundary region between magma reservoir and surrounding medium. This pressure change is about 0.2 % of the lithostatic pressure (367.5 MPa at 15 km depth), but is enough to trigger an eruptions in case the magma is ready to erupt. For Mount Fuji, there is no evidence so far that these earthquakes and crustal deformations did reactivate the volcano, considering the seismicity of deep long period earthquakes.
Quantitative models of pressurized magma reservoirs are used extensively to gain insight into magma plumbing systems, surface displacements, eruption styles and the formation of structural features. Most studies performed to date assume an elastic rheology, but existing model formulations often disagree about what parameters are important and should be examined. As a consequence published modeling results can yield contradictory information. The goal of this paper is to provide a framework within which differences between existing elastic magma reservoir model results can be understood, with a specific focus upon where and under what conditions rupture of the wall occurs. Finite element models of tensile rupture of an internally pressurized ellipsoidal magma reservoir in an axisymmetric elastic half space illustrate that gravity plays a critical role in this process. Failure to incorporate gravitational loading correctly in many published models affects for example: application of corrections accounting for the presence of the free surface in analytical models; inferences about the internal pressure that a reservoir can sustain prior to rupture; conclusions about the importance of magma and host rock densities; and, predictions about the location at which rupture of the reservoir wall will occur and the favored style of intrusion. Analyses that effectively reduce magma reservoirs to a cavity within an unloaded elastic medium, inflated by only an excess pressure component, sacrifice important information and should not be used to interpret reservoir activity or to calibrate more advanced models of volcanic regions and phenomena; an exception to this rule occurs, however, when constraining the pressure that can be inferred from surface displacements for a reservoir of known geometry. In a gravitationally loaded model, the characteristics of the failure process are insensitive to geologically plausible variations in the tensile strength, shear modulus, density structure and gravitational acceleration. The initial results presented here, which will benefit from future expansion to include topography and other factors, can thus yield direct insight into magma reservoirs on Earth as well as those located within the crusts of other solar system bodies.
Results from a finite element model characterizing tensile rupture of an internally pressurized ellipsoidal magma reservoir within an axisymmetric elastic half space illustrate that gravity plays a critical role in this process. Failure to incorporate gravitational loading correctly, which is the case for most published models, affects for example: (a) application of corrections designed to account for the presence of the free surface in analytical models; (b) inferences about the internal pressure that a reservoir can sustain prior to rupture; (c) conclusions about the importance of neutral buoyancy, i.e. the relative host rock and magma density structures; and, (d) predictions about the location at which rupture of the reservoir wall will occur and the style of intrusion which will be favored. Analyses that reduce magma reservoirs to a cavity within an unloaded elastic medium, inflated by only an excess pressure component, sacrifice important information and should not be used to interpret reservoir activity or to calibrate more advanced models of volcanic regions and phenomena; an exception to this rule occurs, however, when constraining the pressure that can be inferred from surface displacements for a reservoir of known geometry. In a gravitationally loaded model, the characteristics of the failure process are insensitive to geologically plausible variations in the tensile strength, shear modulus, density structure and gravitational acceleration. As a result the half-space analysis presented here, which will benefit from future expansion to include topography and other factors, can yield insight into not only magma reservoirs on Earth but those thought to have formed within the crusts of Mars, Venus and other solar system bodies as well.
Geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the Earth's surface. Elastic modeling of the vertical ground-surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth. A mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike. Therefore, slip extending into or beyond the dike cavity must occur either (1) on faults that intersect the dike near its top, above the zone of dike-induced compression, or (2) on faults that slip ahead of the dike as it propagates laterally. Data from Iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike.
We present a model of the triggering of volcanic eruptions based on fractional crystallisation and oversaturation of volatile species in a shallow magma chamber. We calculate the overpressure in the chamber and consequent increase in its volume by deformation of the surrounding rocks as a function of the amount of crystallisation. When the overpressure reaches a value of twice the effective tensile strength of the volcanic edifice, eruption or emplacement of a dyke occurs, and the chamber returns to its original pressure and volume. We show the quantitative effects on the pressure history of the form of the solubility law (depending on the volatile species present), the crystallisation contraction and the presence of some initial mass of gas at t = 0. The most important of these is the solubility law. We show that, once saturated, the more soluble is the volatile species, the more important it is for the development of overpressure in the chamber. Only a few per cent fractional crystallization are required to cause overpressures equal to the fracture criterion for a pure H2O gas phase. A pure CO2 gas phase connot cause important overpressures because it is much less soluble. Only for the improbable case of a pure CO2 gas phase and a large crystallisation contraction do underpressures arise. We calculate the volumetric deformation of the surroundings and the erupted volume of lava as a fraction of the chamber volume. In the cases of Kilauea and Krafla volcanoes where the volume of the magma chamber is known approximately, our results are close to observations of both the amounts of tumescence and volumes of melt ejected from the chamber. In this model the repose time between eruptions is determined by the rate of crystallisation. Estimates for the time required to reach overpressures equal to the fracture criterion are on the order of a few years for basaltic melts and a few hundred years for more viscous systems. Over the course of many eruptions this model predicts an approximately constant output rate of lava. If the chamber is a closed system, after an eruption, the amount of liquid ejected is replaced by an equivalent volume of gas. The amount of gas builds up with each successive eruption and eventually the magma chamber roof can become unstable causing caldera collapse.
Although earthquakes and volcanic eruptions are each manifestations of
large-scale tectonic plate and mantle motions, it is usually thought
that the occurrences of these events are not directly related. There
have been some studies, however, in which triggering of volcanic
eruptions by earthquakes (remote from the volcano) has been proposed,.
The 1992 Landers (southern California) earthquake caused triggered
seismicity at very large distances, including the magmatically active
Long Valley caldera region which also experienced a significant
coincident deformation transient. Motivated by this demonstration of the
ability of a distant earthquake to disturb a volcanic system, and the
earlier studies of specific cases of eruption triggering, we examine
here the historical record of eruptions and earthquakes to see if there
are indeed significantly more eruptions immediately following large
earthquakes. We find that within a day or two of large earthquakes there
are many more eruptions within a range of 750km than would otherwise be
expected. Additionally, it is well known that volcanoes separated by
hundreds of kilometres frequently erupt in unison; the characteristics
of such eruption pairs are also consistent with the hypothesis that the
second eruption is triggered by earthquakes associated with the first.
Destruction of a volcanic edifice by landslides or phreatic explosions unloads the upper crust. Induced changes of stress field around, and of magmatic pressure within, a magma reservoir are investigated with an analytical model for the deformation of a liquid-filled cavity within an elastic half-space. Unloading affects the reservoir pressure, and hence the net result depends on how the liquid-filled reservoir responds to a change of remote stress. Magma compressibility is taken into account and may dampen changes of internal pressure in small volatile-rich reservoirs. The main consequence of edifice destruction is a decrease of magmatic pressure and stresses on the reservoir walls. In some cases, this may be responsible for dyke closure at the reservoir walls, which stops magma withdrawal and may prevent eruption. If an eruption does occur, edifice destruction affects the volume of magma erupted. Depending on edifice size, magma reservoir size and depth, the erupted volume may be smaller or larger than that which would be erupted with no damage to the edifice. These results suggest that major phreatic explosions may prevent magmatic eruptions.
The April 2010 eruption of Eyjafjallajökull volcano (Figure 1), located on Iceland's southern coast, created unprecedented disruptions to European air traffic during 15-20 April, costing the aviation industry an estimated $250 million per day (see the related news item in this issue). This cost brings into focus how volcanoes can affect communities thousands of miles away. Eyjafjallajökull rises to 1666 meters above sea level and hosts agricultural land on its southern slopes, with farms located as close as 7 kilometers from the summit caldera. In the past 1500 years, Eyjafjallajökull has produced four comparatively small eruptions. The eruption previous to 2010 began in December 1821 and lasted for over a year, with intermittent explosive activity spreading a thin layer of tephra (ash and larger ejected clasts) over the surrounding region. In contrast, the explosive 2010 eruption, sourced within the ice-capped summit of the volcano, so far is larger and characterized by magma of a slightly different composition. This may suggest that deep within the volcano, the 1821 magma source is mixing with new melt, or that residual melt from past intrusive events is being pushed out by new magma.
The large-scale volcanic lineaments in Iceland are an axial zone, which is delineated by the Reykjanes, West and North Volcanic Zones (RVZ, WVZ, NVZ) and the East Volcanic Zone (EVZ), which is growing in length by propagation to the southwest through pre-existing crust. These zones are connected across central Iceland by the Mid-Iceland Belt (MIB). Other volcanically active areas are the two intraplate belts of Öræfajökull (ÖVB) and Snæfellsnes (SVB). The principal structure of the volcanic zones are the 30 volcanic systems, where 12 are comprised of a fissure swarm and a central volcano, 7 of a central volcano, 9 of a fissure swarm and a central domain, and 2 are typified by a central domain alone.
Approximate solutions are obtained for the stress and displacement
fields due to a pressurized spherical cavity in an elastic half-space.
The solutions take the form of series expansions in powers of ɛ
= a/d, where a is the cavity radius and d is the depth. The
leading-order term in the expression for the surface uplift, which
arises at O(ɛ3), recovers the well-known result of
Mogi for the response to a point dilatation. The first higher-order
correction accounts for a cavity of finite size and thus offers the
possibility of fitting leveling data for not only the depth but also the
radius and pressure increment. However, this correction is of
O(ɛ6) and, consequently, is weak. The result provides
a formal explanation for the success of the point dilatation model in
representing uplift data even when it is known independently that
ɛ is not small. The higher-order correction causes the surface
uplift to fall off more rapidly in the radial direction, implying that a
fit of the point source solution tends to underestimate the depth d. In
contrast to the surface displacement, the stress field near the cavity
is affected profoundly by the proximity of the free surface. Three
higher-order corrections to the stress field are obtained, which result
in a uniformly valid approximation to O(ɛ5). The hoop
stress at the cavity exhibits a tensile maximum at the circle of
tangency with a cone with its apex at the free surface. This result
appears to be consistent with the locus of fractures radiating outward
from the magma body inferred by seismic methods in Long Valley,
California.
We investigate how surface load variations around volcanoes act on shallow magma chambers. Numerical calculations are carried out in axisymmetric geometry for an elliptical chamber embedded in an elastic medium. Magma compressibility is taken into account. For variable chamber shape, size and depth, we quantify how unloading events induce magmatic pressure change as well as variation of the threshold pressure required for dyke initiation at the chamber wall. We evaluate the triggering effect of these surface events on onset of eruptions and find it depends strongly on the surface load location and the magma chamber shape. We apply this model to two active Icelandic subglacial volcanoes: Grímsvötn and Katla. The 2004 eruption of Grímsvötn was immediately preceded by a jökulhlaup, a glacial outburst flood of 0.5km3. We show that this event may have triggered the eruption only if the system was very close to failure conditions. Katla volcano is covered by the Mýrdalsjökull ice cap. An annual cycle, with up to 6m change in snow thickness, occurs from winter to summer. As the seasonal snow load is reduced, a pressure decrease of the same order of magnitude as the load is induced within the magma storage zone. Our model predicts that, in the case of a spherical or horizontally elongated magma chamber, eruptions are more likely when the snow cover is smallest, which appears consistent with the fact that all the last nine major historical eruptions at Katla occurred during the summer period. The model predicts an increase in Coulomb stress around the caldera, up to 7km from its centre, during unloading periods, enough to trigger earthquakes. Stress due to snow load variations, with focusing of it in weak zones near the caldera boundary, is considered a contributing factor to seasonal seismicity observed beneath Mýrdalsjökull.
Injection of basic magma into acid magma causes superheating of the acid
magma and vigorous convection. Vesiculation induced by convection and
increased magma pressure fractures the volcanic edifice triggering an
explosive acid eruption. The 1875 plinian eruption of Askja, Iceland is
an example of an explosive eruption triggered by magma mixing.
This review begins by highlighting the observations of ancient and modern dikes that lay the groundwork for current modeling. The emphasis then shifts to the dominant physical processes involved - host rock fracture and deformation (elastic and inelastic), magma flow, and heat transfer. This review attempts to provide a framework for thinking about important but poorly understood processes such as dike initiation, the role of dike propagation in the ascent of granitic magmas, and earthquakes accompanying magma transport. -from Author
Injection of basaltic magmas into silicic crustal holding chambers and subsequent magma mingling or mix-ing is a process that has been recognised since the late sev-enties as resulting in explosive eruptions. Detailed recon-struction and assessment of the mixing process caused by such intrusion is now possible because of the exceptional time-sequence sample suite available from the tephra fallout of the 2010 summit eruption at Eyjafjallajökull volcano in South Iceland. Fallout from 14 to 19 April contains three glass types of basaltic, intermediate, and silicic composi-tions recording rapid magma mingling without homogeni-sation, involving evolved FeTi-basalt and silicic melt with composition identical to that produced by the 1821–1823 AD Eyjafjallajökull summit eruption. The time-dependent change in the magma composition suggests a binary mixing process with changing end-member compositions and pro-portions. Beginning of May, a new injection of primitive basalt was recorded by deep seismicity, appearance of Mg-rich olivine phenocrysts together with high sulphur dioxide output and presence of sulphide crystals. Thus, the com-position of the basaltic injection became more magnesian and hotter with time provoking changes in the silicic mix-ing end-member from pre-existing melt to the solid cara-pace of the magma chamber. Finally, decreasing proportions of the mafic end-member with time in the erupted mixed-magma demonstrate that injections of Mg-rich basalt was the motor of the 2010 Eyjafjallajökull explosive eruption, and Correspondence to: O. Sigmarsson (olgeir@raunvis.hi.is) that its decreasing inflow terminated the eruption. Signifi-cant quantity of silicic magma is thus still present in the in-terior of the volcano. Our results show that detailed sam-pling during the entire eruption was essential for deciphering the complex magmatic processes at play, i.e. the dynam-ics of the magma mingling and mixing. Finally, the rapid compositional changes in the eruptive products suggest that magma mingling occurs on a timescale of a few hours to days whereas the interval between the first detected magma injec-tion and eruption was several months.
It has been suggested that deglaciations have influenced volcanism in several areas around the world increasing productivity of mantle melting and eruptions from crustal magma chambers. However, the connection between glaciations and increased volcanism is not straightforward. Investigation of Ar–Ar, U–Pb, and 14 C ages of caldera-forming eruptions for the past million years in the glaciated arc of Kamchatka has lead to the observation that the majority of large-volume ignimbrites, which are associated with the morphologically preserved calderas, correspond in time with "maximum glacial" conditions for the past several glacial cycles. In the field, the main proof is related to the fact that glaciated multi-caldera volcanoes hosted thick glacial ice caps. Additional evidence comes from clustering Kamchatka-derived marine ash layers with glacial moraines in DSDP cores. Here we present a set of new results from numerical modelling using the Finite Element Method that investigate how the glacial load dynamic may affect the conditions for ring-fault formation in such glaciated multi-caldera volcanoes. Different scenarios were simulated by varying: (1) the thickness and asymmetric distribution of the existing ice cap, (2) the depth and size of the magmatic reservoir responsible for the subsequent collapse event, (3) the thickness and mechanical properties of the roof rock due to the alteration by hydrothermal fluids, (4) the existence of a deeper and wider magmatic reservoir and (5) possible gravitational failure triggered, in part, by subglacial rock mass build up and hydrothermal alteration. The results obtained indicate that: (1) Any ice cap plays against ring fault formation; (2) Asymmetric distribution of ice may favour the initiation of trap-door type collapse calderas; (3) Glacial erosion of part of volcanic edifice or interglacial edifice failure may facilitate subsequent ring fault formation; (4) hydrothermal system under an ice cap may lead to a quite effective hydrothermal rotting of the intracaldera roof rocks and hence to variations of their mechanical properties and inhibit/deflect ring fracture propagation; and (5) rock surface topography/load influenced by glacial erosion and ice volume change during the interstadials. Although, the analysis of the stress field may inform us about the possibility of ring-fracture initiation, it does not ensure its complete propagation. Parameters controlling this phenomenon are also discussed here. Overall, the maximal glacial time represent the most dynamic time in a multi-caldera volcano life (as compared to more quiet interglacial) promoting physical and chemical feedbacks. We consider that brief interstadial periods during maximal glacial creates most favourable conditions for initiation of caldera-forming eruption, largely through its influence on surface topography by glacial action, mass wasting, and influencing magma vesiculation/discharge as a function of rapidly changing overload.
Mt Etna lies on the footwall of a large normal fault system, which cuts the eastern coast of Sicily and crosses the volcano eastern flank. These faults are responsible for both large magnitude historical earthquakes and smaller damaging seismic events, closer to the volcano. We investigate here the two-way mechanical coupling between such normal faults and Mt Etna through elastic stress transfer. The comparison between eruptive sequences and historical seismicity reveals that the large earthquakes which struck the eastern Sicily occurred after long periods of activity along the Mt Etna rift zone. The larger the erupted lava volumes, the stronger the earthquake. The smaller earthquakes located on the eastern flank of the volcano occur during periods of rift zone eruptions. We point out that the seismicity rates are well correlated with the rate of erupted lava. By modelling elastic stress changes caused by earthquakes and eruptions in a 3-D elastic half-space, we investigate their interaction. Earthquake dislocations create a vertical stress gradient along fissures oriented perpendicular to the minimum compressive stress and compress shallow reservoirs beneath the volcano. This may perturb the magmatic overpressures in the Etna plumbing system and influence the transport and storage of the magma as well as the style of the eruptions. Conversely, the large rift zone eruptions increase up to several tenths MPa the Coulomb stress along the eastern Sicily normal fault system and may promote earthquakes. We show that the seismic activity of the normal faults that cut the eastern flank of the volcano is likely to be controlled by Coulomb stress perturbations caused by the voiding of shallow reservoirs during flank eruptions.
The migration and accumulation of magma beneath volcanoes often causes surface displacements that can be measured by geodetic techniques. Usually, deformation signals are explained using models with uniform mechanical properties. In this paper, we study surface displacements due to magma chamber inflation, using heterogeneous finite element models. We first present a systematic analysis of the influence of mechanical layering, showing that the stiffness contrast significantly affects the entity and the pattern of vertical and radial displacements. Second, as an example we apply the models to interpret ground displacements at Darwin volcano (Galápagos Islands) as revealed by InSAR data in the period 1992–1998. The considered models suggest that geodetic data interpreted using homogeneous models leads to underestimation of the source depth and volume change. Thus, we propose correction factors for the source parameters estimated by homogeneous models, in order to consider a range of variation due to mechanical layering as analysed in this study. The effect of the mechanical heterogeneities affects the correct understanding of geodetic data and also influences the evaluation of a volcanic hazard potential.
Since the end of the last century many investigations have been carried out concerning the question if stresses due to the tidal forcing of the earth do trigger earthquakes or volcanic activities. Since the publications are so numerous and the results so controversial a complete review is impossible. An attempt is made to describe the different methods of investigation applied in most of the searches for tidal triggering. Summaries of the most important papers and their results are given separately for the different types of geophysical events such as earthquakes, volcanic eruptions and volcanic shocks.
Understanding the state of stress within volcanoes and how it changes with active geophysical processes is important to assess when and where magma may propagate to the surface and initiate an eruption. Observations of volcanic deformation help to constrain parameters of models that can be used to describe these processes and to calculate the resulting stress changes within volcanoes. Here I use geodetic observations to constrain elastic dislocation models of magma accumulation and faulting at Sierra Negra volcano, Galápagos, in the years before the October 2005 eruption, which was the first eruption at Sierra Negra since 1979. Both InSAR and GPS data document a remarkable story of ~ 5 m pre-eruption uplift during 1992–2005, which was accompanied by at least three trapdoor faulting events on an intra-caldera fault system, in January 1998, April 2005, and just before the eruption in October 2005. The pattern of uplift observed in the InSAR data from different time periods during 1992–2005 is consistent with filling and pressurization of a 2.2 km deep sill under the caldera. Modeling results for the trapdoor faulting in April 2005, on the other hand, indicate that slip occurred on a thrust fault that dips steeply (71°) in towards the center of the caldera, and extends from the surface down to the sill at 2.2 km depth. Independently, the calculated stress changes caused by the inflating sill show that the Coulomb Failure Stress change (ΔCFS) at the fault's location is maximized on a 72° dipping thrust fault, agreeing almost exactly with the deformation modeling result, and implying that the inflation triggered the faulting. The calculations also show that the ΔCFS for optimally oriented faults (any strike and dip) is significantly larger in the southern part of the caldera, where the faulting took place, than at any other location. The trapdoor faulting in turn relieved the pressure within the sill, but caused compression to the south of the faults. This suggests that while the faulting provides a mechanism for the sill to thicken and postpone eruptions, it also prevents the sill from growing to the south.
Sheet intrusions are of two main types: local inclined (cone) sheets and regional dykes. In Iceland, the inclined sheets form dense swarms of (mostly) basaltic, 0.5–1 m thick sheets, dipping either at 20–50° or at 75–90° towards the central volcano to which they belong. The regional dykes are (mostly) basaltic, 4–6 m thick, subvertical, subparallel and form swarms, less dense than those of the sheets but tens of kilometres long, in the parts of the volcanic systems that are outside the central volcanoes. In both types of swarms, the intrusion intensity decreases with altitude in the lava pile. Theoretical models generally indicate very high crack-tip stresses for propagating dykes and sheets. Nevertheless, most of these intrusions become arrested at various crustal depths and never reach the surface to supply magma to volcanic eruptions. Two principal mechanisms are proposed to explain arrest of dykes and sheets. One is the generation of stress barriers, that is, layers with local stresses unfavourable for the intrusion propagation. The other is mechanical anisotropy whereby sheet intrusions become arrested at discontinuities. Stress barriers may develop in several ways. First, analytical solutions for a homogeneous and isotropic crust show that the intensity of the tensile stress associated with a pressured magma chamber falls off rapidly with distance from the chamber. Thus, while dyke and sheet injection in the vicinity of a chamber may be favoured, dyke and sheet arrest is encouraged in layers (stress barriers) at a certain distance from the chamber. Second, boundary-element models for magma chambers in a mechanically layered crust indicate abrupt changes in tensile stresses between layers of contrasting Young’s moduli (stiffnesses). Thus, where soft pyroclastic layers alternate with stiff lava flows, as in many volcanoes, sheet and dyke arrest is encouraged. Abrupt changes in stiffness between layers are commonly associated with weak and partly open contacts and other discontinuities. It follows that stress barriers and discontinuities commonly operate together as mechanisms of dyke and sheet arrest in central volcanoes.
The currently two most important surveying tools in volcano monitoring are observations of seismicity and surface deformation. Magma migration within the Earth's crust is frequently associated with seismic activity, often occurring as distinct earthquake swarms. Understanding the exact nature and significance of these swarms is important when evaluating volcanic crisis situations. There seems, however, to be no general correlation between the amount of seismic energy release, and the rate and volume of magma on the move, which may complicate an immediate risk evaluation.It has previously been shown that stressing rate appears as the controlling factor on the occurrence of seismicity during intrusion. However, in this paper we emphasize that this is only true under specific circumstances. Other factors influence the evolution and resulting seismic energy release, and each intrusion scenario needs to be evaluated separately.Three Icelandic cases, where seismic swarms have been confirmed through deformation measurements to be related to magmatic movements, were selected. We show how the relationship between volume change and resulting seismicity varies greatly between cases, and suggest that, in a general approach, the most important factor governing the level of seismic energy release is the background stress state. Tectonic setting, regional stresses and tectonic history are therefore all of great importance when evaluating a magmatically induced seismic crisis. The stressing rate only plays the dominant role when either a) stress in the intruded area is close to the yield strength before intrusion or possibly b) when the intrusion is extraordinarily voluminous and rapid.
Around 19°W64°N, in the Eastern Volcanic Zone of south Iceland, the southwestern extremity of the tholeiitic Veidivötn fissure swarm abuts the northern periphery of the mildly alkaline silicic central volcano Torfajökull. Effusive mixed-magma eruptions in this area have been initiated by crustal rifting associated with lateral injection of tholeiitic magma into the rhyolitic Torfajökull chamber. Mixed rocks, in which cm- to dm-sized mafic inclusions constitute of the rhyolite lava are distinguished from hybrid rocks which are thorough mixtures having an homogeneous fabric.