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Decades of studies at divergent plate margins have revealed networks of magmatic sills at the crust-mantle boundary. However, a lack of direct observations of deep magma motion limits our understanding of magma inflow from the mantle into the lower crust and the mechanism of sill formation. Here, satellite geodesy reveals rift-scale deformation caused by magma inflow in the deep crust in the Afar rift (East Africa). Simultaneous inflation of four sills, laterally separated by 10s of km and at depths ranging 9–28 km, caused uplift across a ~ 100-km-wide zone, suggesting the sills are linked to a common mantle source. Our results show the supply of magma into the lower crust is temporally episodic, occurring across a network of sills. This process reflects inherent instability of melt migration through porous mantle flow and may be the fundamental process that builds the thick igneous crust beneath magmatic rifts and rifted margins globally [ABSTRACT FROM AUTHOR]

Dykes are magma‐filled fractures propagating through the brittle crust. Understanding the physics of dyking process is essential to mitigate the volcanic hazard associated with the opening of new eruptive fissures at the surface. Often, physics‐based models view either fracturing of the host rock or viscous flow of the magma as the dominating energy sink during dyke propagation. Here, we provide a numerical model that captures the coupling of fracturing at the crack tip and the transport of a viscous fluid. Built with the boundary element technique, our model allows for computation of the shape and velocity of a growing fluid‐filled crack accounting for the viscosity of the fluid: the fluid flow induces a viscous pressure drop acting at the crack walls, and modifies the shape of the crack. The energy conservation equation provides the constraints to solve for the crack growth velocity, assuming that brittle fracturing and viscous flow are the main processes that dissipate energy. Using a parameter range that represents typical magmatic intrusions, we obtain crack shapes displaying some typical characteristics, including a tear‐drop head and an open tail that depend on rock rigidity, magma viscosity, and buoyancy. We show that viscous forces significantly contribute to the energy dissipated during the propagation of magmatic dykes. Applied to the 1998 intrusion at Piton de la Fournaise (La Réunion Island), we provide ranges of dyke lengths and openings by adjusting the numerical velocity to the one deduced from the migration of volcano‐tectonic events. Plain Language Summary: Magma is a viscous fluid that can propagate through the crust by fracturing rocks and flowing through them. These magma‐filled fractures are called dykes. Magma pressure is the force ensuring the opening of the fracture and maintaining the flow of magma. Although physics‐based models provide simplified but reliable representations of dykes, few consider both fracture creation and viscous flow simultaneously. Here, we present a new model for magmatic dyke propagation, implementing viscous flow equations into an existing model, such that the dyke shape, trajectory, and velocity are determined together. We show that the viscous dissipation within the magma is a major contribution to the energy balance for a range of viscosity and velocity values which are typical for magmatic intrusions. The dyke shapes obtained with our model compare well with the ones obtained with previous models. The velocity derived from the model has been compared with that derived from the spatio‐temporal evolution of seismic events recorded before the 1998 eruption at Piton de la Fournaise. Our new modeling scheme may represent a step forward in predicting the timing and location of future eruptions at monitored volcanoes. Key Points: We present a new modeling scheme to compute the shape and velocity of a growing fluid‐filled crackOur magmatic dykes show a tear drop head and open tail, on a wide range of propagation velocitiesWe reproduce the velocity and fit important parameters for the 1998 Piton de la Fournaise intrusion [ABSTRACT FROM AUTHOR]

Surface deformation accompanying dike intrusions is dominated by uplift and horizontal motion directly related to the intrusions. In some cases, it includes subsidence due to associated magma reservoir deflation. When reservoir deflation is large enough, it can form, or reactivate preexisting, caldera ring‐faults. Ring‐fault reactivation, however, is rarely observed during moderate‐sized eruptions. On February 21, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 m of coeruptive uplift, as measured by InSAR, synthetic aperture radar correlation, and Multiple Aperture Interferometry. Here, we show that an average of ∼40 cm of slip occurred on a normal caldera ring‐fault during this moderate‐sized (VEI < 3) event, which intruded a volume of ∼24 × 106 m3 and erupted ∼9.3 × 106 m3 of lava (DRE). Using the 3D Mixed Boundary Element Method, we explore the stress change imposed by the opening dike and the depressurizing reservoir on a passive, frictionless fault. Normal fault slip is promoted when stress is transferred from a depressurizing reservoir beneath one of Ambrym's main craters. After estimating magma compressibility, we provide an upper bound on the critical fraction (f = 7%) of magma extracted from the reservoir to trigger fault slip. We infer that broad basaltic calderas may form in part by hundreds of subsidence episodes no greater than a few meters, as a result of magma extraction from the reservoir during moderate‐sized dike intrusions. Plain Language Summary: Many volcanoes feature large depressions, called calderas. Calderas form when enough magma leaves a deep reservoir, and the solid rock lid above this reservoir can no longer support its own weight. Caldera faults, or cracks surrounding the reservoir which extend from the reservoir to Earth's surface, form as the lid collapses. Ambrym volcano (Vanuatu) has a 12‐km wide caldera, and researchers propose it formed during an explosive eruption 2,000 years ago. However, in 2018, Ambrym's caldera sunk along caldera faults during a nonexplosive eruption. This observation questions whether an explosive eruption was necessary to form Ambrym's caldera in the first place. Furthermore, in February 2015, an eruption 10 times smaller than in 2018 also caused the ground to sink along caldera faults. Utilizing ground motion data obtained from satellite radar systems to model magma reservoir outflow and fault displacement, we conclude that, in 2015, the ground sank along caldera faults. This sinking is explained by the removal of as little as <7% of the stored magma from the reservoir. We therefore propose that Ambrym's wide caldera may have formed as a result of many frequently occurring medium‐sized eruptions. This challenges the thought that wide calderas mainly form as a result of large eruptions. Key Points: Ground displacement at Ambrym in February 2015 was caused by a dike intrusion, deflating reservoir, and normal slip on a caldera ring‐faultExtracting at most 7% of the magma from Ambrym's reservoir suffices to reactivate the caldera ring‐faultsNormal slip along Ambrym's ring‐fault can occur during moderate‐sized eruptions, resulting in subsidence and further caldera development [ABSTRACT FROM AUTHOR]

On January 12, 2020, Taal volcano, Philippines, erupted after 43 years of repose, affecting more than 500,000 people. Using interferometric synthetic aperture radar (InSAR) data, we present the pre‐ to post‐eruption analyses of the deformation of Taal. We find that: (1) prior to eruption, the volcano experienced long‐term deflation followed by short‐term inflation, reflecting the depressurization‐pressurization of its ∼5 km depth magma reservoir; (2) during the eruption, the magma reservoir lost a volume of 0.531 ± 0.004 km3 while a 0.643 ± 0.001 km3 lateral dike was emplaced; and (3) post‐eruption analyses reveal that the magma reservoir started recovery approximately 3 weeks after the main eruptive phase. We propose a conceptual analysis explaining the eruption and address why, despite the large volume of magma emplaced, the dike remained at depth. We also report the unique and significant contribution of InSAR data during the peak of the crisis. Plain Language Summary: Taal volcano in the Philippines erupted on January 12, 2020. Here, we present the pre‐, co‐, and post‐eruption data, model, and analyses using interferometric synthetic aperture radar (InSAR) data acquired by various satellite systems. We find that: (1) prior to the eruption, the volcano experiences a sequence of long‐term (>1 year) deflation followed by short‐term (≤1 year) inflation as a result of the depressurization‐pressurization of its ∼5 km depth magma reservoir; (2) during the eruption, the magma reservoir lost a volume of 0.531 ± 0.004 km3 while a 0.643 ± 0.001 km3 lateral dike was emplaced; and (3) post‐eruption analyses reveal that the magma reservoir is in recovery starting ∼3 weeks after the main eruptive phase. We propose a conceptual analysis to explain the 2020 Taal eruption and the dike emplacement. We also report the unique and significant contribution of remote sensing data, particularly InSAR during the peak of the crisis. Key Points: We present a comprehensive interferometric synthetic aperture radar (InSAR)‐based data, analyses, and models of Taal's pre‐ to post‐eruptive stateDuring the eruptive crisis, Taal's magma reservoir lost 0.531 ± 0.004 km3 of volume while a 0.643 ± 0.001 km3 lateral dike was emplacedLow‐latency InSAR‐derived products provided crucial and significant information to PHIVOLCS during the January 2020 eruptive event [ABSTRACT FROM AUTHOR]

Mixed‐mode fluid‐filled cracks represent a common means of fluid transport within the Earth's crust. They often show complex propagation paths which may be due to interaction with crustal heterogeneities or heterogeneous crustal stress. Previous experimental and numerical studies focus on the interplay between fluid overpressure and external stress but neglect the effect of other crack parameters. In this study, we address the role of crack length on the propagation paths in the presence of an external heterogeneous stress field. We make use of numerical simulations of magmatic dike and hydrofracture propagation, carried out using a two‐dimensional boundary element model, and analogue experiments of air‐filled crack propagation into a transparent gelatin block. We use a 3‐D finite element model to compute the stress field acting within the gelatin block and perform a quantitative comparison between 2‐D numerical simulations and experiments. We show that, given the same ratio between external stress and fluid pressure, longer fluid‐filled cracks are less sensitive to the background stress, and we quantify this effect on fluid‐filled crack paths. Combining the magnitude of the external stress, the fluid pressure, and the crack length, we define a new parameter, which characterizes two end member scenarios for the propagation path of a fluid‐filled fracture. Our results have important implications for volcanological studies which aim to address the problem of complex trajectories of magmatic dikes (i.e., to forecast scenarios of new vents opening at volcanoes) but also have implications for studies that address the growth and propagation of natural and induced hydrofractures. Plain Language Summary: Fluids move within the Earth by means of different mechanisms. One of the most relevant mechanisms, particularly for magma transport within the lithosphere, is the propagation through fluid‐filled fractures: the fluid (or magma) can create its own path through the crustal rocks by fracturing them. If the density of the fluid is lower than the density of the rocks, the fluid would be pushed upward by buoyancy (similarly to a gas bubble in water). However, the propagation path followed by these fluid‐filled fractures may be complex. This may be due to several factors, including the forces (stresses) acting within the crust because of plate tectonic or because of remarkable topographic features. Here we make use of computer simulations and laboratory experiments to test how fluid‐filled fractures interact with such crustal stresses. We quantify how the competition between (i) crustal stresses, (ii) fluid (or magma) pressure, and (iii) the length of a fluid‐filled fracture may affect its direction of propagation. We define a critical range of values for a parameter which may help identifying the path of a fluid‐filled fracture propagating through the Earth crust. Our results may have important implications for volcanological studies which aim to forecast scenarios of new eruption locations. Key Points: We make use of analogue experiments and numerical simulations to study the propagation path of fluid‐filled cracks in interaction with crustal stressesWe show and quantify how the competition between crustal stresses, fluid pressure, and crack length affect the path of fluid‐filled cracksWe provide a critical range of values for a parameter which may help predicting the propagation path of a fluid‐filled crack [ABSTRACT FROM AUTHOR]

Magma propagation is an unsteady process controlled by magma‐crust interaction. To provide information on its dynamics, we invert complementary ground deformation data spanning the 8 hr preceding the 26 May 2016 eruption at Piton de la Fournaise (PdF) volcano (La Réunion, France). Data are inverted using 3‐D boundary element models combined with a Monte Carlo inversion method. The final geometry of the displacement source is determined based on four interferograms spanning the whole propagation phase while the dynamics of the propagation is inferred from temporal inversion of continuous Global Navigation Satellite System (GNSS) data, using the final geometry as an a priori to constrain the source. The best modeled magma path consists in a 2,700‐m‐long sill located 800 m above sea level and connected to the eruptive fissure by a subvertical dike. The quick opening of the horizontal part of the intrusion could have been favored by limited flank sliding during the early stage of propagation. The intrusion then stalled for ∼5 hr, while pressure increased slightly, until final upward propagation and eruption. Volume budget suggests that the eruption was fed by a single batch of magma quickly disconnected from its source. The delay prior to the eruption may reflect a limited magma supply. Finally, two mechanisms, potentially acting together, might have favored the eruption: a driving role of magmatic gas and/or, as often observed at Piton de la Fournaise, an eastward flank slip. Plain Language Summary: Basaltic magma stored beneath volcanoes reaches the surface by fracturing the Earth's crust. As experienced in May 2018 at Kilauea volcano (Hawaii, USA), magma can travel kilometers from the reservoir and fissure opening may threaten man‐made structures. Anticipating where and when eruptive fissures open requires better understanding of the factors controlling magma propagation. During the 26 May 2016 eruption of Piton de la Fournaise, Réunion Island,the preeruptive crisis spanned 8hr25min from the first signal recorded by the observatory to the eruption onset.We determine the magma paths and propagation timing, which led to this eruption using complementary satellite data of ground surface displacement, combining radar interferometry, which provides high spatial resolution, with GPS, which provides high temporal resolution. We highlight complex magma propagation within the subareal volcano, showing two direction changes, an arrest and an acceleration. Flank slip and magma degassing seem to play a key role in controlling both the geometry and the timing. Based on this scenario, this event was close to turn into a failed eruption as there was a 5‐hr pause in propagation before magma finally reached the surface. Understanding such unusual eruptions is a challenge for observatories as it may lead to repeated "false" alerts. Key Points: Magma feeding the May 2016 eruption propagated laterally as a sill before turning into a dikeThe sill propagation is stepwise with an initial acceleration followed by a 5‐hr pauseThe eruption was fed by a single batch of magma quickly disconnected from its source [ABSTRACT FROM AUTHOR]

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Surges are common at all the major ice caps in Iceland. Ice masses of gigatons may shift from the upper part of the outlet glacier towards the terminus in a few months, advancing the glacier front by up to several kilometres. The advancing ice front may be up to 100 m thick, increasing the load on crustal rocks correspondingly. We use the observed change in crustal loading during a surge of the western part of the Vatnajökull ice cap, Iceland, during 1993–1995 and the corresponding elastic crustal deformation, surveyed with interferometric synthetic aperture radar, to investigate the material properties of the solid Earth in this region. Crustal subsidence due to the surge reaches ∼75 mm at the edge of the Síðujökull outlet glacier. This signal is mixed with a broad uplift signal of ∼12 mm yr−1, relative to our reference area, caused by the ongoing retreat of Vatnajökull in response to climate change. We disentangle the two signals by linear inversion. Finite element modelling is used to investigate the elastic Earth response of the surge, as well as to confirm that no significant viscoelastic deformation occurred as a consequence of the surge. The modelling leads to estimates of the Young's modulus and Poisson's ratio of the underlying Earth. Comparison between the observed and modelled deformation fields is made using a Bayesian approach that yields the estimate of a probability distribution for each of the free parameters. Residuals indicate a good agreement between models and observations. One-layer elastic models result in a Young's modulus of 43.2–49.7 GPa (95 per cent confidence) and Poisson's ratio of 0–0.27, after removal of outliers. Our preferred model, with two elastic layers, provides a better fit to the whole surge signal. This model consists of a 1-km-thick upper layer with an average Young's modulus of 12.9–15.3 GPa and Poisson's ratio of 0.17, overlying a layer with an average Young's modulus of 67.3–81.9 GPa and Poisson's ratio of 0.25. [ABSTRACT FROM AUTHOR]

The coseismic surface displacement field and slip distribution at depth due to the Kashmir earthquake (Mw = 7.6, 2005) have been analysed by different authors using subpixel correlation of synthetic aperture radar (SAR) images and optical images, teleseismic analysis, GPS measurements, as well as in situ field measurements. In this paper, first, we use 23 sets of measurement from subpixel correlation of SAR images and differential interferometry to retrieve the 3-D coseismic surface displacement field. The obtained horizontal and vertical components along the fault trace are then compared, respectively, to equivalent measurements obtained from subpixel correlation of two optical ASTER images and in situ field measurements. Second, the coseismic fault geometry parameters and slip distribution at depth are estimated. In addition to the one segment slip model as reported in previous work, a two segments slip model that better fits the surface fault break is proposed. The improvement of the two segments slip model in interpreting the measured displacement field is highlighted through comparison of residuals of both slip models. Taking advantage of differential interferometry measurements that provide precise and continuous information in the far field of the fault, firstly, a wedge thrust according to Bendick et al. to the Northwest of the main rupture built on our two segments model is tested. According to the obtained results, the residual of the two segments main rupture plus wedge thrust model is slightly smaller than the residual of the two segments model to the Northwest of the Balakot–Bagh fault. Secondly, we test the sensitivity of our slip model to the presence of slip along a décollement as evidenced by Jouanne et al. through post-seismic analysis. The results indicate that the estimations of the coseismic displacement field and slip distribution in this paper are not significantly biased by such post-seismic displacement and that most coseismic displacement is located on a ∼40° NE-dipping fault, as previously reported. [ABSTRACT FROM AUTHOR]

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 0.5 km3. 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 6 m 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 7 km 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. [ABSTRACT FROM AUTHOR]

Surface displacements induced by ice load variation through time are calculated by spatial integration of Green's function for both end-members: an elastic half-space and a thick elastic plate lying over an inviscid mantle. The elastic half-space model allows the consideration of displacements caused by short-term (seasonal) variations. The thick plate model describes the final relaxed state. The transition between these two stages is dominated by an effective relaxation time which depends on mantle viscosity. This behaviour is considered to estimate displacements induced by long-term load changes (ice retreat over decades). We apply these methods to the Mýrdalsjökull ice cap, Iceland, where an annual cycle in ice load occurs as well as a gradual ice retreat as a consequence of climate warming. Seasonal vertical displacements measured from 2000 to 2006 at two continuous GPS stations located near the edge of Mýrdalsjökull ice cap fit well to a model of an elastic response to the annual variation in ice load. A comparison of model displacements and observations provides a minimum value of for the effective static local value of the Young's modulus. We infer long-term displacements induced by ice retreat over the last 115 yr using a combination of the instantaneous elastic response and the final relaxed state. Results are compared to GPS measurements used to monitor the Katla volcano lying beneath the Mýrdalsjökull ice cap. A forward model considering an elastic thickness of 5 km can explain a fraction of the uplift recorded from 1999 to 2004, but it cannot account for the observed horizontal velocities. The study confirms that magma inflow is required to explain observed inflation of the Katla volcano 1999–2004. [ABSTRACT FROM AUTHOR]