Description: We develop a mathematical model describing dyke propagation in proximity of an elastic discontinuity of the embedding medium. The dyke is modelled as a fluid-filled crack in plane strain configuration employing the boundary element method. The pressure gradient along the crack is assumed proportional to the difference between the densities of the host rock and the fluid. Mass conservation is imposed during propagation and fluid compressibility is taken into account. The path followed by the crack is found by maximizing the total energy release, given by the sum of the elastic and gravitational contributions. The mathematical simulations provide a sort of ‘refraction phenomenon’, that is a sudden change in the direction of propagation when the crack crosses the boundary separating different rigidities: if the dyke enters a softer medium, its path deviates towards the vertical, if the dyke enters a harder medium its path deviates away from the vertical and may even become arrested as a horizontal sill along the interface, if the rigidity contrast is large. Gravitational energy plays a major role during propagation; in particular, in proximity of layer boundaries, this role is enhanced by the shift of the centre of mass due to changes of dyke shape. Mathematical results were validated by laboratory experiments performed injecting tilted air-filled cracks through gelatin layers with different rigidities.

Description: Published

Description: 1107-1123

Description: 2V. Struttura e sistema di alimentazione dei vulcani

Description: JCR Journal

Description: It has been posited that the 1975–1984 Krafla rifting episode in northern Iceland was responsible for a significant drop in the rate of earthquakes along the Húsavík-Flatey Fault (HFF), a transform fault that had previously been the source of several magnitude 6–7 earthquakes. This compelling case of the existence of a stress shadow has never been studied in detail, and the implications of such a stress shadow remain an open question. According to rate-state models, intense stress shadows cause tens of years of low seismicity rate followed by a faster recovery phase of rate increase. Here, we compare the long-term predictions from a Coulomb stress model of the rifting episode with seismological observations from the SIL catalog (1995–2011) in northern Iceland. In the analyzed time frame, we find that the rift-induced stress shadow coincides with the eastern half of the fault where the observed seismicity rates are found to be significantly lower than expected, given the historical earthquake activity there. We also find that the seismicity rates on the central part of the HFF increased significantly in the last 17 years, with the seismicity progressively recovering from west to east. Our observations confirm that rate-state theory successfully describes the long-term seismic rate variation during the reloading phase of a fault invested by a negative Coulomb stress. Coincident with this recovery, we find that the b-value of the frequency-magnitude distribution changed significantly over time. We conclude that the rift-induced stress shadow not only decreased the seismic rate on the eastern part of the HFF but also temporarily modified how the system releases seismic energy, with more large magnitude events in proportion to small ones. This behavior is currently being overturned, as rift-induced locking is now being compensated by tectonic forcing.

Description: Published

Description: 1109-1121

Description: 5V. Processi eruttivi e post-eruttivi

Description: JCR Journal

Description: Dikes and sills are the moving building blocks of the plumbing system of volcanoes and play a fundamental role in the accretionary processes of the crust. They nucleate, propagate, halt, resume propagation, and sometimes change trajectory with drastic implications for the outcome of eruptions (Sigmundsson et al., 2010). Their dynamics is still poorly understood, in particular when different external influencing factors are interacting. Here we apply a boundary element model to study dike and sill formation, propagation and arrest in different scenarios. We model dikes as finite batches of compressible fluid magma, propagating quasi-statically in an elastic medium, and calculate their trajectories by maximising the energy release of the magma-rock system. We consider dike propagation in presence of density layering, of density plus rigidity layering, of a weakly welded interface between layers, under the action of an external stress field (of tectonic or topographic origin). Our simulations predict sill formation in several situations: i) when a horizontal weak interface is met by a propagating dike; ii) when a sufficiently high compressive tectonic environment is experienced by the ascending dike and iii) in case a dike, starting below a volcanic edifice, propagates away from the topographic load with a low dip angle. We find that dikes halt and stack when they become negatively buoyant and when they propagate with low overpressure at their upper tip toward a topographic load. Neutral buoyancy by itself cannot induce dikes to turn into sills, as previously suggested.

Description: Published

Description: 39-50

Description: 2V. Struttura e sistema di alimentazione dei vulcani

Description: JCR Journal

Description: Magma stored beneath volcanoes is sometimes transported out of the magma chambers by means of laterally propagating dikes, which can lead to fissure eruptions if they intersect the Earth’s surface. The driving force for lateral dike propagation can be a lateral tectonic stress gradient, the stress gradient due to the topographic loads, the overpressure of the magma chamber, or a combination of those forces. The 2000 dike intrusion at Miyakejima volcano, Izu arc, Japan, propagated laterally for about 30 km and stopped in correspondence of a strike-slip system, sub-perpendicular to the dike plane. Then the dike continued to inflate, without further propagation. Abundant seismicity was produced, including five M 〉 6 earthquakes, one of which occurred on the pre-existing fault system close to the tip of the dike, at approximately the time of arrest. It has been proposed that the main cause for the dike arrest was the fault-induced stress. Here we use a boundary element numerical approach to study the interplay between a propagating dike and a pre-stressed strike-slip fault and check the relative role played by dike–fault interaction and topographic loading in arresting the Miyakejima dike. We calibrate the model parameters according to previous estimates of dike opening and fault displacement based on crustal deformation observations. Bycomputing the energy released during the propagation, our model indicates whether the dike will stop at a given location. We find that the stress gradient induced by the topography is needed for an opening distribution along the dike consistent with the observed seismicity, but it cannot explain its arrest at the prescribed location. On the other hand, the interaction of dike with the fault explains the arrest but not the opening distribution. The joint effect of the topographic load and the stress interaction with strike-slip fault is consistent with the observations, provided the pre-existing fault system is pre-loadedwith a significant stress, released gradually during the dike–fault interplay. Our results reveal how the mechanical interaction between dikes and faults may affect the propagation of magmatic intrusions in general. This has implications for our understanding of the geometrical arrangement of rift segments and transform faults in Mid Ocean Ridges, and for the interplay between dikes and dike-induced graben systems.

Description: Published

Description: 64-74

Description: 4V. Processi pre-eruttivi

Description: JCR Journal

Description: When continents are stretched over a long period of time, deep elongated rift valleys form at Earth’s surface and zones of ponded magma, centred beneath the rift, form at the crust–mantle boundary 1,2 . Ascending magma sometimes erupts within the rift valley 3,4 or, counterintuitively, at volcanic fields away from the rift valley that are offset by tens of kilometres from the source of magma at depth 5–8 . The controls on the distribution of this off-rift volcanism are unclear. Here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley, whereas others are confined to the valley. We find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure, caused by crustal thinning and faulting along the rift borders. When gravitational unloading dominates over tectonic stretching forces, dykes ascending from the ponded magma are steered towards the rift sides, eventually causing off-rift eruptions. Our model also predicts the formation of stacked magma sills in the lower crust above the magma-ponding zone, as well as the along-rift propagation of shallow dykes during rifting events, consistent with observations of magmatism and volcanism in rift zones globally. We conclude that rift topography-induced stress changes provide a fundamental control on the transfer of magma from depth to the surface.

Description: Published

Description: 2V. Struttura e sistema di alimentazione dei vulcani

Description: JCR Journal

Description: This article has been accepted for publication in Geophysical Journal International ©: The Authors 2015. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Uploaded in accordance with the publisher's self-archiving policy.

Description: ectonic earthquake swarms challenge our understanding of earthquake processes since it is difficult to link observations to the underlying physical mechanisms and to assess the hazard they pose. Transient forcing is thought to initiate and drive the spatio-temporal release of energy during swarms. The nature of the transient forcing may vary across sequences and range from aseismic creeping or transient slip to diffusion of pore pressure pulses to fluid redistribution and migration within the seismogenic crust. Distinguishing between such forcing mechanisms may be critical to reduce epistemic uncertainties in the assessment of hazard due to seismic swarms, because it can provide information on the frequency–magnitude distribution of the earthquakes (often deviating from the assumed Gutenberg–Richter relation) and on the expected source parameters influencing the ground motion (for example the stress drop). Here we study the ongoing Pollino range (Southern Italy) seismic swarm, a long-lasting seismic sequence with more than five thousand events recorded and located since October 2010. The two largest shocks (magnitude M w = 4.2 and M w = 5.1) are among the largest earthquakes ever recorded in an area which represents a seismic gap in the Italian historical earthquake catalogue. We investigate the geometrical, mechanical and statistical characteristics of the largest earthquakes and of the entire swarm. We calculate the focal mechanisms of the M l 〉 3 events in the sequence and the transfer of Coulomb stress on nearby known faults and analyse the statistics of the earthquake catalogue. We find that only 25 per cent of the earthquakes in the sequence can be explained as aftershocks, and the remaining 75 per cent may be attributed to a transient forcing. The b-values change in time throughout the sequence, with low b-values correlated with the period of highest rate of activity and with the occurrence of the largest shock. In the light of recent studies on the palaeoseismic and historical activity in the Pollino area, we identify two scenarios consistent with the observations and our analysis: This and past seismic swarms may have been ‘passive’ features, with small fault patches failing on largely locked faults, or may have been accompanied by an ‘active’, largely aseismic, release of a large portion of the accumulated tectonic strain. Those scenarios have very different implications for the seismic hazard of the area.

Description: Published

Description: 1553–1567

Description: 4T. Sismicità dell'Italia

Description: JCR Journal

Description: Knowledge of the spatio-temporal changes in the characteristics and distribution of subsurface fluids is key to properly addressing important societal issues, including: sustainable management of energy resources (e.g., hydrocarbons and geothermal energy), management of water resources, and assessment of hazard (e.g., volcanic eruptions). Gravimetry is highly attractive because it can detect changes in subsurface mass, thus providing a window into processes that involve deep fluids. However, high cost and operating features associated with current instrumentation seriously limits the practical field use of this geophysical method. The NEWTON-g project proposes a radical change of paradigm for gravimetry through the development of a fieldcompatible measuring system (the gravity imager), able to real-time monitor the evolution of the subsurface mass changes. This system includes an array of lowcosts microelectromechanical systems-based relative gravimeters, anchored on an absolute quantum gravimeter. It will provide imaging of gravity changes, associated with variations in subsurface fluid properties, with unparalleled spatio-temporal resolution. During the final ∼2 years of NEWTON-g, the gravity imager will be field tested in the summit of Mt. Etna volcano (Italy), where frequent gravity fluctuations, easy access to the active structures and the presence of a multiparameter monitoring system (including traditional gravimeters) ensure an excellent natural laboratory for testing the new tools. Insights from the gravity imager will be used to i) improve our knowledge of the causeeffect relationships between volcanic processes and gravity changes observable at the surface and ii) develop strategies to best incorporate the gravity data into hazards assessments and mitigation plans. A successful implementation of NEWTON-g will open new doors for geophysical exploration.

Description: Published

Description: 573396

Description: 1IT. Reti di monitoraggio e sorveglianza

Description: JCR Journal

Description: We developed a hybrid numerical model of dike propagation in two dimensions solving both for the magma trajectory and velocity as a function of the source overpressure, the magma physical properties (density and viscosity), and the crustal density and stress field. This model is used to characterize the influence of surface load changes on magma migration toward the surface. We confirm that surface loading induced by volcanic edifice construction tends both to attract the magma and to reduce its velocity. In contrast, surface unloading, for instance, due to caldera formation, tends to divert the magma to the periphery-retarding eruption. In both cases the deflected magma may remain trapped at depth. Amplitudes of dike deflection and magma velocity variation depend on the ratio between the magma driving pressure (source overpressure as well as buoyancy) and the stress field perturbation. Our model is then applied to the July 2001 eruption of Etna, where the final dike deflection had been previously interpreted as due to the topographic load. We show that the velocity decrease observed during the last stage of the propagation can also be attributed to the local stress field. We use the dike propagation duration to estimate the magma overpressure at the dike bottom to be less than 4 MPa. This approach can be potentially used to forecast if, where, and when propagating magma might reach the surface when having knowledge on the local stress field, magma physical properties, and reservoir overpressure.

Description: Published

Description: 1107-1125

Description: 2V. Struttura e sistema di alimentazione dei vulcani

Description: JCR Journal

Description: Calderas are topographic depressions formed by the collapse of a partly drained magma reservoir. At volcanic edifices with calderas, eruptive fissures can circumscribe the outer caldera rim, be oriented radially and/or align with the regional tectonic stress field. Constraining the mechanisms that govern this spatial arrangement is fundamental to understand the dynamics of shallow magma storage and transport and evaluate volcanic hazard. Here we show with numerical models that the previously unappreciated unloading effect of caldera formation may contribute significantly to the stress budget of a volcano. We first test this hypothesis against the ideal case of Fernandina, Galápagos, where previous models only partly explained the peculiar pattern of circumferential and radial eruptive fissures and the geometry of the intrusions determined by inverting the deformation data. We show that by taking into account the decompression due to the caldera formation, the modeled edifice stress field is consistent with all the observations. We then develop a general model for the stress state at volcanic edifices with calderas based on the competition of caldera decompression, magma buoyancy forces and tectonic stresses. These factors control: 1) the shallow accumulation of magma in stacked sills, consistently with observations; 2) the conditions for the development of circumferential and/or radial eruptive fissures, as observed on active volcanoes. This top-down control exerted by changes in the distribution of mass at the surface allows better understanding of how shallow magma is transferred at active calderas, contributing to forecasting the location and type of opening fissures.

Description: Published

Description: 257-293

Description: 2V. Struttura e sistema di alimentazione dei vulcani

Description: JCR Journal

Description: Understanding shallow magma transfer and the related vent distribution is crucial for volcanic hazard. Here we investigate how the stress induced by topographic scarps linked to normal faults affects the distribution of monogenic volcanoes at divergent plate boundaries. Our numerical models of dyke propagation below a fault scarp show that the dykes tend to propagate toward and erupt on the footwall side. This effect, increasing with the scarp height, is stronger for dykes propagating underneath the hanging wall side and decreases with the distance from the scarp. A comparison to the East African Rift System, Afar and Iceland shows that (1) the inner rift structure, which shapes the topography, controls shallow dyke propagation; (2) differential loading due to mass redistribution affects magma propagation over a broad scale range (10 0 –10 5 m). Our results find application to any volcanic field with tectonics- or erosion-induced topographic variations and should be considered in any volcanic hazard assessment.

Description: Published

Description: 7507–7512

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: JCR Journal