Rivalta, Eleonora

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.

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.

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.

Processes occurring in volcanic conduits, the pathways through which magma travels from its storage region to the surface, have a fundamental control on the nature of eruptions and associated phenomena. It has been well established that magma flows, crystallizes, degasses, and fragments in conduits, that fluids migrate in and out of conduits, and that seismic and acoustic waves are generated and travel within conduits. A better understanding of volcanic conduits and related processes is of paramount importance for improving eruption forecasting, volcanic hazard assessment and risk mitigation. However, despite escalating advances in the characterization of individual conduit processes, our understanding of their mutual interactions and the consequent control on volcanic activity is still limited. With the purpose of addressing this topic, a multidisciplinary workshop led by a group of international scientists was hosted from 25 to 27 October 2014 by the Pisa branch of the Istituto Nazionale di Geofisica e Vulcanologia under the sponsorship of the MeMoVolc Research Networking Programme of the European Science Foundation. The workshop brought together the experimental, theoretical, and observational communities devoted to volcanological research. After 3 days of oral and poster presentations, breakout sessions, and plenary discussions, the participants identified three main outstanding issues common to experimental, analytical, numerical, and observational volcanology: unsteadiness (or transience), disequilibrium, and uncertainty. A key outcome of the workshop was to identify the specific knowledge areas in which exchange of information among the subdisciplines would lead to efficient progress in addressing these three main outstanding issues. It was clear that multidisciplinary collaboration of this sort is essential for progressing the state of the art in understanding of conduit magma dynamics and eruption behavior. This holistic approach has the ultimate aim to deliver fundamental improvements in understanding the underlying processes generating and controlling volcanic activity.

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.

Magma is transported in the crust mainly by dike intrusions. In volcanic areas, dikes can ascend toward the free surface and also move by lateral propagation, eventually feeding flank eruptions. Understanding dike mechanics is a key to forecasting the expected propagation and associated hazard. Several studies have been conducted on dike mechanisms and propagation; however, a less in-depth investigated aspect is the relation between measured dike-induced deformation and the seismicity released during its propagation. We individuated a simple x that can be used as a proxy of the expected mechanical energy released by a propagating dike and is related to its average thickness. For several intrusions around the world (Afar, Japan, and Mount Etna), we correlate such mechanical energy to the seismic moment released by the induced earthquakes. We obtain an empirical law that quantifies the expected seismic energy released before arrest. The proposed approach may be helpful to predict the total seismic moment that will be released by an intrusion and thus to control the energy status during its propagation and the time of dike arrest.

Tectonic earthquake swarms (TES) often coincide with aseismic slip and sometimes precede damaging earthquakes. In spite of recent progress in understanding the significance and properties of TES at plate boundaries, their mechanics and scaling are still largely uncertain. Here we evaluate several TES that occurred during the past 20 years on a transform plate boundary in North Iceland. We show that the swarms complement each other spatially with later swarms discouraged from fault segments activated by earlier swarms, which suggests efficient strain release and aseismic slip. The fault area illuminated by earthquakes during swarms may be more representative of the total moment release than the cumulative moment of the swarm earthquakes. We use these findings and other published results from a variety oftectonic settings to discuss general scaling properties for TES. The results indicate that the importance of TES in releasing tectonic strain at plate boundaries may have been underestimated.

The Rhenish Massif in Central Europe, which includes the Eifel Volcanic Fields, has shown ongoing ground deformation and signs of possible unrest. A buoyant plume exerting uplift forces at the bottom of the lithosphere was proposed to explain such deformation; the hypothesis of (possibly concurrent) melt accumulation in the crust/lithospheric mantle has not been explored yet. Here, we test deformation models in an elastic half-space considering sources of varying aspect ratio, size and depth. We explore the effects of data coverage, noise and uncertainty on the inferred source parameters. We find that the observed deformation would require melt accumulation in sub-horizontal sill-like structures expanding at the rate of up to ∼0.045 km3/yr. We discuss feasibility, limitations and possible interpretations of our resulting models and elaborate on further observations which may help constrain the structure of the Rhenish Massif magmatic system.

The occurrence time of earthquakes can be anticipated or delayed by external phenomena that induce strain energy changes on the faults. ‘Anticipated’ earthquakes are generally called ‘triggered’; however, it can be controversial to label a specific earthquake as such, mostly because of the stochastic nature of earthquake occurrence and of the large uncertainties usually associated to stress modelling. Here we introduce a combined statistical and physical approach to quantify the probability that a given earthquake was triggered by a given stress-inducing phenomenon. As an example, we consider an earthquake that was likely triggered by a natural event: the M = 6.2 13 Jan 1976 Kópasker earthquake on the Grímsey lineament (Tjörnes Fracture Zone, Iceland), which occurred about 3 weeks after a large dike injection in the nearby Krafla fissure swarm. By using Coulomb stress calculations and the rate-and-state earthquake nucleation theory, we calculate the likelihood of the earthquake in a scenario that contains only the tectonic background and excludes the dike and in a scenario that includes the dike but excludes the background. Applying the Bayes’ theorem, we obtain that the probability that the earthquake was indeed triggered by the dike, rather than purely due to the accumulation of tectonic strain, is about 60 to 90 %. This methodology allows us to assign quantitative probabilities to different scenarios and can help in classifying earthquakes as triggered or not triggered by natural or human-induced changes of stress in the crust.

Volcanism in continental rifts is generally observed to shift over time from the inside of the basin to its flanks and vice versa, but the controls on these switches are still unclear. Here we use numerical simulations of dike propagation to test the hypothesis that the spatio-temporal evolution of rift volcanism is controlled by the crustal stresses produced during the development of the rift basin. We find that the progressive deepening of a rift rotates the direction of the principal stresses under the basin, deflecting ascending dikes. This causes an early shift of volcanism from the inside of the graben to its flanks. The intensification of this stress pattern, due to further deepening of the basin, promotes the formation of lower crustal sill-like intrusions that can stack under the rift, shallowing the depth at which dikes nucleate, eventually causing a late stage of in-rift axial volcanism. Given the agreement between our model results and observations, we conclude that the temporal shifts in the location of rift volcanism are controlled to first order by the elastic stresses developing in the crust as the rift matures. We thereby suggest that geodynamic models should account for elasticity and the redistribution of surface loads in order to effectively reproduce rift-related magmatism.