Common mode signals and vertical velocities in the great Alpine area from GNSS data

We study time series of vertical ground displacements from continuous GNSS stations to investigate the spatial and temporal contribution of different geophysical processes to the time-varying displacements that are superimposed on vertical linear trends across the European Alps. We apply a multivariate statistics-based blind source separation algorithm to both GNSS displacement time series and to ground displacements associated with atmospheric and hydrological loading processes, as obtained from global reanalysis models. This allows us to associate each retrieved geodetic vertical deformation signal with a corresponding forcing process. Atmospheric loading is the most important one, reaching amplitudes larger than 2 cm. Besides atmospheric loading, seasonal displacements with amplitudes of about 1 cm are associated with temperature-related processes and with hydrological loading. We find that both temperature and hydrological loading cause peculiar spatial features of GNSS ground displacements. For example, temperature-related seasonal displacements show different behaviour at sites in the plains and in the mountains. Atmospheric and hydrological loading, besides the first-order spatially uniform feature, are associated also with NS and EW displacement gradients. We filter out signals associated with non-tectonic deformation from the raw time series to study their impact on both the estimated noise and linear rates in the vertical direction. While the impact on rates appears rather limited, given also the long-time span of the time-series considered in this work, the uncertainties estimated from filtered time-series assuming a power law + white noise model are significantly reduced, with an important increase in white noise contributions to the total noise budget. Finally, we present the filtered velocity field and show how vertical ground velocities are positively correlated with topographic features of the Alps.

Minimum Detectable Mass and Volume Fluxes During Magmatic Recharge at High Prominence Volcanoes: An Application to Erciyes Dağ Volcano (Turkey)

Magma reservoir recharge is widely recognised as a precursor of eruptive activity. However, the causative relationships between reservoir rejuvenation and surface observables such as gravitational potential field changes and ground deformation are still poorly understood. At intermediate and silicic intra-plate volcanoes where crustal mechanical heterogeneity combined with high-prominence are expected to fundamentally affect the crustal stress and strain relationship, protracted period of repose and absence of monitoring data raise questions about the detectability of magma recharge. Here we report results from integrated geodetic forward modelling of ground displacements and gravity changes from reservoir recharge at Erciyes Dağ, a large prominence (∼2,800 m), yet poorly studied, stratovolcano of the Central Anatolian Volcanic Province in Turkey. The most recent eruption at ∼7000 BC, close proximity to the Kayseri Metropolitan Area and absence of dedicated volcano monitoring set a precedent to explore stealth magmatic processes at the volcano. Using finite element analysis we systematically explore the influence of subsurface mechanical heterogeneities and topography on surface deformation and gravity changes from magmatic recharge of Erciyes Dağ’s reservoir. We show that whilst crustal heterogeneity amplifies ground displacements and gravity variations, the volcano’s substantial prominence has the opposite effect. For generic reservoir pressure and density changes of 10 MPa and 10 kg m−3 predicted vertical displacements vary by a factor of 5 while residual gravity changes vary by a factor of 12 between models ignoring topography or mechanical heterogeneity and those that do not. We deduce reservoir volume and mass changes of order 10–3 km3 and 1010 kg, respectively, at the detectability limit of conventional surveying techniques at the volcano. Though dependent on model assumptions, all results indicate that magma recharge at Erciyes Dağ may go undetected at fluxes 1) sufficient to maintain an active reservoir containing eruptable magma and 2) similar to those reported for intermediate/silicic volcanoes with repose times of 100–1,000s of years (e.g., Parinacota) and persistently active mafic volcanoes such as Mt. Etna and Stromboli. Our findings may be utilised to inform integrated geodetic and gravimetric monitoring at Erciyes Dağ and other large prominence silicic volcanoes and could provide early insights into reservoir rejuvenation with implications for the development of disaster risk reduction initiatives.

Ground Displacements due to the Deformations of Shallow Tunnels with Arbitrary Cross Sections in Soft Ground

Nowadays, a huge number of shield-driven tunnels with noncircular cross sections are constructed in urban areas all around the world. However, the ground displacements associated with tunneling still form a difficult issue, especially for noncircular tunnels. In this study, an analytical solution is derived to estimate the ground displacements induced by the deformations of shallow noncircular tunnels in soft ground. First, a solution for the stresses and displacements around a deep tunnel in a full plane is formulated by imposing a specified convergence pattern over the cavity boundary. Subsequently, this solution is validated using finite element simulations in a case study of an elliptical tunnel with four different convergence patterns. Afterward, the solution in the full plane is extended to a half plane using the virtual image technique to estimate the ground displacements around shallow tunnels. The solution is also validated using finite element simulations.

Simplified Analytical Method for Predicting the Lateral Ground Displacements due to Shield Tunnelling

Earth pressure balance or slurry shield tunnelling will squeeze the subsoils and lead to lateral outward ground displacement. However, current methods to estimate the shield tunnelling-induced ground displacements generally use the methods based on the face unsupported tunnelling (e.g., New Austrian tunnelling and open shield excavation), which cannot predict the lateral ground movement due to shield tunnelling. In this paper, a novel simplified analytical method is proposed to predict the ground lateral displacement during the shield advancing process. The key shield tunnelling operation factors, including the additional pressure of cutter head, the friction forces around shield body, the back-fill grouting pressure, and the soil volume loss are all considered. The lateral ground displacements induced by the four former factors are calculated by using Mindlin’s solutions. The soil volume loss-induced lateral ground displacement is calculated by employing the expression introduced by Pinto and Whittle. Combining with the displacement obtained from all the factors, the analytical method for lateral ground displacement induced by shield tunnelling is obtained. The applicability of the proposed analytical approach is verified with three well-documented case histories involving slurry shield and EPB shield machines.

Could the Liquiñe-Ofqui fault zone promote the 2011 Cordon Caulle eruption ?

<p>The 2011-2012 Cordon-Caulle eruption was the largest subaerial eruption of the 21th century. An inflation captured from InSAR between 2007 and 2009 was related to a volume of magma injection too small to have triggered this eruption. Here, we benefit from SAR imagery acquired by ALOS-1, ENVISAT and SENTINEL-1 data, to analyze the temporal and spatial behavior of ground displacements before, during and after the eruption. We find that a similar prolate spheroidal source explains the data for the pre-eruptive and post-eruptive periods. Then we explore two tectonically-related hypotheses to explain the observed displacements during the explosive phase of the eruption. Therefore, first we model InSAR data using standard inversion models to evaluate how slip motion along specific structures explain surface observations. Our results show that the explosive phase's ground displacements could have been produced either by the collapse of the caldera and the graben overriding the reservoir, or by slip motion along a dextral-strike slip fault zone related to the North-South trending Liquiñe-Ofqui fault zone. Second, we use 3D numerical models and elasto-plasticity to assess the failure conditions along both structures resulting from an overpressure applied at the wall of the prolate-spheroidal reservoir. Our results show that a magma injection consistent with the 2007-2009  inflation signal rather promotes constriction at the roof of the reservoir, which tends to impede fluid flow towards the surface. The presence of a relatively weak graben-caldera structure in our models show that this constrictional area is enhanced. On the other hand, the elasto-plastic pattern resulting from the application of a dextral-slip motion along the LOFZ branch-fault generates a dilatational plastic zone that connects the reservoir wall to the surface, where it coincides with the location of the 2011 eruption. Hence we propose that the LOFZ branch-fault, weakened during the pre-eruptive inflation phase, destabilized and slipped two years later in a way that it served as open channels for fluid migration from the magma reservoir up to the surface.</p>