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TOUGH2-SEED: A COUPLED FLUID FLOW MECHANICAL-STATISTICAL MODEL FOR THE STUDY OF INJECTION-INDUCED SEISMICITY
Full text linkThermo-poro-visco-elastic response of a disk shaped inclusion
Full text linkThe study of deformation sources in volcanic and geothermal fields is a topic of great impor-
tance that generates a large debate in the scientific literature. A correct interpretation of the
deformation sources acting in a volcanic context is crucial to distinguish between the mechan-
ical effects due to the tectonic of the area, the intrusion of new magma and/or the mechanical
response of rocks to temperature or pore pressure changes. In the recent literature, thermo-
poro-elastic (TPE) inclusions were proposed as possible deformation sources that can explain
seismicity and displacements even in absence of the emplacement of new magma. In fact, TPE
inclusions allow us to compute the mechanical effects due to temperature and pore-pressure
changes brought by the arrival of hot and pressurized fluids permeating a closed volume. In the
present work, we improve the modellization of such deformation sources to include the effects
of viscoelasticity, which should be expected in high temperature and fluid saturated rocks due
to thermall y acti v ated and pressure-solution creep. The analytical thermo-poro-viscoelastic
(TPVE) solutions for a disc-shaped inclusion embedded in a uniform viscoelastic medium are
obtained through the correspondence principle. Our results can be useful to represent transient
effects of both deformation and stress fields that can occur in both volcanic and geothermal
areas, which would be difficult to explain otherwise. In fact, TPE inclusion models predict
that an increase of uplift occurs simultaneously with an increase of stress, and vice versa.
Instead, we shall see that a TPVE inclusion can provide an increase of uplift even in presence
of a strongly decreasing deviatoric stress. For this reason, a TPVE inclusion can be suitable
to represent a decrease in seismicity rate accompanied by an increase in surface uplift, as
observed, for example, during the ’82–’84 unrest phase of Campi Flegrei in Italy
Stress and deformation induced in layered media by cylindrical thermo-poro-elastic sources: An application to Campi Flegrei (Italy)
No full textIn recent literature thermo-poro-elastic (TPE) inclusions are proposed to contribute to deformation and stress in volcanic and hydrothermal areas. With respect to previous works, the present study implements the modeling improvements requested for a more realistic description of a TPE source and the embedding medium. Firstly we propose a numerical method to represent inclusions with an arbitrary geometry. In the case of cylindrical-shaped sources, the proposed approach allows us to study the effects of the source thickness on both deformation and stress fields. Moreover, we are allowed to consider depth dependent pore pressure and temperature changes within the inclusion, as expected during the transient stage of hot fluid propagation. Secondly, we take into account elastic stratification of the crust, in order to perform unbiased inversion of ground deformation data. The inversion of geodetic data collected during the 1982–84 unrest episode in the Campi Flegrei caldera (Italy), performed in a layered elastic medium, leads to a lower misfit and a deeper deformation source with respect to the one obtained inverting data in a homogeneous half-space. Furthermore, the geometry and location of the thermo-poro-elastic inclusion retrieved from the inversion in the layered medium leads to a shear stress distribution in good agreement with the relocated seismicity observed at Campi Flegrei. Most hypocenters are found to be located near and inside the TPE inclusion boundaries, where the greatest shear stress is predicted. With respect to previous works we also found that vertical gradients of pore pressure and temperature and elastic layering can promote both normal and thrust earthquakes within the inclusion
Fault dip variations related to elastic layering
Full text linkIn this paper, we model the crack growth in an elastic medium constituted by two welded half-spaces with different rigidities. We implement a 2-D boundary element method (BEM) computing shear and normal tractions acting on the crack and the slip accommodating stress drop from an arbitrary initial configuration to a final frictional configuration. The direction of crack growth follows the criterion of maximum energy release (strain and gravitational energy) provided that it overcomes the surface fracture energy and the work dissipated by friction. The energetic criterion leads to estimates of the dip angle of seismic faults depending on the amplitude of the initial stress and it includes the classical Anderson's results as a particular case. Moreover, in presence of a sharp rigidity contrast, the direction of crack growth is strongly deflected. The model simulates non-planar, complex, fault geometries, as in the case of detachment and listric faults and it explains the increase of dip angles for both normal and reverse faults, when they enter soft sedimentary layers
TO SEEP OR NOT TO SEEP? SOME CONSIDERATIONS REGARDING WATER INFILTRATION IN VOLCANIC LAKES
No full textThe interaction between displacements and water level changes due to natural and anthropogenic effects in the Po Plain (Italy): the different point of view of GNSS and piezometers
No full textLike other sedimentary plains, the Po Plain in Northern Italy has largely subsided due to natural processes and human activities. Displacements of the Earth surface of hydrological origin are caused by groundwater changes, which in turn, are expected to be related to rainfall changes. In the Bologna metropolitan area (located in the Southeastern border of Po Plain), the 2010 politic decision of dismissing civil water supply from the groundwater withdrawal has provided us the opportunity to test a methodology for the retrieval of an anthropic effect in two different data sets: vertical displacements measured by continuous GNSS sites and piezometric water table fluctuations. The data sets have been analyzed by means of the Principal Component Analysis (PCA) and compared to rainfall time series from the Po Plain rain gauges. Several piezometers undergo a clear increase in the water level following the withdrawal decrease. Differently, the anthropic induced surface displacements are significantly smaller than the ones induced by rainfall. Accordingly, without a multivariate analysis such an effect on vertical displacements would have remained hidden in the raw time series. Only looking at the spatial distribution of the principal components we have highlighted that anthropic effects are local and present even in GNSS data, entailing for the 2010 case a decrease of about 4 mm/y of vertical velocity in some sites closest the withdrawal wells. Moreover, the multivariate analysis allowed us to assess that, on time scales larger than months, the rainfall-related hydrological response of vertical displacement depends on the geological setting. In the Apennines chain a water level increase causes subsidence, in agreement with the predictions of elastic models, whereas in the Po Plain it causes uplift, suggesting a dominant poro-elastic response, in agreement with the guess that the subsidence of the Po Plain is related to soil compaction. Our results suggest that in cases of the aquifers over-exploitation, a PCA analyses and the combined use of different observables such as GNSS, piezometers time series, rainfall data, geological setting allow getting a correct identification of the anthropic and natural signals
Detecting natural and anthropic effects on displacements and water level changes: a combined observation from rain gauges, piezometers and CGNSS
Full text linkThe Po Plain (Northern Italy) has largely subsided due to natural processes and human activities. In particular, in order to reduce subsidence, in the Bologna metropolitan area a politic decision in 2010, imposed a significant reduction of civil water supply from groundwater withdrawal wells. The study area is characterized by an excellent monitoring activity which provides a good spatial and temporal distribution of data coming from continuous GNSS sites, piezometers and rain gauges.
In the present work we analyze both GNSS and piezometric data by means of the Principal Component Analysis (PCA). The results of the analysis are then compared with the rainfall time series measured by rain gauges. Thanks to the PCA analysis we can identify: i) a clear increase in the water level following the withdrawal decrease started in 2010 and ii) an anthropic induced surface displacement, which is smaller in magnitude than that induced by rainfall variations. Without the PCA analysis, such a small, but still significant, anthropic effect on vertical displacements would have remained hidden in the raw time series.
Our analysis reveals a decrease of about 4 mm/y of vertical velocity in some GNSS sites closest the withdrawal wells. We also found that on large time scales (> 1 month), the vertical displacement induced by rainfall strongly depends on the geological setting: in the mountains a water level increase causes subsidence (elastic response), whereas in the Po Plain it causes uplift (poro-elastic response). Thanks to the PCA analyses, the combined observations of different kind of instruments (GNSS, piezometers and rain gauges) and a basic knowledge of the geological context, we can correctly identify both the anthropic and natural signals on the data
Deformation and stress in hydrothermal regions: The case of a disk-shaped inclusion in a half-space
Full text linkHydrothermal regions are affected by a wide variety of phenomena, including ground inflation and deflation episodes. Among them, calderas offer the opportunity to study the complex interactions between magmatic processes at depth and permeable rocks saturated with fluids in the upper sedimentary layers. One of such regions is the Campi Flegrei caldera in southern Italy, where several source models have been applied over the years to reproduce the ground displacement and seismicity observed during the most recent phase of major unrest (1982–1984). The present work aims at introducing a new source model consisting of a thermo-poro-elastic inclusion embedded in a homogeneous poroelastic half-space. The inclusion is meant to represent a permeable rock layer stressed and strained by hot and pressurized volatiles released upward by an underlying magmatic reservoir and is modeled as a thin horizontal disk inside which a sudden change of temperature and pore pressure occurs. We provide semi-analytical solutions for the displacement and stress fields both within and outside the source and check them by comparison with those obtained through a fully numerical approach. Results provided by our model are compared with two other deformation source models often used to describe volcanic environments in terms of pressurized cavities describing a spherical magma chamber (Mogi source) or a sill-like magma intrusion (Fialko source). For the Campi Flegrei 1982–84 unrest, our model provides a better reproduction of ground deformation data and manages to explain the widespread presence of compressive focal mechanisms, since the stress field promoted both inside and outside the thermo-poro-elastic inclusion is very different from pressurized cavities
The ”Salse di Nirano” mud volcanoes: hints from gravity data
No full textMud volcanoes are distributed throughout the globe, both on- and offshore. Mud volcanism has been widely investigated from the geological, geophysical, and geochemical points of view. The study of mud volcanoes has important implications in energy resource exploration, geohazard identification, and greenhouse gas emissions assessment (mainly CH4 and CO2). Mud volcano eruptions are mainly driven by a gravitative instabilities and fluid overpressure, due to the overall low density of clay/water/gas mixtures with respect to surrounding units. The geohazard of mud volcanoes is to date underrated despite the violent eruptive examples occurred in the past. For instance, the eruption of the Piparo mud volcano (1997, island of Trinidad) damaged electrical and water infrastructures and killed animals and livestock. In 2014, the eruption of the Macalube di Aragona (Italy) mud volcano killed two children. The understanding of the mechanisms regulating mud volcanoes is, therefore, important also in terms of hazard evaluation. To date, a physical conceptual model of the Nirano Salse, Italy, ascribes the eruptions to the presence of over-pressurized fluids that are expelled from a main deep reservoir. The latter is put into communication with the surface due to the episodically reactivation of pre-existing faults or pipes. The debate about this conceptual model is still open. To improve our current understanding, a new high-resolution dataset of gravimetric data was acquired. Our goal is to provide an insight about the subsurface structure of the investigated domain. The gravimetric inversion aims to identify the structural setting of Nirano and the presence of gas traps and faults. The gravity inversion results indicate the existence of a low-density zone (1200-1500 m long, 100-200 m wide, 800 m deep) with an almost planar shape aligned along a NW-SE structural trend, typical of the Northern Apennines chain. This zone likely represents the intrusion of mud/gas in the damage zone of a sub-vertical fault, which feeds shallow fluid reservoirs
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