1,720,979 research outputs found
A Model for Crack Initiation in Solidifying Lava
No full textWe study the thermomechanical evolution of a stationary lava body cooling from the upper surface. The body progressively solidifies and develops a boundary layer made of an upper solid thermoelastic crust and a lower viscoelastic layer. The presented approach to the problem is one-dimensional and quasi-static. Under the assumption that the crust is a brittle solid with a given strength, we study the generation of tensile fractures. We consider a single fracture event and we assume that a fracture is produced when the thermal stress exceeds the strength over a certain depth interval, inferred from the observation of striae. The fracture propagates abruptly across the brittle layer, driven by the difference between the thermal stress and the tensile strength. The model can contribute to explain the formation of striae, surfaces caused by crack advance, in particular, of the first fracture. We find that the Young modulus controls the crack growth. The model provides an explanation for the presence of the rough bands on striae. For its higher values the fracture extends partially into the viscoelastic region; while for lower values the crack is confined to the brittle layer. The maximum opening width is typically in the order of a few thousandths of the vertical length of the crack. The resumption of crack process must await further cooling of the lava body
Role of mechanical erosion in controlling the effusion rate of basaltic eruptions
Full text linkIn many basaltic eruptions, observations show that the effusion rate of magma has a typical dependence on time: the effusion rate curves show first a period of increasing and later a decreasing phase by a maximum value. We present a model to explain this behavior by the emptying of a magma reservoir through a vertical cylindrical conduit with elliptical cross section, coupled with the its widening due to mechanical erosion, produced by the magma flow. The model can reproduce the observed dependence on time of effusion rate in basaltic eruptions. Eruption duration and the maximum value of effusion rate depend on the size of magma chamber, on lava viscosity and strongly on erosion rate per unit traction
Fault slip controlled by gouge rheology: A model for slow earthquakes
No full textDuring 1997 several slow earthquakes have been recorded by a geodetic interferometer located beneath Gran Sasso, central Italy. The strain rise times of the events range from tens to thousands of seconds and strain amplitudes are of the order of 10−9. Amplitudes scale with the square root of the rise time and this suggests a diffusive behaviour of the slip propagation along the fault. In this work, we develop a model in which slip diffusion is the result of the presence of a gouge layer between fault faces, with a viscoplastic rheology. The fluid velocity field in the gouge layer diffuses in the directions of fault length and fault thickness, with different characteristic times. This model reproduces the relation between amplitude and rise time of measured strain signals. Synthetic straingrams, obtained for a horizontally layered, flat Earth and a source located a few kilometres from the instrument, are in agreement with observed signals
Monte Carlo inversion of DInSAR data for dislocation modeling: Application to the 1997 Umbria-Marche seismic sequence (Central Italy)
No full textThe study of surface deformation due to seismic activity is often made using dislocations with uniform slip and simple geometries. A better modeling of coseismic and postseismic surface displacements can be obtained by using dislocations with variable slip and nonregular shapes. This is consistent with the asperity model of fault surfaces, assuming a friction distribution on faults made of locked zones with much higher friction than surrounding zones. In this paper we consider the 1997–1998 Colfiorito seismic sequence. The coseismic surface displacements in the Colfiorito zone are used in order to infer the slip distribution on the fault surface at different stages of the sequence. The displacement field has been modeled varying the slip distribution on the fault, and comparing the deformation observed by SAR and GPS techniques with model results. The slip distribution is calculated by Monte Carlo simulations on a normal fault with the dip angle equal to 40°. A good approximation is obtained by using square asperity units of 1.5 times 1.5 km2. In the first stage, we employed a simplified model with uniform slip, in which each asperity unit is allowed to slip a constant amount or not to slip at all, and in the second stage, we evaluate the slip distribution in the dislocation area determined by the Monte Carlo inversion: in this case we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. The results show that the 1997 seismic events of the sequence can be modeled by irregular dislocations, obtaining a good fit to the DInSAR and GPS observations. The model also confirms the results of previous studies by a different methodology, defining the distribution of asperities on the fault plane using the fault geometry, the geodetic data and the seismic moment of the 1997–1998 Colfiorito seismic sequence. Furthermore, the analysis of 1997 aftershocks in the seismogenic region shows a strong correlation between most events and the asperity distribution, which can be considered as an independent test of the validity of the model
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
Thermal anomaly at the Earth's surface associated with a lava tube
Full text linkLava tubes are frequently encountered in volcanic areas. The formation of lava tubes has strong implications on the volcanic hazard during effusive eruptions. The thermal dissipation of lava flowing in a tube is reduced in respect to the lava flowing in an open channel so the lava may threaten areas that would not be reached by flows in open channels: for this reason it is important to detect the presence of lava tubes.
In this work we propose a model to detect the presence and the characteristics of lava tubes by their thermal footprint at the surface. We model numerically the temperature distribution and the heat flow, both in the steady and the transient state, and we take into account the principal thermal effects due to the presence of an active lava tube, i.e. the conduction to the ground and the atmosphere, the convection and the radiation in the atmosphere. We assume that lava fluid is at high temperature, in motion inside a sloping tube under the gravity force.
The thermal profile across the tube direction, in particular the width of the temperature curve, allows to evaluate the depth of the tube. The values of maximum temperature and of tube depth allow to estimate the area of the tube section. The shape of the temperature curve and its asymmetry can give information about the geometry of the tube.
If we observe volcanic areas at different times by thermal cameras, we can detect anomalies and evaluate their causes during an eruption; in particular, we can evaluate whether they are due to active lava flows or not and what is their state. For lava tubes, we can connect thermal anomalies with lava tube position, characteristics and state
Applications and future developments of the (thermo-) poro-elastic theory in geophysics
Full text linkISSN:0012-8252ISSN:1872-6828ISSN:1872-682
Gravity Data Allow to Image the Shallow‐Medium Subsurface Below Mud Volcanoes
Full text linkThe debate about the conceptual model of mud volcanoes functioning is still alive in the literature. A large part of the literature focuses on the characterization of the deep reservoir where expelled
fluids are expected to originate. Another part of literature is focused on the study of the shallow system of mud volcanoes, which could influence the short-term variations in mud volcanoes activity. We present and analyze a new data set of micro-gravimetric data to study the area of the Nirano Salse, Italy. Unlike what is commonly assumed for the study area, our results suggest that the geomorphology of the Nirano Salse is not related to a caldera collapse above a shallow mud chamber, but to the surface expression of slip distribution of a fault termination along which the fluids ascended to the surface. We believe that gravimetric data can significantly improve the study of hydrocarbon seeps and mud volcanism
Fluid conduits and shallow-reservoir structure defined by geoelectrical tomography at the Nirano Salse (Italy)
Full text linkMud volcanoes are fluid escape structures allowing for surface venting of hydrocarbons (mostly gas but also liquid condensates and oils) and water–sediment slurries. For a better understanding of mud volcano dynamics, the characterization of the fluid dynamics within mud volcano conduits; the presence, extent, and depth of the fluid reservoirs; and the connection among aquifers, conduits, and mud reservoirs play a key role. To this aim, we performed a geoelectricalsurvey in the Nirano Salse Regional Nature Reserve, located at the edge of the northern Apennines (Fiorano Modenese, Italy), an area characterized by several active mud fluid vents. This study, for the first time, images the resistivity structure of the subsoil along two perpendicular cross sections down to a depth of 250 m. The electrical models show a clear difference between the northern and southern sectors of the area, where the latter hosts the main discontinuities. Shallow reservoirs, where fluid muds accumulate, are spatially associated with the main fault/fracture controlling the migration routes associated with surface venting and converge at depth towards a common clayey horizon. There is no evidence of a shallow mud caldera below the Nirano area. These findings represent a step forward in the comprehension of the Nirano Salse plumbing system and in pinpointing local site hazards, which promotes safer tourist access to the area along restricted routes
- …
