992 research outputs found
Assumptions in the evaluation of lava eusion rates from heat radiation
No full textThe availability of high-resolution thermal imagery of active lava flows has stimulated the use of radiance maps for the evaluation of lava effusion rates. This is made possible by simple formulae relating the lava flow rate to the energy radiated per unit time from the planimetric surface of the flow. Such formulae are based on a specific flow model and, consequently, their validity is subject to the model assumptions. An analysis of these assumptions reveals that the current use of the formulae is not consistent with the model. The reason why they provide reasonable, although very rough, values for effusion rates appears to be that the actual radiated energy is controlled by a feature (the nonuniform temperature of flow surface) which is not accounted for by the model and which counterbalances the effect of inconsistent use of the formulae. Citation: Dragoni, M., and A. Tallarico (2009), Assumptions in the evaluation of lava effusion rates from heat radiation, Geophys. Res. Lett., 36, L08302, doi:10.1029/2009GL037411
Introduzione
No full textIntroduzione al Supplemento "L'archeologia pubblica prima e dopo l'archeologia pubblica".Introduction to the Supplement on "Public Archaeology before and after Public Archaeology"
Physical modelling of lava flows
No full textLava flows are not only a fascinating scientific problem, involving many branches of continuum mechanics and thermodynamics, but are natural events having a strong social impact. A reliable evaluation of volcanic hazard connected with lava flows depends on the availability of physical models allowing us to predict the evolution of these phenomena. In this regard, the rheological properties of lavas are of major importance in controlling the dynamics of lava flows. Lava is a multi-phase and chemically heterogeneous system. This entails a characteristic, non-Newtonian behaviour of lava flows, which is emphasized by the fact that the rheological parameters are strongly temperature dependent and are therefore affected by the progressive cooling of lava after effusion. Physical models of lava flows show us the complex relationships between the many quantities governing this process and in the near future they may allow us to predict the dynamics of lava flows and to take effective measures for the reduction of volcanic risk
Gravity in Earth's Interior
No full textThe acceleration of gravity in Earth's interior is determined by the density distribution in Earth. A remarkable result is that the acceleration is approximately constant all over the mantle, which amounts to about 84% of Earth's volume. This result can be explained by a simple two-layer model of Earth, showing that the constancy of the acceleration in the mantle is a consequence of the particular size and density of Earth's core with respect to the size and density of the whole Earth. In other planets, with different mass distributions, the dependence of acceleration on depth could be very different
Discrete Fault Models
No full textFault surfaces are characterized by an inhomogeneous friction distribution, that can be represented with asperity models. Fault mechanics is dominated by asperities, so that a fruitful approach is to use discrete models, where asperities are the basic elements and the state of the fault is described by the average values of stress, friction and slip on each asperity. Under reasonable assumptions, the equations of motion can be solved analytically, with a deeper understanding of the behavior of the system. Fault dynamics has a sticking mode, where asperities are stationary, and a number of slipping modes, corresponding to the separate or simultaneous motion of asperities. Any seismic event is a sequence of slipping modes and a large variety of source functions is possible. Many large earthquakes are observed to be the consequence of the failure of two asperities: a discrete two-asperity model shows a rich dynamics and allows a detailed study of interaction between asperities. In this framework, fault evolution during coseismic and interseismic intervals can be calculated in terms of fault slip, stress state, energy release and seismic spectrum, including viscoelastic relaxation, fault creep and stress perturbations from other faults. Discrete models may include interaction between neighboring faults, allowing to assess conditions for the occurrence of seismic sequences in a fault system. A review of recent work on this subject is presented with applications to real earthquakes
Contribution of the 2010 Maule Megathrust Earthquake to the Heat Flow at the Peru-Chile Trench
No full textThe 2010 Maule earthquake was a megathrust event that occurred along the Peru–Chile Trench. The earthquake source can be modelled as a fault with two asperities with different areas and strengths. By employing a discrete fault model, where asperities are the basic elements, the event can be described as a sequence of three dynamic modes involving simultaneous asperity slip. Interaction between asperities by mutual stress transfer plays a crucial role during fault slip. With a careful choice of values for the model parameters, the mode durations, the slip distribution, the seismic moment rate and the final moment calculated from the model are found to be consistent with the observed values. An important amount of frictional heat is produced by an event of this size and is calculated by summing up the contributions of each asperity. The seismic event produces a heat pulse propagating through the Earth’s crust and contributing to the average heat flow in the region. The calculated heat production is equal to about 2 × 10^(17) J and the peak value of the heat pulse is equal to 6 × 10^(−3) mW m^(−2) or about 10^(−4) of the average surface heat flow density, with a characteristic diffusion time in the order of 10^6 a
A dislocation model of aseismic fault slip under nonuniform friction
No full textA model is proposed for studying the mechanical behaviour of faults during their interseismic periods. The model considers a plane fault surface in an elastic medium, subject to a uniform shear stress which increases slowly with time. A1‐D friction distribution is assumed on the fault, characterized by asperities and a weaker zone. The traction vector on the fault plane has an arbitrary orientation: in particular, it can be nonperpendicular to the asperity borders. Aseismic fault slip takes place when the applied stress exceeds the frictional resistance: slip starts in weak zones and is confined by asperities, where it propagates at increasing velocity. Propagation into asperities is characterized by a dislocation front, advancing perpendicularly to the asperity border. Fault slip does not take prate in the direction of traction, except when traction is perpendicular or parallel to the asperity border. The propagation of such aseismic dislocations produces a stress redistribution along the fault and can play a key role in determining the conditions which give rise to earthquakes
Changes in lava effusion rate from a volcanic fissure due to pressure changes in the conduit
No full textWe calculate the change in effusion rate of lava from a volcanic fissure due to pressure changes in the volcanic conduit. The conduit is modelled as a cylinder with elliptical cross-section, embedded in an elastic medium. The elliptical shape can represent a wide range of cross-sections, according to the value of eccentricity, from almost circular vents to very long and narrow fissures. A 2-D problem is considered assuming invariance of pressure changes and conduit geometry with depth. The problem is solved analytically and expressions for the displacement and the stress fields in the elastic medium are provided. The displacement of the conduit wall is proportional to the ratio between the pressure change and the rigidity of surrounding rocks. The flow rate is a nonlinear function of the pressure change and increases with increasing pressure, due to the elastic deformation of the conduit wall.We consider flow rate oscillations with periods ranging from several minutes to hours, as are often observed during effusive eruptions. Assuming pressure oscillations with these periods, flow rate oscillations resulting from the elastic deformation of the conduit are calculated. The greatest oscillations in flow rate are obtained for very large values of the conduit eccentricity, corresponding to long and narrow volcanic fissures. For example, if a fissure is 100 m long and 2 m large, a pressure oscillation with an amplitude of 1 MPa yields a maximum displacement of the conduit wall equal to about 6 cm and an amplitude of flow rate oscillations of about 20 per cent
Conditions for large earthquakes in a two-asperity fault model
No full textA fault with two asperities is modelled as a system made of two blocks coupled by a spring and sliding on a plane under the same values of static and dynamic friction. An analytical solution is given for the simultaneous motion of the blocks and the corresponding orbits are plotted in the phase space. It is proven that, whichever the initial state is, the long-term behaviour of the system is one of an infinite number of limit cycles, characterized by a particular pattern of forces. The region where the system is located when the blocks are stationary can be divided into narrow stripes corresponding to different orbits of the points belonging to them. This implies that the system is sensitive to perturbations and has relevant implications for a fault, which is subject to stress transfers from earthquakes generated by neighbouring faults. In this case, the fault may experience a larger earthquake, with the simultaneous failure of the two asperities, which restores a stress distribution compatible with periodic behaviour. The seismic moment associated with simultaneous asperity failure is always greater than the maximum value that can be released in a limit cycle. For strongly coupled asperities, the moment can be several times larger
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