1,721,179 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
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
Feeding of a magma chamber by an ascending magma bubble
Full text linkThere is evidence that magma chambers are not fed by a continuous flow, but in the form of discrete magma batches. The possibility is considered that the chamber is fed by a magma bubble ascending through the region underneath, driven by buoyancy force. Due to the high ambient temperatures, it is assumed that the bubble moves through a viscoelastic medium with temperature-dependent viscosity. The motion of a spherical magma bubble and its inflow into the chamber are studied. The bubble volume is assumed to be at least equal to magma volumes in typical effusive eruptions on Mount Etna, corresponding to bubble radii of a few hundred metres. Under some simplifying assumptions, the problem is solved analytically. The bubble velocity is directly proportional to the square of its radius and inversely to the viscosity of surrounding rocks, but it is independent of magma viscosity. Velocity can reach values of the order of tens of metres per year in the proximity of the chamber. Since the characteristic time for heat diffusion is several hundred years, the bubble can cover several kilometres with only moderate cooling. During ascent, forced convection takes place in the bubble. Equations for convection streamlines are obtained and traveltimes of magma are calculated, giving a mixing time of the order of hundred years below the chamber. Inflow of the bubble in the magma chamber produces a pulse in flow rate. Under reasonable assumptions, pulse shape and duration are calculated analytically. Pulse duration can be several tens of years and can give rise to a sequence of eruptions, depending on the size of the bubble and the critical overpressure for eruption
Stress relaxation at the lower dislocation edge of great shallow earthquakes
No full textThe problem of post-seismic stress evolution at the lower dislocation edge of great shallow earthquakes is investigated by a model reproducing a transcurrent boundary zone subject to a constant strain rate. Dislocation models with frictional stress threshold for slip arrest must be considered in order to study post-seismic stress evolution in the proximity of dislocation edges. It is found that the boundary zone is vertically divided in two regions: an upper region, where stress increases with time and a lower region, where stress decreases. Dislocations which are nucleated in the seismogenic region can propagate downward into the aseismic region: in this case earthquakes are followed by relaxation of the stress concentration at the lower dislocation edge. This may have effects on the space-time distribution of aftershocks
Heavy-lepton decay into a massive neutrino with various spin assignments
No full textSince the neutrino-induced dimuon events with opposite charges were discovered, several mechanisms of production have been proposed. In the present paper the hypothesis is considered that the heavy lepton-associated neutrino is a massive particle
A model of interseismic fault slip in the presence of asperities
No full textA 2‐D model which represents a slipping fault with non‐uniform Coulomb friction is studied. The fault plane is subject to a uniform ambient shear stress, slowly increasing with time. Aseismic fault creep is assumed to start in a weak zone, when the ambient stress reaches a strength threshold. The solution for the resulting dislocation is worked out analytically using a technique based on Chebyshev polynomials. It is found that the dislocation partially propagates into the adjacent asperities, concentrating stress onto them and preparing the conditions which will produce the asperity failure and the accompanying earthquake. Propagation is not self‐similar and occurs at increasing velocity. A non‐linear slip hardening effect is reproduced. The nearness to earthquake instability is measured by a dimensionless parameter which depends on Coulomb friction and ambient shear stress and decreases to zero with time. An upper boundary to the critical value of this parameter, at which instability may occur, is estimated and is found to depend on the ratio between the sizes of the asperity and the weak zone
A model of interseismic stress evolution in a transcurrent shear-zone
No full textIt has been observed that most shallow earthquakes occur in a seismogenic layer which extends to a depth of a few tens of kilometres, while at greater depths the relative plate motion must take place aseismically. Such behaviour is reproduced by a model where a wide, deformed fault region is encompassed by two transcurrent plates and subjected to a constant strain rate. The shear zone is treated as a viscoelastic body, for which a power-law constitutive relation is employed. Temperature and, therefore, rheology depend on depth z. Also rigidity depends on depth. The model determines a maximum depth H for earthquake nucleation on the faults in the shear zone, if a frictional resistance linearly increasing with depth is assumed. The interseismic shear stress evolution on a vertical fault is obtained analytically for n = 1,2,3 and 4, where n is the power-law exponent. It is found that the rate of stress increase does not change appreciably as a function of n for z < H, while the effect of non-linearity becomes more sensible at larger z. Moreover, for z < H, the state of stress as obtained from this model is very different from estimates obtained from purely viscous models even for much longer times than are considered in seismology
A dynamical model of lava flows cooling by radiation
No full textThe behaviour of a lava flow is reproduced by a two-dimensional model of a Bingham liquid flowing down a uniform slope. Such a liquid is described by two rheological parameters, yield stress and viscosity, both of which are strongly temperature-dependent. Assuming a flow rate and an initial temperature of the liquid at the eruption vent, the temperature decrease due to heat radiation and the consequent change in the rheological parameters are computed along the flow. Both full thermal mixing and thermal unmixing are considered. The equations of motion are solved analytically in the approximation of a slow downslope change of the flow parameters. Flow height and velocity are obtained as functions of the distance from the eruption vent; the time required for a liquid element to reach a certain distance from the vent is also computed. The gross features of observed lava flows are reproduced by the model which allows us to estimate the sensitivity of flow dynamics to changes in the initial conditions, ground slope and rheological parameters. A pronounced increase in the rate of height increase and velocity decrease is found when the flow enters the Bingham regime. The results confirm the observation according to which lava flows show an initial rapid advance, followed by a marked deceleration, while the final length of a flow is such that the Graetz number is in the order of a few hundreds
Post-seismic stress evolution in a boundary-zone model with depth-dependent rheology
No full textTectonic boundary zones are characterized by a shallow, brittle region (seismogenic region) where frictional slip occurs on fault planes, and by a lower, ductile and aseismic region. The nucleation depth and the extent of slip planes in great interplate earthquakes are determined by the relationships between frictional resistance, applied stress and stress drop. The post-seismic stress evolution on a fault plane is investigated by a model which reproduces a transcurrent boundary zone subject to a constant strain rate. Dislocation models with frictional stress threshold for slip arrest must be considered in order to study post-seismic stress evolution in the proximity of dislocation edges. It is found that dislocations which are nucleated in the seismogenic region can propagate downward into the aseismic region: in this case earthquakes are followed by relaxation of the stress concentration at the lower dislocation edge. This has effects on the space-time distribution of aftershocks
Moment rate of the 2018 Gulf of Alaska earthquake
No full textThe 2018 Gulf of Alaska earthquake (Mw 7.9) occurred in a region of the Pacific plate southwest of the Alaskan subduction zone. The earthquake was a strike-slip event, with the hypocenter located at a depth of about 25 km and a seismic moment equal to 0.96 × 1021 Nm. Two observed moment rates have been obtained by the Geoscope Observatory, France, and by the United States Geological Survey (USGS). Both of them can be interpreted as due to the failure of two asperities on the fault surface. We consider a discrete fault model, with two asperities of different areas and strengths, and show that the observed moment rates can be reproduced by appropriate values of the model parameters, as inferred from the available data. A good fit to the observed moment rates is obtained by a sequence of three dynamic modes of the system, including a phase of simultaneous slip of the asperities. The two moment rates are however characterized by different initial conditions, in terms of different initial shear stress distributions on the fault. Shear stresses on the asperities are calculated as functions of time during the event and show a similar evolution in the two cases, but with different final values. The model results show that the presence of simultaneous asperity motion can significantly increase the seismic moment of a large earthquake
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