1,356,048 research outputs found
A model for the formation of lava tubes by roofing over a channel
No full textA model for tube formation by roofing of a channel is proposed and involves first describing lava as a Bingham liquid flowing down a slope. It is further assumed that lava flows in a channel with rectangular cross section: as a result of heat loss into the atmosphere, a crust is gradually formed on the upper surface of the flow and this crust eventually welds to the channel levees. The model predicts that if the flow rate is constant, the thickness of the flow increases as the crust fragments grow and weld to each other, and the velocity of the crust decreases to zero. Once the lava tube is formed, the initial flow rate can be achieved by a flow thickness smaller than the vertical size of the tube, with the same viscous dissipation. -from Author
Effusion Rate From a Volcanic Conduit Subject to Pressure Oscillations in a Viscoelastic Medium
No full textEffusion rate in basaltic eruptions typically depends on time: there is an initial, relatively fast increase followed by a much slower decrease until the eruption vanishes; in addition, changes are observed in the effusion rate having durations much shorter than the total duration of the eruption. For an effusive eruption, we calculate the deformation of the volcanic conduit due to short-term pressure oscillations. The model considers an elliptical conduit embedded in a viscoelastic medium, described by a Maxwell body. As a consequence of pressure oscillations, the semi axes of the conduit are quasi periodic functions of time with the same period as pressure. For volcanic fissures the viscoelastic rheology entails a remarkable increase in oscillation amplitude of flow rate with respect to the elastic case and a time delay in flow rate oscillation with respect to overpressure oscillation. For a given value of overpressure amplitude, this effect is controlled by the conduit eccentricity and the ratio between overpressure period and Maxwell time; for larger values of this ratio and/or for eccentricity values closer to unity, flow rate oscillates around a value larger than its initial value and can vary from 5% to 30% with respect to it. The model can approximate the in-situ observations of short-time fluctuations of flow rate during the 2018 eruption of Kı̄lauea Volcano
A Model for the Effusion Rate Produced by a Magma Pulse
No full textWe assume that a magma pulse enters a magma chamber from the plumbing system, producing an overpressure triggering an effusive volcanic eruption. The chamber is modeled as a spherical cavity in an elastic half-space, connected to the Earth's surface by a vertical conduit, and the magma as an incompressible Newtonian liquid. Overpressure in the chamber is calculated as a function of time during the eruption and has different behaviors depending on the characteristics of the magma pulse and of the volcanic system. The time history of effusion rate is controlled by the pulse duration and the geometrical and rheological parameters of the model. The calculated effusion rates are compared with different classes of effusion rates extrapolated from historical data of Mount Etna, which are asymmetric functions of time. For appropriate values of the model parameters, the calculated time histories are found to fit well those derived from observation
Displacement and stress fields around a fault jog: Effects on fault mechanics
No full textWe consider a fault surface which differs slightly from a plane due to a jog. The fault is placed in an elastic space and is subject to a uniform stress field. The orientation of the fault is such that the normal traction is greater on the jog determining a higher static friction. A considerable increase in friction and the formation of a strong asperity can occur due to repeated slip episodes on the fault. Slip produces an elastic deformation of the fault faces in correspondence of the asperity, causing an increase in normal traction and hence in friction. This process can be described as a tensile Somigliana dislocation, accompanied by partial fracturing of the fault face material, which can produce fault gouge
Frictional changes induced by seismic slip on a non-planar fault
No full textWe propose a model which describes the formation of a strong asperity on a fault. We consider a fault surface which differs slightly from a plane due to a jog-like topographic variation. The fault is placed in an elastic space and is subject to a uniform stress field. The orientation of the fault is such that the normal traction (which is compressive) is greater on the topographic variation, determining a higher static friction and hence an asperity. The value of friction on this asperity depends on the magnitude of shear stress. For times of seismological interest, the increase in shear stress, at rates typical of tectonic processes, does not produce a sensible increase in friction with respect to the adjacent fault segments. A considerable increase in friction and the formation of a strong asperity (or even a barrier) can occur due to repeated seismic-slip episodes on the fault. Slip results in an elastic medium deformation, causing an increase in normal traction on the asperity and hence in friction. This process is described with the aid of a tensile Somigliana dislocation. Regions with high friction undergo partial fracturing of the fault-face material, which can produce fault gouge. The tensile dislocation introduces a small non-double-couple component in the seismic moment of the seismic event, the magnitude of this component depending mainly on the relative size of the asperity
Dynamics of a seismogenic fault subject to variable strain rate
No full textThe behaviour of seismogenic faults is generally investigated under the assumption that they are subject to a constant strain rate. We consider the effect of a slowly variable strain rate on the recurrence times of earthquakes generated by a single fault. To this aim a spring-block system is employed as a low-order analog of the fault. Two cases are considered: a sinusoidal oscillation in the driver velocity and a monotonic change from one velocity value to another. In the first case, a study of the orbit of the system in the state space suggests that the seismic activity of the equivalent fault is organized into cycles that include several earthquakes and repeat periodically. Within each cycle the recurrence times oscillate about an average value equal to the recurrence period for constant strain rate. In the second case, the recurrence time changes gradually from the value before the transition to the value following it. Asymptotic solutions are also given, approximating the case when the amplitude of the oscillation or of the monotonic change is much smaller than the average driver velocity and the period of oscillation or the duration of the transition is much longer than the recurrence times of block motions. If the system is not isolated but is subject to perturbations in stress, the perturbation anticipates or delays the subsequent earthquake. The effects of stress perturbations in the two cases of strain rate oscillations and monotonic change are considered
Ground displacement in a fault zone in the presence of asperities
No full textFriction on faults controls slip distribution in response to tectonic stress: the friction distribution can be simplified by considering locked zones (asperities) surrounded by aseismic slipping zones. The aseismic slip of fault sections has an important role in concentrating stress on the asperities and in producing their failure. The slow ground displacement in fault zones is measurable through classic or spatial geodetic techniques and may help to localize the greater asperities on faults. Therefore accurate geodetic measurements in fault zones may be used to evaluate the seismic hazard in the region. We represent the Earth's crust by an elastic, homogeneous and isotropic half-space, including a plane normal fault. A locked asperity is considered on the fault, while the surrounding area of the fault surface undergoes a uniform slip. The surface displacement field is analyzed in the presence and in the absence of the asperity; the influence of the asperity shape, size and depth is studied also varying the dip angle of the fault. We conclude that an asperity, whose area is about 1 km2, determines a surface displacement of mm order, when its centre is placed at depths ranging from 5 to 10 km and the surrounding fault area slips by tens of centimeters: in this case an asperity with an area of about 5×5 km2 could be reasonably localized by current geodetic measurements
A model for the formation of wrinkle ridges in volcanic plains on Venus
No full textRidged plains, the most abundant geologic terrain on Venus, are volcanic plains deformed by wrinkle ridges after their emplacement. It has been suggested that the ridges are the product of thermal stresses induced in the lithosphere by the increase of surface temperature due to the greenhouse effect of gases released during the emplacement of the volcanic plains. A model for the formation of wrinkle ridges is proposed where the ridges are assumed to be the surface effect of dislocations produced by compressive stress in the Venusian lithosphere. The lithosphere is modeled as a thermoelastic half-space, the surface of which is subject to a linear temperature increase during 100 Ma. The state of stress is calculated and the maximum distance is obtained at which dislocations can be placed in order that the compressive stress at the Venusian surface is everywhere lower than the rock strength. The model predictions in terms of dislocation density and crustal shortening appear to be consistent with observations
Propagation of an aseismic dislocation through asperities with smooth borders
No full textA 2-D model is presented for the propagation of a Somigliana dislocation along a fault with nonuniform friction. Fault slip is driven by a uniform ambient shear stress, slowly increasing with time. The dislocation is nucleated in the lowest-friction region of the fault plane and is confined by the surrounding higher-friction regions (asperities). The case studied involves asperities which have smooth borders, characterized by a constant friction gradient. For values of ambient shear stress near to the weak-zone friction, the propagation of dislocation is slowed down by the presence of asperities. Only when it goes beyond the border does the dislocation front move at increasing velocity. The model shows that, at a given value of ambient shear stress, the slip amplitude is larger in the case of finite and constant friction gradient than in the case of asperities with sharp borders. Unlike the propagation velocity, slip rate is ever increasing during the dislocation process. The model shows to what extent a dislocation is influenced by the distribution of friction on the fault. A detailed knowledge of slip rate and slip history is needed to understand the mechanism of frictional instability on faults. © 1993
Role of viscous dissipation in the dynamics of lava flows with power-law rheology
No full textWe model a lava flow as a one-dimensional flow of a pseudoplastic fluid with viscous dissipation. The flow is horizontally unbounded and is driven downslope by the gravity force. We consider a power-law constitutive equation and we take into account the temperature dependence of the rheological parameters. Given an effusion rate and an initial temperature at the eruption vent, the flow is assumed to cool down by heat radiation. We calculate the heat produced by viscous dissipation as a function of lava temperature and effusion rate. The cooling rate is calculated as a function of the surface temperature and flow rate. Viscous dissipation reduces the cooling rate by an amount which is independent of flow rate. We evaluate the effect of viscous dissipation on the flow thickness and velocity. The effect of dissipation is to decrease the flowthickness and to increase the flowvelocity. The effect on flow thickness is greater for smaller flow rates, while the effect on velocity is greater for larger effusion rates. In principle, themodel provides a method for estimating the flowrate fromin-field measurements of distances and temperatures
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