290 research outputs found
Fixed Points in the Ambient Logic
We present an extension of the ambient logic with fixed points
operators in the style of the mu-calculus. We give a simple
syntactic condition for the equivalence between minimal and maximal
fixpoint formulas and show how to subsume spatial analogues of the
usual box and diamond operators
Horizontal mantle flow controls subduction dynamics
It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age âthe main control on buoyancyâ exhibits little correlation with most of the present-day subduction velocities and slab dips. âWestâ-directed subduction zones are on average steeper (~65°) than âEastâ-directed (~27°). Also, a âwesterlyâ-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an âeasterlyâ-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth
A Typed Calculus for Querying Distributed XML Documents
We study the problems related to querying large, distributed XML documents. Our proposal takes the form of a new process calculus in which XML data are processes that can be queried by means of concurrent pattern-matching expressions. What we achieve is a functional, strongly-typed programming model based on three main ingredients: an asynchronous process calculus in the style of Milner's pi-calculus and existing semantics for concurrent-ML; a model where documents and expressions are both represented as processes, and where evaluation is represented as a parallel composition of the two; a static type system based on regular expression types
A Bisimulation for the Blue Calculus
The Blue calculus is a direct extension of both the lambda and the pi calculi. In this report, we define an equivalence for this calculus based on barbed congruence, and we prove the validity of the replication laws. For example, we prove that a replicated resource, shared by many processes, can be safely copied and distributed
Automated Polyhedral Abstraction Proving
We propose an automated procedure to prove polyhedral abstractions for Petri
nets. Polyhedral abstraction is a new type of state-space equivalence based on
the use of linear integer constraints. Our approach relies on an encoding into
a set of SMT formulas whose satisfaction implies that the equivalence holds.
The difficulty, in this context, arises from the fact that we need to handle
infinite-state systems. For completeness, we exploit a connection with a class
of Petri nets that have Presburger-definable reachability sets. We have
implemented our procedure, and we illustrate its use on several examples
Structural Evolution of Orogenic Wedges: Interplay Between Erosion and Weak DĂ©collements
Orogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak dĂ©collements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using twoâdimensional, viscoâelastoâplastic numerical models. Our experiments assume a basal and an intermediate dĂ©collement within the wedge. Experiments with a frictional strength of the basal dĂ©collement lower or equal to that of the intermediate dĂ©collement show a structural evolution of foldâandâthrust belts dominated by outâofâsequence thrusting. Conversely, when the intermediate dĂ©collement is weaker than the basal dĂ©collement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (âŒ0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper dĂ©collement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of âŒ0.18. Foldâandâthrust belts worldwide exhibit various architectures in which different dĂ©collement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise
Structural Evolution of Orogenic Wedges: Interplay Between Erosion and Weak DĂ©collements
Orogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak dĂ©collements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using twoâdimensional, viscoâelastoâplastic numerical models. Our experiments assume a basal and an intermediate dĂ©collement within the wedge. Experiments with a frictional strength of the basal dĂ©collement lower or equal to that of the intermediate dĂ©collement show a structural evolution of foldâandâthrust belts dominated by outâofâsequence thrusting. Conversely, when the intermediate dĂ©collement is weaker than the basal dĂ©collement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (âŒ0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper dĂ©collement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of âŒ0.18. Foldâandâthrust belts worldwide exhibit various architectures in which different dĂ©collement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise
Unraveling scaling properties of slow-slip events
A major debate in geophysics is whether earthquakes and slowâslip events (SSEs) arise from similar failure mechanisms. Recent observations from different subduction zones suggest that SSEs follow the same momentâduration scaling as earthquakes, unlike qualitatively different scaling proposed by earlier studies. Here, we examine the scaling properties using dynamic simulations of frictional sliding. The resulting sequences of SSEs match observations from the Cascadia subduction zone, including the earthquakeâlike cubic momentâduration scaling. In contrast to conventional and widely used assumptions of magnitudeâinvariant rupture velocities and stress drops, both simulated and natural SSEs have rupture velocities and stress drops that increase with event magnitudes. These findings support the same frictional origin for both earthquakes and SSEs while suggesting a new explanation for the observed SSEs scaling
Trapping light with micro lenses in thin film organic photovoltaic cells.
We demonstrate a novel light trapping configuration based on an array of micro lenses in conjunction with a self aligned array of micro apertures located in a highly reflecting mirror. When locating the light trapping element, that displays strong directional asymmetric transmission, in front of thin film organic photovoltaic cells, an increase in cell absorption is obtained. By recycling reflected photons that otherwise would be lost, thinner films with more beneficial electrical properties can effectively be deployed. The light trapping element enhances the absorption rate of the solar cell and increases the photocurrent by as much as 25%. (C) 2008 Optical Society of Americ
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