Degree Of Mechanical Coupling Research Articles

Artificial Partners to Understand Joint Action: Representing Others to Develop Effective Coordination.

In the last years, artificial partners have been proposed as tools to study joint action, as they would allow to address joint behaviors in more controlled experimental conditions. Here we present an artificial partner architecture which is capable of integrating all the available information about its human counterpart and to develop efficient and natural forms of coordination. The model uses an extended state observer which combines prior information, motor commands and sensory observations to infer the partner's ongoing actions (partner model). Over trials, these estimates are gradually incorporated into action selection. Using a joint planar task in which the partners are required to perform reaching movements while mechanically coupled, we demonstrate that the artificial partner develops an internal representation of its human counterpart, whose accuracy depends on the degree of mechanical coupling and on the reliability of the sensory information. We also show that human-artificial dyads develop coordination strategies which closely resemble those observed in human-human dyads and can be interpreted as Nash equilibria. The proposed approach may provide insights for the understanding of the mechanisms underlying human-human interaction. Further, it may inform the development of novel neuro-rehabilitative solutions and more efficient human-machine interfaces.

Quasilocalized states of self stress in packing-derived networks

.States of self stress (SSS) are assignments of forces on the edges of a network that satisfy mechanical equilibrium in the absence of external forces. In this work we show that a particular class of quasilocalized SSS in packing-derived networks, first introduced by D.M. Sussman, C.P. Goodrich, A.J. Liu (Soft Matter 12, 3982 (2016)), are characterized by a decay length that diverges as 1/sqrt{z_c-z} , where z is the mean connectivity of the network, and z_c equiv 4 is the Maxwell threshold in two dimensions, at odds with previous claims. Our results verify the previously proposed analogy between quasilocalized SSS and the mechanical response to a local dipolar force in random networks of relaxed Hookean springs. We show that the normalization factor that distinguishes between quasilocalized SSS and the response to a local dipole constitutes a measure of the mechanical coupling of the forced spring to the elastic network in which it is embedded. We further demonstrate that the lengthscale that characterizes quasilocalized SSS does not depend on its associated degree of mechanical coupling, but instead only on the network connectivity.Graphical abstract

Obduction: Why, how and where. Clues from analog models

Obduction is an odd geodynamic process characterized by the emplacement of dense oceanic “ophiolites” atop light continental plates in convergent settings. We herein present analog models specifically designed to explore the conditions (i.e., sharp increase of plate velocities — herein coined as ‘acceleration’, slab interaction with the 660 km discontinuity, ridge subduction) under which obduction may develop as a result of subduction initiation.The experimental setup comprises an upper mantle modeled as a low-viscosity transparent Newtonian glucose syrup filling a rigid Plexiglas tank and high-viscosity silicone plates. Convergence is simulated by pushing a piston with plate tectonics like velocities (1–10 cm/yr) onto a model comprising a continental margin, a weakness zone with variable resistance and dip (W), an oceanic plate (with or without a spreading ridge), a preexisting subduction zone (S) dipping away from the piston and an upper active continental margin, below which the oceanic plate is being subducted at the start of the model (as for the Neotethyan natural example).Several configurations were tested over thirty-five parametric models, with special emphasis on comparing different types of weakness zone and the degree of mechanical coupling across them. Measurements of displacements and internal deformation allow for a precise and reproducible tracking of deformation. Models consistently demonstrate that once conditions to initiate subduction are reached, obduction may develop further depending on the effective strength of W. Results (1) constrain the range of physical conditions required for obduction to develop/nucleate and (2) underline the key role of such perturbations for triggering obduction, particularly plate ‘acceleration’. They provide an explanation to the short-lived Peri-Arabic obduction, which took place along thousands of km almost synchronously (within ∼50–10 Myr), from Turkey to Oman, while the subduction zone beneath Eurasia became temporarily jammed. They also demonstrate that the emplacement of dense, oceanic material on continental lithosphere is not a mysterious process requiring extraordinary boundary conditions but results from large-scale, normal (oceanic then continental) subduction processes.

Three-dimensional dynamic laboratory models of subduction with an overriding plate and variable interplate rheology

Subduction zones are complex 3-D features in which one tectonic plate sinks underneath another into the deep mantle. During subduction the overriding plate (OP) remains in physical contact with the subducting plate and stresses generated at the subduction zone interface and by mantle flowforce the OP to deform.We present results of 3-D dynamic laboratory models of subduction that include an OP.We introduce newinterplate materials comprising homogeneous mixtures of petrolatum and paraffin oil to achieve progressive subduction. The rheology of these mixtures is characterized by measurements using a strain rate controlled rheometer. The results show that the strength of the mixture increases with petrolatum content, which can be used as a proxy for the degree of mechanical coupling along the subduction interface. Results of subduction experiments are presented with different degrees of mechanical coupling and the influence this has on the dynamics and kinematics of subduction. The modelling results show that variations in the degree of mechanical coupling between the plates have a major impact on subduction velocities, slab geometry and the rate of OP deformation. In all experiments the OP is displaced following trench migration and experiences overall extension localized in the plate interior. This suggests that OP deformation is driven primarily by the toroidal component of subduction-related mantle return flow. The subduction rate is always very slow in experiments with medium mechanical coupling, and subduction stops prematurely in experiments with very high coupling. This implies that the shear forces along the plate interface in natural subduction zone systems must be relatively low and do not vary significantly. Otherwise a higher variability in natural subduction velocities should be observed for mature, nonperturbed subduction zones. The required low shear force is likely controlled by the rheology of highly hydrated sedimentary and basaltic rocks.

Varying mechanical coupling along the Andean margin: Implications for trench curvature, shortening and topography

Convergent margins often exhibit spatial and temporal correlations between trench curvature, overriding plate shortening and topography uplift that provide insights into the dynamics of subduction. The Andean system, where the Nazca plate plunges beneath continental South America, is commonly regarded as the archetype of this class of tectonics systems. There is distinctive evidence that the degree of mechanical coupling between converging plates, i.e. the amount of resistive force mutually transmitted in the direction opposite to their motions, may be at the present-day significantly higher along the central Andean margin compared to the northern and southern limbs. However quantitative estimates of such resistance are still missing and would be desirable. Here we present laboratory models of subduction performed to investigate quantitatively how strong lateral coupling variations need to be to result in trench curvature, tectonic shortening and distribution of topography comparable to estimates from the Andean margin. The analogue of a two-layers Newtonian lithosphere/upper mantle system is established in a silicone putty/glucose syrup tank-model where lateral coupling variations along the interface between subducting and overriding plates are pre-imposed. Despite the simplicity of our setup, we estimate that coupling in the central margin as large as 20% of the driving force is sufficient to significantly inhibit the ability of the experimental overriding plate to slide above the subducting one. As a consequence, the central margin deforms and shortens more than elsewhere while the trench remains stationary, as opposed to the advancing lateral limbs. This causes the margin to evolve into a peculiar shape similar to the present-day trench of the Andean system.

Ductile deformation and rheology of sub-continental mantle in a hot collisional orogeny: Example from the Bohemian Massif

Fabric patterns of strongly serpentinized peridotite were determined using eigenvector analysis and eigenvalue classification of lattice preferred orientation of olivine and orthopyroxene. This approach has been applied to a rootless fold-shaped body of mylonitized spinel to garnet peridotite surrounded by fine-grained and partially retrogressed ky–kfs granulite. The EBSD data show either axial [010] or [100](0kl) pattern, both characteristic for ‘dry’ slip systems. The former pattern occurs predominantly along the inner margin and southern limb while the latter is mainly developed in the hinge of the fold shaped body. Foliations and lineations deciphered from the LPO data suggest that the [100](0kl) pattern reflects constrictional deformation (prolate strain ellipsoid) in the hinge of the peridotite fold while the axial [010] pattern reflects pure flattening (oblate strain ellipsoid) inherited from the period of emplacement of the peridotite sheet in the crust. Similarity in finite strain pattern of peridotite and surrounding granulites indicates their common thermal and mechanical evolution during folding. The petrology and structural data result in a model of burial of peridotite below thickened crustal root, its exhumation and folding. The burial stage is associated with prograde metamorphism resulting in a coarse-grained microstructure and development of spinel and garnet zones. The emplacement of peridotite into lower crustal granulites occurred along a shear zone associated with grain size reduction in both peridotite and granulite and rapid cooling of mylonitized peridotite to the ambient temperatures of lower crust. Further ascent to mid-crustal levels occurred within the vertical granulite channel. Final fold shape of the peridotite developed during subsequent indentation of the weak vertically anisotropic crust by the adjacent continental promontory. The degree of mechanical coupling between folded peridotite and granulite in mid-crustal levels is estimated using comparison of studied microstructures with experimental data.

The Ordovician Sierras Pampeanas–Puna basin connection: Basement thinning and basin formation in the Proto-Andean back-arc

The Ordovician Sierras Pampeanas, located in a continental back-arc position at the Proto-Andean margin of southwest Gondwana, experienced substantial mantle heat transfer during the Ordovician Famatina orogeny, converting Neoproterozoic and Early Cambrian metasediments to migmatites and granites. The high-grade metamorphic basement underwent intense extensional shearing during the Early and Middle Ordovician. Contemporaneously, up to 7000 m marine sediments were deposited in extensional back-arc basins covering the pre-Ordovician basement. Extensional Ordovician tectonics were more effective in mid- and lower crustal migmatites than in higher levels of the crust. At a depth of about 13 km the separating boundary between low-strain solid upper and high-strain lower migmatitic crust evolved to an intra-crustal detachment. The detachment zone varies in thickness but does not exceed about 500 m. The formation of anatectic melt at the metamorphic peak, and the resulting drop in shear strength, initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies P–T conditions. P–T–d–t data in combination with field evidence suggest significant (ca. 52%) crustal thinning below the detachment corresponding to a thinning factor of 2.1. Ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25% and 44%, constituting total crustal thinning factors of 1.7–2.0. While the migmatites experienced retrograde decompression during the Ordovician, rocks along and above the detachment show isobaric cooling. This suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface. Upper crustal extension and thinning is compensated by newly deposited sediments, maintaining constant pressure at detachment level. Thinning of the migmatitic lower crust is compensated by elevation of the crust–mantle boundary. The degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension, basin formation, and sediment thickness in the back-arc basin. The initiation of crustal extension in the back-arc, however, crucially depends on the presence of anatectic melt in the middle and lower crust. Consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension.

Resonant sensor assembly

A triple beam resonator ( 10 ) is provided with three beams or tines ( 12, 14, 16 ) aligned in parallel alongside each other and joined at a decoupling zone ( 18 ) at each end, which is in turn connected to the surrounding material. The central beam ( 14 ) is twice the width of the two outer beams ( 12, 16 ). The resonating element has a length of 15.5 mm, a thickness of 0.25 mm and beam widths of 2 mm and 1 mm. The distance between the beams is 0.5 mm. Finite element analysis predicts the modal behavior with stress distribution and eigenfrequencies of the resonator ( 10 ). Thick-film PZT elements ( 20, 22 ) were printed on separate regions at each end of the central beam ( 14 ), where maximum stresses exist as the resonator ( 10 ) operates in its favored mode of vibration. The PZT element ( 20 ) at one end drives the vibrations, while the PZT element ( 22 ) at the other end detects them. Positioning the PZT driving and sensing elements ( 20,22 ) on the regions of maximum stresses maximizes the degree of mechanical coupling between the active piezoelectric layer and the resonator for generation of both driving forces and sensing signals. Very high quality factors of 3100 and higher have been experienced. Furthermore, the resonator can be manufactures by batch production techniques while maintaining high reliability and reproducibility.

Asperity distribution and earthquake occurrence in the southern Kurile Islands arc

The seismicity in the southern Kurile Islands arc is characterized by a complex overlapping of aftershock zones of a few great and many large earthquakes ( M s ≧ 7.0 ). The spatial relationships of these aftershock zones can be explained in terms of a simple pattern of fault plane heterogeneity. The four largest earthquakes in this region are, from north to south, the 1963 Kurile Islands ( M w = 8.5), the 1958 Etorofu ( M w = 8.3), the 1969 Kurile Islands ( M w = 8.2) and the 1973 Nemuro-Oki ( M w = 7.8) events. Detailed body wave studies indicate that the moment release associated with each event is concentrated in a small portion of the aftershock area, with scale lengths < 100 km. These regions of high moment release are the largest asperities. The areas that surround the large asperities slip in two modes: (1) coseismically during the great events; and (2) as distinct large ( M s ≧ 7.0 ) earthquakes both before and after the great events. The second mode involves either rupture of relatively weak regions which results in low-stress-drop events, or rupture of smaller asperities. The earthquakes produced by failure of small asperities tend to repeatedly rupture the same regions, and they have impulsive source time functions indicating rupture of highly stressed regions of the fault surface. By analysis of all M s ≧ 7.0 events since 1946 in this region we find a segmentation of the southern Kurile Islands thrust zone into strongly coupled regions with two scale lengths (dominant and secondary asperities) and weakly coupled regions (the surrounding areas generating the low-stress-drop events). The largest earthquakes and the smaller asperity earthquakes occur at the down-dip portion of the lithospheric interface, while the low-stress-drop events occur much closer to the trench axis. This implies a difference in the degree of mechanical coupling between the subducting and overriding plates, with the deeper portions having greater strength than the shallower parts.