Slab

About: Slab is a research topic. Over the lifetime, 31617 publications have been published within this topic receiving 318693 citations.

Upper mantle structure from teleseismic P wave arrivals in Washington and northern Oregon

Abstract: Teleseismic P wave travel time residuals are used to detect lateral velocity heterogeneities in the upper mantle beneath Washington and northern Oregon. The results of an inversion for three-dimensional velocity variations resolves an east dipping high-velocity zone that we interpret as the subducting Juan de Fuca plate. The plate is characterized by 3–8% higher velocities than those in the surrounding upper mantle. Inversion of the travel time data and ray trace modeling indicate that the plate extends to a depth of 200–300 km. The plate dips at a moderate angle of 45° to the east-northeast beneath the central Washington Cascade Range north of Mount Rainier, with 5% faster velocities than the surrounding upper mantle. Beneath the North Cascade Range of Washington, the plate strikes to the northwest and has 6–8% faster velocities than the upper mantle to the west. South of 47°N, beneath the Cascade Range in southern Washington and northern Oregon, the plate dips steeply to the east and has 3–4% faster velocities than the surrounding upper mantle. Based on changes in the geometry and velocity structure of the subducted Juan de Fuca plate east of about 123°W, we propose that the subducted slab is segmented into three sections beneath Washington and northern Oregon.

Tomographic image of the Juan de Fuca Plate beneath Washington and western Oregon using teleseismic P‐wave travel times

Abstract: Backprojection tomography is applied to teleseismic P-wave travel-time residuals to produce a detailed image of the upper mantle beneath Washington and western Oregon. Beneath the Cascades volcanic arc from central Oregon to central Washington the subducted Juan de Fuca plate is imaged as a high-velocity quasi-planar feature dipping ∼65°E. Resolution analysis shows that beneath southern Washington, where resolution is best, the steeply dipping slab extends to a depth of only about 300 km. The slab may end near this depth, or low-velocity structure in the vicinity of the slab or a shallowing of the slab dip may produce this result. Beneath Oregon, where resolution is poor, the slab extends steeply into the mantle to at least 150 km depth. The uppermost mantle beneath northeastern Washington is high in velocity, preventing the resolution of the slab geometry there.

Complex slab subduction beneath northern Sumatra

Abstract: [1] New data provided by the 2004–2005 Sumatra-Andaman great earthquake sequences allow us to image with improved detail the P-wave velocity structure beneath Sumatra and adjacent regions. Below northern Sumatra, we find that the slab is folded at depth, exhibiting geometry similar to that of the volcanic arc and the trench at the surface. We speculate that this fold plays a major role in the segmentation of the Sumatra megathrust, and may impede rupture propagation in the region. North of Sumatra, significant slab material in the mantle transition zone is imaged for the first time, and we infer the presence of a major tear between the upper mantle and transition zone there.

Measurement of fracture mechanical properties of snow and application to dry snow slab avalanche release

Abstract: A dry snow slab avalanche is released by a sequenceof failure processes in the snow cover. When a weak layer in the snow cover is damaged over a certain area, the weak layer Starts to fail progressively in slope parallel direction. This shear failure disconnectsthe overlaying slab from the basal layer. Finally, a tensile fracture occurs in slope normal direction across the layering which releases the slab. For a better understanding of these mechanisms a well founded understanding of the fracture mechanical properties of homogeneous and layered snow is essential. The aim of this work was to investigate the fracture mechanical properties of snow under tension (mode I) and under shear (mode II) with homogeneous and layered snow samples on the basis of experiments in the cold laboratoryand in the field and to relate the results to dry slab avalanche release. The experimental work was structured in three groups of fracture experiments: experiments in mode I with homogeneous snow samples in the cold laboratory, experiments in mode II with layered snow samples in the cold laboratoryand mode II experiments with in-situ snow beams in the field. For the mode I experiments, beam-shaped snow specimenscut from homogeneous layers of naturally deposited snow were subjected to three-point bending and cantilever beam tests. Uncracked specimenswere used to determine the tensile strength of snow and notched specimensto determine the critical stress intensity factor in mode I. The threepoint bendingtests provided higher values than the cantilever beam tests. Furthermore the cantilever beam tests depended on cantilever length. The differences betweenthe test methods were significant and were attributed to non-negligible size and shape effects. The fracture process zone was experimentally determined and was estimated to be in the order of several centimeters, implying that snow has to be considered as a quasibrittle material at the scale of our experiments. For a quasi-brittle material linear elastic fracture mechanics is applicable only with a size correction. As a method to correct the critical stress intensity factor to the size-independent fracture toughness, Kjc, which is a material property, the equivalent fracture toughness, Kc[c, was determined aecording to Bazant and Planas (1998) and a size correction function was proposed. The results for Kfü ranged from 0.8kPa>/m for a density of p = 150kg/m3 up to GkPu^Jm for a density of p = 350 kg/m3 for typical slab layers. It was confirmed that snow has an extremely low value of Kjc. Fracture toughness is expected to be size dependent up to the scale of a slab avalanche. Layered snow samples including a weak layer were tested in mode II to determine the energy release rate of a crack propagating along the weak layer. A new experimental setup based on a cantilever beam experiment was designed and proved to be applicable for layered snow samples. In absence of an analytical Solution, the finite element method (FEM) was used to simulate the experiments and determine the energy release rate numerically. A critical energy release rate Gf = 0.04 ± 0.02J/m2 was found for the tested weak layers. Gf was primarily a material property of the weak layer. For similar snow densities, mode I fracture toughness results were about two times as large as for the tested weak layers in mode II. Two analytical approacheswere tested and compared to the FEM results. Both analytical approaches, a homogeneous cantilever beam with a deep crack, and a bilayer beam with interface crack were highly correlated with the results obtained from the FE model. The analytical results of both approaches were too large by a factor of about two. Due to the higher coefficient of determination, the cantilever beam approachshould be preferred. In addition, the dynamic Young'smodulus of the tested snow sampleswas determined. The results for the Young's modulus were strongly correlated with an index for the Young's modulus derived from a penetration resistance profile recorded with a snow micro-penetrometer SMP. A field test was developed in which a weak layer in an isolated snow beam was tested in mode II in-situ on a slope. The critical energy release rate Gf was determined numerically in a FEM Simulation. The result for the tested weak layers was Gf = 0.07±0.01 J/m2. It was found that slope normal bending of the slab contributed considerably to the energy release rate G of our tests and was more important than the component due to shear loading for angles between 30° and 45°. Critical crack sizes of about 25 cm were required to start fracture propagation along the weak layer of the isolated beams. By applying new test methods to snow and acquiring a considerable data set of fracture mechanical properties of snow in laboratory and field tests, it was possible to improve the knowledge and the understanding of the fracture mechanical behaviour of snow. It could be shown that for fracture propagation the material properties of the weak layer as well as of the overlayingslab play an important role. Whereas the energy to fracture a weak layer depends on the material properties of the weak layer, the available energy for crack propagation depends mainly on the material properties of the overlaying slab and the slope normal collapse height of a weak layer. It is expected that this behaviour holds also for the scale of a slab avalanche.

Simulations of non-neutral slab systems with long-range electrostatic interactions in two-dimensional periodic boundary conditions.

Abstract: We introduce a regularization procedure to define electrostatic energies and forces in a slab system of thickness h that is periodic in two dimensions and carries a net charge. The regularization corresponds to a neutralization of the system by two charged walls and can be viewed as the extension to the two-dimensional (2D)+h geometry of the neutralization by a homogeneous background in the standard three-dimensional Ewald method. The energies and forces can be computed efficiently by using advanced methods for systems with 2D periodicity, such as MMM2D or P3M/ELC, or by introducing a simple background-charge correction to the Yeh-Berkowitz approach of slab systems. The results are checked against direct lattice sum calculations on simple systems. We show, in particular, that the Madelung energy of a 2D square charge lattice in a uniform compensating background is correctly reproduced to high accuracy. A molecular dynamics simulation of a sodium ion close to an air/water interface is performed to demonstrate that the method does indeed provide consistent long-range electrostatics. The mean force on the ion reduces at large distances to the image-charge interaction predicted by macroscopic electrostatics. This result is used to determine precisely the position of the macroscopic dielectric interface with respect to the true molecular surface.