Stress Release Mechanism Research Articles

Stresses along the metastable wedge of olivine in a subducting slab: possible explanation for the Tonga double seismic layer

A numerical calculation of the stresses associated with changes in volume during phase transitions of olivine in a descending slab results in a double layer of high shear stress along the metastable olivine wedge in the depth range 350–460 km. Stress in the upper layer is in-plane tensional and stress in the lower layer is down-dip compressional. The modeled stress field agrees with observations of stress in the Tonga double seismic zone. High shear stress also exists in the slab at depths below the metastable wedge. This stress distribution involves down-dip compression and trench-parallel tension, which agrees with about half of the focal mechanisms in the Tonga slab at depths of 460–690 km. The model supports the idea that at least two possible stress release mechanisms for deep earthquakes may act in the Tonga subducting slab. One, transformational faulting, is restricted to the metastable wedge while the other one acts below the metastable wedge.

Low-Frequency Properties of Intermediate-Focus Earthquakes

Intermediate-focus earthquakes are known to show features such as non-double-couple focal mechanisms that indicate source complexities. To characterize these features, we have systematically studied the low-frequency radiation from 108 intermediate-focus earthquakes recorded by high-performance seismic networks from 1989–1997 whose total moment was > 3 × 10 18 N m. We determined frequency-dependent focal mechanisms and source phase and amplitude spectra for each earthquake, estimating the uncertainties for all parameters. Frequency-dependent focal mechanisms were obtained from vertical-component, free-oscillation data in 1-mHz bands over the range of 1–11 mHz. Source amplitude and phase-delay spectra were determined to 20 mHz from a combination of free-oscillation and surface-wave data. The population of intermediate-focus earthquakes in our catalog is not equally divided between compressional and tensional stress-release mechanisms; instead, 59% are down-dip tensional, 25% are down-dip compressional, and 16% are neither. We have assessed the statistical significance of any non-double-couple component of the source for every earthquake in the catalog. We represent a deviatoric focal mechanism by its principle axes and a scalar ξ that ranges from –1, for a compensated linear vector dipole (CLVD) with a compressional axis of symmetry through 0 for a double couple, to +1, for a CLVD with a tensional axis of symmetry. ξ varies from –0.91 to +0.64 in this dataset. Fifty-six of the 108 events have a significant CLVD component (ξ ≠ 0) at the 95% confidence level; 15 events have an unusually large CLVD component of the focal mechanism (|ξ| ≥ 0.4). We do not observe a correlation between the CLVD component and seismic moment or depth. T -type CLVD mechanisms correlate weakly with slab stress state, but no correlation was similarly found for P -type CLVD events. Seven earthquakes in the catalog are slow or compound, and three show strong frequency dependence of the moment tensor.

An Integrated AFM and SANS Approach toward Understanding Void Formation in Conductive Composite Materials

The morphology of voids within carbon black-filled polyethylene conductive composites has been studied by both atomic force microscopy and small-angle neutron scattering. Both void size and polyethylene crystalline lamellae thickness were measured with both techniques, and the results were compared. Atomic force micrographs indicate that voids form at the carbon black−polyethylene interface as previously hypothesized from the interpretation of neutron scattering experiments alone. This result supports the conclusion that voids provide a stress release mechanism during polyethylene crystallization and that variables such as filler morphology, volume fraction, and polymer crystallinity may be used to quantitatively predict the extent of void incorporation as composite composition is varied.

Micro-scratch analysis and mechanical properties of plasma-deposited silicon-based coatings on polymer substrates

Advanced optical applications require multifunctional coatings with specific mechanical properties, such as resistance to damage and good adhesion to different types of substrates, including polymers. In the present study we deposited amorphous hydrogenated silicon nitride (SiN 1.3) and oxide (SiO 2) films on polycarbonate and on silicon substrates by plasma enhanced chemical vapor deposition (PECVD), using a dual-mode microwave/radio frequency plasma system. The film adhesion was determined by the micro-scratch test. Depth-sensing indentation and substrate curvature measurements were used to evaluate the microhardness. Young's modulus and residual stresses of the films. The adhesion strength, represented by the critical load, L c, when the film starts to delaminate, was determined as a function of the substrate material and the energy of bombarding ions. A direct correlation between the L c values and the mechanical properties of the films was found. The formation of different crack patterns in the coatings during the scratch procedure is explained in terms of stress release mechanism depending on the mechanical properties of the film, the substrate and the interface region. In addition, different models applicable to the evaluation of the work of adhesion in the case of hard coatings on soft substrates are critically reviewed.

Patterns of deep seismicity reflect buoyancy stresses due to phase transitions

Thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies. Finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near ∼400 km depth, down‐dip compression over ∼400–690 km (peaking at ∼550 km), and transition to rapidly fading tension below ∼690 km. Such features, even when complicated by olivine metastability, are consistent with observed patterns of deep seismicity. That such a simple model, neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations, successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs. It also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release.

Residual stress and thermal properties of zirconia/metal (nickel, stainless steel 304) functionally graded materials fabricated by hot pressing

To analyse the residual stress and the thermal properties of functionally graded materials (FGMs), disc-type tetragonal zirconia polycrystal (TZP)/Ni- and TZP/stainless steel 304 (SUS304)-FGM were hot pressed, and compared with directly jointed materials (DJMs). The continuous interface and the microstructure of FGMs were characterized by electron probe microanalysis, wavelength dispersive spectrometry, optical microscopy and scanning electron microscopy. It has been verified that the defect-like cracks in FGMs are induced by the preferential shear stress and shown to cause fracture. These facts have corresponded well with the residual stress distribution analysed by the finite element method. The thermal diffusivity and the thermal conductivity of FGMs and DJMs were also measured by the laser flash technique. As a consequence, this work has described the interfacial stability, the residual stress release mechanism and the thermal protection characteristics of FGMs via a compositional gradient.

ＦｅＲｈ合金薄膜の再結晶過程と磁気相転移

The recrystallization process in Fe51.2Rh48.8 thin films was studied to clarify the relationship between the film microstructure and the magnetic transition. It was found that high-temperature annealing results in grain growth and a plate-like structure. The transition temperature and the thermal hysteresis width decrease from 70 to 10°C and from 140 to 3°C, respectively, as the annealing temperature increases from 600 to 1200°C. The film annealed at 1200°C exhibits complete ferromagnetic-to-antiferromagnetic transition as bulk FeRh alloy does. The thin plate-like structure and the lattice distortion strongly affect the thermal hysteresis. It was found that the stress release mechanism is closely related to the thin plate martensitic transformation.

X‐ray photoelectron spectroscopy study of the atomic structure change of amorphous carbon films annealed in vacuum

Amorphous carbon films deposited at near room temperature on Si substrates by a direct current plasma chemical vapor deposition technique using CH4 gas have been annealed in vacuum. X‐ray photoelectron spectroscopy measurements show that, as the annealing temperature increases from room temperature to 300 °C, a significant shift (0.5 eV) of the C 1s electron binding energy is observed. The observed shift may be the indication of an atomic structure change of the amorphous films upon the thermal annealing. On the other hand, a large shift in the sp2 bond peaks of the Raman spectra only occurs at annealing temperatures higher than 300 °C, which may be the indication of a structural change of the amorphous carbon films by the annealing. A two stage stress release mechanism is suggested: One for the atomic structure change and one for the film structure change.

A Kinetic model of crack fusion

SUMMARY The paper presents a kinetic approach to the problem of fusion of cracks. A kinetic equation for the size distribution function of cracks is derived. By division of all cracks into three groups a set of three simple equations is obtained. From a numerical analysis of the simple model we conclude that periodic solutions appear, if the intensity of nucleation of little cracks is big enough and a stress release mechanism at the crack source is included. the periodic solutions can be interpreted as repetitive episodes of seismicity.

A model for repetitive cycles of large earthquakes

The theory of the fusion of small cracks into large ones reproduces certain features also observed in the clustering of earthquake sequences. By modifying our earlier model to take into account the stress release associated with the occurrence of large earthquakes, we obtain repetitive periodic cycles of large earthquakes. A preliminary conclusion is that a combination of the stress release or elastic rebound mechanism plus time delays in the fusion process are sufficient to destabilize the crack populations and, ultimately, give rise to repetitive episodes of seismicity.