Impact of galactic and intergalactic dust on the stellar EBL

Current theories assume that the low intensity of the stellar extragalactic background light (stellar EBL) is caused primarily by finite age of the Universe because the finite age limits the number of photons pumped into the space by galaxies and thus the sky is dark in the night. We oppose this opinion and show that two main factors are responsible for the extremely low intensity of the observed stellar EBL: (1) a low mean surface brightness of galaxies, which causes a low luminosity density in the local Universe, and (2) light extinction due to absorption by galactic and intergalactic dust. Dust produces a partial opacity of galaxies and of the Universe. The galactic opacity reduces the intensity of light from more distant background galaxies obscured by foreground galaxies. The effective extinction AV for light passing through a galaxy is 0.2 mag. This causes that distant background galaxies do not contribute to the EBL significantly. In addition, light of distant galaxies is dimmed due to absorption by intergalactic dust. Even a minute intergalactic opacity of 1x10^(-2) mag per Gpc is high enough to produce significant effects on the EBL. The absorbed starlight heats up the galactic and intergalactic dust and is further re-radiated at the IR, FIR and micro-wave spectrum. Assuming static infinite universe with no galactic and intergalactic dust, the stellar EBL should be as high as the surface brightness of stars. However, if dust is considered, the predicted stellar EBL is about 290 nWm^(-2)sr^(-1), which is only 5 times higher than the observed value. Hence, the presence of dust has higher impact on the EBL than currently assumed. In the expanding universe, the calculated value of the EBL is further decreased, because the obscuration effect and intergalactic absorption become more pronounced at high redshifts when the matter was concentrated at smaller volume than at present.Comment: 9 pages, 3 figure

Closing crack earthquakes within the Krafla caldera, North Iceland

Moment tensor analysis with a Bayesian approach was used to analyse a non-double-couple (non-DC) earthquake ($M_w$ ~ 1) with a high isotropic (implosive) component within the Krafla caldera, Iceland. We deduce that the earthquake was generated by a closing crack at depth. The event is well located, with high signal-to-noise ratio and shows dilatational $P$-wave first arrivals at all stations where the first arrival can be picked with confidence. Coverage of the focal sphere is comprehensive and the source mechanism stable across the full range of uncertainties. The non-DC event lies within a cluster of microseismic activity including many DC events. Hence, we conclude that it is a true non-DC closing crack earthquake as a result of geothermal utilization and observed magma chamber deflation in the region at present.Natural Environment Research Council (Grant ID: NE/H025006/1

Damage patterns, stress rotations and pore fluid pressures in strike-slip fault zones

Active faults unfavorably oriented with respect to the regional maximum compressive stress have been labeled as "weak." The seismic hazards posed by these faults make understanding this apparent weakness a priority. Stress rotations in these fault zones, together with an increase in mean stress, could enable high pore fluid pressures to weaken a fault zone. Such a model requires a foundation in the physics and mechanics of damage. This paper presents a new model for stress rotations in fault zones by combining the Effective Medium Theory with anisotropic poroelasticity. This approach enables the quantitative characterization of crack damage and the prediction of progressive changes in the elastic properties of rocks across the fault zone. The processes of fault growth and wear will lead to distinct patterns of crack damage, with different effects on the elastic properties. Elevated pore fluid pressures have long been known to change the effective normal and shear stresses of anisotropic rocks, and this work incorporates these effects into a multilayer fault zone model. It is shown that high pore fluid pressures in the anisotropic rocks of the core zone can generate large stress rotations (i.e., more fault-parallel), and increases in mean stress, sufficient to weaken the fault. Stress rotations in the damage zones of unfavorably oriented faults tend to be away from the fault (i.e., more fault-normal) for likely combinations of damage patterns and pore fluid pressure

Wavenumber sampling strategies for 2.5-D frequency-domain seismic wave modelling in general anisotropic media

The computational efficiency of 2.5-D seismic wave modelling in the frequency domain depends largely on the wavenumber sampling strategy used. This involves determining the wavenumber range and the number of the sampling points, and overcoming the singularities in the wavenumber spectrum when taking the inverse Fourier transform to yield the frequency-domain wave solution. In this paper, we employ our newly developed Gaussian quadrature grid numerical modelling method and extensively investigate the wavenumber sampling strategies for 2.5-D frequency-domain seismic wave modelling in heterogeneous, anisotropic media. We show analytically and numerically that the various components of the Green's function tensor wavenumber-domain solutions have symmetric or antisymmetric properties and other characteristics, all of which can be fully used to construct effective and efficient sampling strategies for the inverse Fourier transform. We demonstrate two sampling schemes—called irregular and regular sampling strategies for the 2.5-D frequency-domain seismic wave modelling technique. The numerical results, which involve calibrations against analytic solutions, comparison of the different wavenumber sampling strategies and validation by means of 3-D numerical solutions, show that the two sampling strategies are both suitable for efficiently computing the 3-D frequency-domain wavefield in 2-D heterogeneous, anisotropic media. These strategies depend on the given frequency, elastic model parameters and maximum wavelength and the offset distance from the source.Bing Zhou, Stewart Greenhalgh and Mark Greenhalg