Influence of the zone of weakness on dip angle and shear heating of subducted slabs

We study the importance of the zones of weakness and the pattern of downgoing flow in steady-state models of subducting lithosphere, which interacts mechanically and thermally with the ambient mantle. The non-linear system of governing equations consists of (i) the momentum equation in stream function formulation and (ii) the steady-state heat transfer equation including conduction and advection of heat and dissipation. A finite element method has been applied to this system. We consider the viscosity to be a non-linear function of both the temperature and the stream function. In steady-state two-dimensional (2D) flow, the stream function isolines follow material trajectories. They are used to follow the top of the subducting slab, which because of its possible increase in water content, is assumed to have a lower viscosity. The zone of weakness has been thus obtained in the self-consistent fashion since the stream function as well as the temperature are the output from our modeling and no a priori assumptions about the shape of the bending lithosphere are taken into account. It was shown that several orders decrease of viscosity in the zone of weakness is required to obtain the dip angle of about 45°. If the decrease of viscosity is not sufficient enough, the subducted slab either sinks almost vertically or does not exhibit a plate-like behavior. We have also demonstrated that shear heating can unrealistically increase at the zone of weakness for fast subductions if decrease of viscosity is underestimated.     

Evaluation of Methods for Determining Crack Initiation in Compression Tests on Low-Porosity Rocks

Laboratory testing of rocks is traditionally carried out to determine the peak strength using the ISRM Suggested Methods or other suitable standards. However, it is well known that in low-porosity crystalline rocks there are at least three distinct stages of compressive loading that can be readily identified if the stress–strain response is monitored during the loading process: (1) crack initiation, (2) unstable crack growth, i.e., crack coalescence and (3) peak strength. Crack initiation is noted as the first stage of stress-induced damage in low-porosity rocks, yet the suggested guidelines of the ISRM for compression tests make no mention of crack initiation. In addition, recent research suggests that crack initiation can be used as an estimate for the in situ spalling strength, commonly observed around underground excavations in massive to moderately jointed brittle rocks. Various methods have been proposed for identifying crack initiation in laboratory tests. These methods are evaluated using ten samples of ?sp? Diorite and the results are compared with a simplified method, lateral strain response. Statistically, all methods give acceptable crack-initiation values. It is proposed that the ISRM Suggested Methods be revised to include procedures suitable for establishing the crack-initiation stress.     

Noise reduction and detection of weak, coherent signals through phase-weighted stacks

We present a new tool for efficient incoherent noise reduction for array data employing complex trace analysis. An amplitude-unbiased coherency measure is designed based on the instantaneous phase, which is used to weight the samples of an ordinary, linear stack. The result is called the phase-weighted stack (PWS) and is cleaned from incoherent noise. PWS thus permits detection of weak but coherent arrivals. The method presented can easily be extended to phase-weighted cross-correlations or be applied in the τ p domain. We illustrate and discuss the advantages and disadvantages of PWS in comparison with other coherency measures and present examples. We further show that our non-linear stacking technique enables us to detect a weak lower-mantle P -to- S conversion from a depth of approximately 840 km on array data. Hints of an 840 km discontinuity have been reported; however, such a discontinuity is not yet established due to the lack of further evidence.     

Corruption in cyclone preparedness and relief efforts in coastal Bangladesh: Lessons for climate adaptation?

This article seeks to draw possible lessons for adaptation programmes in Bangladesh by examining whether cyclone preparedness and relief interventions are subject to corrupt practices. Based on a random sample survey of 278 households, three focus-group discussions and seven key-informant interviews, the article investigates the nature and extent of corruption in pre- and post-disaster interventions in Khulna before and after Cyclone Aila in May 2009. Ninety nine percent of households reported losses from corrupt practices. Post-disaster interventions (such as food aid and public works schemes) suffered from greater levels, and worse types, of corruption than pre-disaster interventions (such as cyclone warning systems and disaster-preparedness training). Using an asset index created using principal component analysis, the article assesses how corruption affected wealth quartiles. Ultra-poor households were affected more by corruption in pre-disaster interventions, the wealthiest quartile more in certain post-disaster interventions, in particular public works and non-governmental interventions. These findings may hold lessons for attempts to increase resilience as current adaptation measures mirror some cyclone preparedness and relief efforts.     

Climate variability and trends in downscaled high-resolution simulations and projections over Metropolitan France

In order to fulfill the society demand for climate information at the spatial scale allowing impact studies, long-term high-resolution climate simulations are produced, over an area covering metropolitan France. One of the major goals of this article is to investigate whether such simulations appropriately simulate the spatial and temporal variability of the current climate, using two simulation chains. These start from the global IPSL-CM4 climate model, using two regional models (LMDz and MM5) at moderate resolution (15–20 km), followed with a statistical downscaling method in order to reach a target resolution of 8 km. The statistical downscaling technique includes a non-parametric method that corrects the distribution by using high-resolution analyses over France. First the uncorrected simulations are evaluated against a set of high-resolution analyses, with a focus on temperature and precipitation. Uncorrected downscaled temperatures suffer from a cold bias that is present in the global model as well. Precipitations biases have a season- and model-dependent behavior. Dynamical models overestimate rainfall but with different patterns and amplitude, but both have underestimations in the South-Eastern area (Cevennes mountains) in winter. A variance decomposition shows that uncorrected simulations fairly well capture observed variances from inter-annual to high-frequency intra-seasonal time scales. After correction, distributions match with analyses by construction, but it is shown that spatial coherence, persistence properties of warm, cold and dry episodes also match to a certain extent. Another aim of the article is to describe the changes for future climate obtained using these simulations under Scenario A1B. Results are presented on the changes between current and mid-term future (2021–2050) averages and variability over France. Interestingly, even though the same global climate model is used at the boundaries, regional climate change responses from the two models significantly differ.     

Analysis of a “flickering” Geminid fireball

Abstract— In the early morning hours of December 13, 2002, a bright Geminid fireball with an absolute magnitude of ?9.2 ± 0.5 was observed from Southern Saskatchewan, Canada. The fireball displayed distinct small‐scale oscillations in brightness, or flickering, indicative of the parent meteoroid being both non‐spherical and rotating. Using the light curve derived from a calibrated radiometer, we determine a photometric mass of 0.429 ± 0.15 kg for the meteoroid, and we estimate from its initial rotation rate of some 6 Hz that the meteoroid was ejected from the parent body (3200) Phaethon some 2500 ± 500 years ago. We find that 70% of Geminid fireballs brighter than magnitude ?3 display distinct flickering effects, a value that is in stark contrast to the 18% flickering rate exhibited by sporadic fireballs. The high coincidence of flickering and the deep atmospheric penetration of Geminid fireballs are suggestive of Geminid meteoroids having a highly resilient structure, a consequence, we suggest, of their having suffered a high degree of thermal processing. The possibility of Gemind material surviving atmospheric ablation and being sampled is briefly discussed, but the likelihood of collecting and identifying any such material is admittedly very small.     

Anisotropies of in-phase,out-of-phase,and frequency-dependent susceptibilities in three loess/palaeosol profiles in the Czech Republic; methodological implications

The relationship between the anisotropy of frequency-dependent magnetic susceptibility (fdAMS) and the anisotropy of out-of-phase magnetic susceptibility (opAMS) was investigated theoretically and also empirically at three loess/palaoesol profiles in Prague and in Southern Moravia. The data treatment was made in terms of mean susceptibility, degree of AMS, and orientations of principal susceptibilities. It has shown that the fdAMS and opAMS can serve as indicators of the preferred orientations of ultrafine magnetic particles that are on transition between superparamagnetic and stable single domain states in rocks, soils and environmental materials. In loess/palaeosol sequences, the fdAMS and opAMS correlate reasonably, because they are due to magnetic particles of similar grain sizes. The fdAMS and opAMS can be both coaxial with standard AMS (i.e. anisotropy of in-phase susceptibility - ipAMS) or non-coaxial indicating slightly different orientations of viscous magnetic particles.     

Passive seismic control of cable-stayed bridges with damped resonant appendages

A study is conducted to investigate the effectiveness of attaching to cable-stayed bridges resonant appendages with a relatively small mass and a high damping ratio as a means to reduce their response to earthquake excitations. The study is based on a previously developed formulation that shows that the use of these appendages may increase the inherent damping of building structures and, as a result, may reduce their response to seismic disturbances. It includes numerical and experimental tests that are conducted to assess the validity of such a formulation for the case of cable-stayed bridges and the extent to which such appendages can reduce their seismic response. In the numerical study, an actual cable-stayed bridge is modelled with finite elements and analysed with and without the proposed appendages under different earthquake ground motions. Appendages with damping ratios of 10, 15, 20 and 30 per cent and masses that, respectively, represent 0.67, 1.5, 2.7 and 6.0 per cent of the total mass of the bridge are considered. In the experimental test, a 3.7 m long cable-stayed bridge and an appendage consisting of a small mass, a small spring and a small viscous damper are built and the bridge tested, without and with the appendage, on a pair of shaking tables which are set to reproduce ground acceleration records from past earthquakes. The damping ratio of the appendage in this test is 32 per cent and its mass represents about 8 per cent of the total mass of the bridge model. In the numerical test, it is found that the appendages reduce the longitudinal response of the bridge deck up to 88 per cent. Similarly, in the experimental test it is found that the appendage reduces the longitudinal bridge deck response by about 41 per cent. From these studies, it is concluded that the suggested appendages may indeed be effective in reducing the seismic response of cable-stayed bridges as they are for building structures.     

Ultra-Deep Optical Spectroscopy with PMAS. Using the Nod-and-Shuffle Technique

PMAS, the Potsdam Multi-Aperture Spectrophotometer, is a new integral field spectrograph in the optical, which is optimized for good transmission and high image quality from 350 nm to 1 μm. We present our plan to implement a CCD charge-shuffle mode to allow for beam switching with a very high degree of sky subtraction accuracy for faint object 3-D spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.