Biotic controls on shallow translational landslides

In undisturbed tropical montane rainforests massive organic layers accommodate the majority of roots and only a small fraction of roots penetrate the mineral soil. We investigated the contribution of vegetation to slope stability in such environments by modifying a standard model for slope stability to include an organic layer with distinct mechanical properties. The importance of individual model parameters was evaluated using detailed measurements of soil and vegetation properties to reproduce the observed depth of 11 shallow landslides in the Andes of southern Ecuador. By distinguishing mineral soil, organic layer and above‐ground biomass, it is shown that in this environment vegetation provides a destabilizing effect mainly due to its contribution to the mass of the organic layer (up to 973 t ha^{? 1} under wet conditions). Sensitivity analysis shows that the destabilizing effect of the mass of soil and vegetation can only be effective on slopes steeper than 37.9°. This situation applies to 36% of the study area. Thus, on the steep slopes of this megadiverse ecosystem, the mass of the growing forest promotes landsliding, which in turn promotes a new cycle of succession. This feedback mechanism is worth consideration in further investigations of the impact of landslides on plant diversity in similar environments. Copyright © 2012 John Wiley & Sons, Ltd.     

Reliability of short seismic links in shear

This paper presents the results of research in reliability of short seismic links in shear based on tests. Four types of short links were analysed, each having the same cross section and the same length, but with different number of web stiffeners. The main purpose of the stiffeners is to preserve buckling of the seismic link web, i.e. to achieve plastification of the cross section by shear. The design model of shear resistance according to Eurocode 8 is applicable only to short links without web stiffeners. By adding the web stiffeners nonlinear inelastic behaviour of short seismic links differs depending on the number of stiffeners, so that the calculation model of shear resistance according to Eurocode 8 for short seismic link with stiffeners should be improved. This fact is considered by introducing the improve factors that were determined from the laboratory tests conducted on 16 specimens. On the base of tests in the second part of this paper the reliability of short seismic link is performed by forming limit state equations. These equations are formed by using the stochastic model, i.e. by describing the statistical nature of basic variables calculating the reliability index as an operational value of failure probability. The reliability level was determined by using the probabilistic analysis based on the first order reliability method which resulted with the conclusion that the short seismic links with two and three couples of web stiffeners designed according to requirements of Eurocode 8 have enough reliability for the reliability class RC2 and the mean recurrence interval of 50 years.     

Deflection of Coronal Rays by Remote CMEs: Shock Wave or Magnetic Pressure?

We analyze five events of the interaction of coronal mass ejections (CMEs) with the remote coronal rays located up to 90° away from the CME as observed by the SOHO/LASCO C2 coronagraph. Using sequences of SOHO/LASCO C2 images, we estimate the kink propagation in the coronal rays during their interaction with the corresponding CMEs ranging from 180 to 920 km s⁻¹ within the interval of radial distances from 3 R _⊙ to 6 R _⊙. We conclude that all studied events do not correspond to the expected pattern of shock wave propagation in the corona. Coronal ray deflection can be interpreted as the influence of the magnetic field of a moving flux rope within the CME. The motion of a large-scale flux rope away from the Sun creates changes in the structure of surrounding field lines, which are similar to the kink propagation along coronal rays. The retardation of the potential should be taken into account since the flux rope moves at a high speed, comparable with the Alfvén speed.     

Modification by lateral mixing of the Warm Deep Water entering the Weddell Sea in the Maud Rise region

Deep water originating in the North Atlantic is transported across the Antarctic Circumpolar Current by eddies and, after circumnavigating of the Antarctic, enters the Weddell Gyre south of Africa. As it does so, it rises up from mid-depth towards the surface. The separate temperature and salinity maxima, the Upper and Lower Circumpolar Deep Waters, converge to form the Warm Deep Water. Cores of this water mass on the southern flank of the eastern Weddell Gyre show a change in characteristic as they flow westward in the Lazarev Sea. Observations have been made along four meridional sections at 3° E, 0°, 3° W and 6° W between 60 and 70° S during the Polarstern Cruise ANTXXIII/2 in 2005/2006. These show that a heterogeneous series of warm and salty cores entering the region from the east both north and south of Maud Rise (65° S, 3° W) gradually merge and become more homogeneous towards the west. The gradual reduction in the variance of potential temperature on isopycnals is indicative of isopycnic mixing processes. A multiple regression technique allows diagnosis of the eddy diffusivities and, thus, the relative importance of isopycnic and diapycnic mixing. The method shows that the isopycnic diffusivity lies in the range 70–140 m² s⁻¹ and the diapycnic diffusivity reaches about 3 × 10⁻⁶ m² s⁻¹. Scale analysis suggests that isopycnic diffusion dominates over diapycnic diffusion in the erosion of the Warm Deep Water cores.     

Multiband photometric detection of a huge flare on the M9 dwarf 2MASSW J1707183+643933

We present simultaneous UV , G , R and I monitoring of 19 M dwarfs that reveal a huge flare on the M9 dwarf with an amplitude in the UV of at least 6 mag. This is one of the strongest detections ever of an optical flare in an M star and one of the first in an ultracool dwarf (spectral types later than about M7). Four intermediate-strength flares  (Δ m _UV < 4 mag)  were found in this and three other targets. For the whole sample we deduce a flare probability of 0.013 (rate of  0.022 h⁻¹  ), and  0.049 (0.090 h⁻¹)  for 2M1707+64 alone. Deviations of the flare emission from a blackbody is consistent with strong  Hα  line emission. We also confirm our previously found rotation period for 2M1707+64 and determine it more precisely to be  3.619 ± 0.015 h  .     

Circulation and transport of water masses in the Lazarev Sea, Antarctica, during summer and winter 2006

The distribution and circulation of water masses in the region between 6°W and 3°E and between the Antarctic continental shelf and 60°S are analyzed using hydrographic and shipboard acoustic Doppler current profiler (ADCP) data taken during austral summer 2005/2006 and austral winter 2006. In both seasons two gateways are apparent where Warm Deep Water (WDW) and other water masses enter the Weddell Gyre through the Lazarev Sea: (a) a probably topographically trapped westward, then southwestward circulation around the northwestern edge of Maud Rise with maximum velocities of about 20 cm s⁻¹ and (b) the Antarctic Coastal Current (AntCC), which is confined to the Antarctic continental shelf slope and is associated with maximum velocities of about 25 cm s⁻¹.Along two meridional sections that run close to the top of Maud Rise along 3°E, geostrophic velocity shears were calculated from CTD measurements and referenced to velocity profiles recorded by an ADCP in the upper 300 m. The mean accuracy of the absolute geostrophic velocity is estimated at ±2 cm s⁻¹. The net baroclinic transport across the 3°E section amounts to 20 and 17 Sv westward for the summer and winter season, respectively. The majority of the baroclinic transport, which accounts for ∼60% of the total baroclinic transport during both surveys, occurs north of Maud Rise between 65° and 60°S.However, the comparison between geostrophic estimates and direct velocity measurements shows that the circulation within the study area has a strong barotropic component, so that calculations based on the dynamic method underestimate the transport considerably. Estimation of the net absolute volume transports across 3°E suggests a westward flow of 23.9±19.9 Sv in austral summer and 93.6±20.1 Sv in austral winter. Part of this large seasonal transport variation can be explained by differences in the gyre-scale forcing through wind stress curl.     

A phase-by-phase upstream scheme that converges to the vanishing capillarity solution for countercurrent two-phase flow in two-rock media

We discuss the convergence of the upstream phase-by-phase scheme (or upstream mobility scheme) towards the vanishing capillarity solution for immiscible incompressible two-phase flows in porous media made of several rock types. Troubles in the convergence were recently pointed out by Mishra and Jaffré (Comput. Geosci. 14, 105–124, 2010) and Tveit and Aavatsmark (Comput. Geosci. 16, 809–825, 2012). In this paper, we clarify the notion of vanishing capillarity solution, stressing the fact that the physically relevant notion of solution differs from the one inferred from the results of Kaasschieter (Comput. Geosci. 3, 23–48, 1999). In particular, we point out that the vanishing capillarity solution depends on the formally neglected capillary pressure curves, as it was recently proven in by Andreianov and Cancès (Comput. Geosci. 17, 551–572, 2013). Then, we propose a numerical procedure based on the hybridization of the interfaces that converges towards the vanishing capillarity solution. Numerical illustrations are provided.