ALBERT

Description: Following earlier studies, we present forward and inverse simulations of heat and fluid transport of the upper crust using a local 3-D model of the Kola area. We provide best estimates for palaeotemperatures and permeabilities, their errors and their dependencies. Our results allow discriminating between the two mentioned processes to a certain extent, partly resolving the non-uniqueness of the problem. We find clear indications for a significant contribution of advective heat transport, which, in turn, imply only slightly lower ground surface temperatures during the last glacial maximum relative to the present value. These findings are consistent with the general background knowledge of (i) the fracture zones and the corresponding fluid movements in the bedrock and (ii) the glacial history of the Kola area.

Description: Along the central segment of the San Andreas Fault (SAF) near Parkfield, California, displacement occurs by a combination of aseismic creep and micro-earthquake slip. To constrain the strength and parametrize a constitutive relation for the creeping behaviour of the central segment of the SAF, we conducted friction experiments on clay-rich gouge retrieved by coring the Central Deforming Zone (CDZ) of the SAF at 2.7 km vertical depth. The gouge was flaked rather than powdered to preserve the natural scaly microfabric, and formed into 2-mm-thick layers that were sheared using a triaxial deformation apparatus. Experiments were conducted at in situ effective normal stress (100 MPa), pore pressure (25 MPa) and temperature (80–120 °C) conditions using brine pore fluid with the ionic composition of the in situ formation fluid. Velocity-stepping (0.006–0.6 μm s –1 ) and temperature-stepping experiments were conducted on brine-saturated gouge, and slide-hold-slide experiments were conducted on brine-saturated and room-dry gouge. Results are used to quantify the effects of rate, state, temperature and pore fluid on the strength of the CDZ gouge. We find that the gouge is extremely weak ( μ  〈 0.13) and rate-strengthening, consistent with findings of previous studies on the CDZ gouge. We also find that, in a rate and state friction framework, slip history has a negligible effect on strength ( b 0) under both saturated and dry conditions. The CDZ gouge is temperature-weakening from 80 to 120 °C and weakens 17 per cent when saturated with brine compared to room-dry conditions. Employing the laboratory-derived friction constitutive parameters, and including the temperature weakening and the strain-rate strengthening effects, we determine an approximate in situ friction coefficient of μ 0.11. For μ 0.11, aseismic creep under normal pore fluid conditions is permitted for angles up to 79° between the maximum horizontal stress and the plane of the SAF, consistent with nearby stress orientation measurements.

Description: Laboratory experiments in which synthetic, partially molten rock is subjected to forced deformation provide a context for testing hypotheses about the dynamics and rheology of the mantle. Here our hypothesis is that the aggregate viscosity of partially molten mantle is anisotropic, and that this anisotropy arises from deviatoric stresses in the rock matrix. We formulate a model of pipe Poiseuille flow based on theory by Takei & Holtzman and Takei & Katz. Pipe Poiseuille is a configuration that is accessible to laboratory experimentation but for which there are no published results. We analyse the model system through linearized analysis and numerical simulations. This analysis predicts two modes of melt segregation: migration of melt from the centre of the pipe towards the wall and localization of melt into high-porosity bands that emerge near the wall, at a low angle to the shear plane. We compare our results to those of Takei & Katz for plane Poiseuille flow; we also describe a new approximation of radially varying anisotropy that improves the self-consistency of models over those of Takei & Katz. This study provides a set of baseline, quantitative predictions to compare with future laboratory experiments on forced pipe Poiseuille flow of partially molten mantle.

Description: Electromagnetic (EM) measurements were performed 1 yr after the most recent eruption of the Eyjafjallajökull volcano (2010 March–May), southern Iceland, to investigate the geometries of structures of the volcano system through imaging lateral and vertical electrical resistivity variations. High quality magnetotelluric (MT) and transient EM data were acquired at 26 sites around Eyjafjallajökull and the southern part of Myrdalsjökull (the glacier covering the Katla volcano). For some locations the steep topography has influence on the MT responses, but this can be compensated by static shift correction using the transient EM data and/or including topography in the modelling mesh. As expected, qualitative indicators, such as phase tensor ellipses and induction arrows, infer a concentration of conductive material beneath Eyjafjallajökull. 2-D resistivity models are presented from data along three profiles: Along the river valley of Markarfljót in the north, along the coast to the south of Eyjafjallajökull and across the mountain ridge Fimmvörðuháls between Eyjafjallajökull and Katla. In numerous previous studies elsewhere in Iceland a conductive layer at about 10–30 km depth was identified. From our data, such a conductor is also present in the northeastern part of the investigated area. Additionally, all profiles show a conductive, near-surface layer at about 1–2 km depth, as seen previously for example at the Hengill geothermal region. A connection between those two conductive layers is indicated by the resistivity models, and the dyke (flank eruption) and the conduit (summit eruption) appear as vertical conductive structures. It is uncertain if the vertical connection is permanent or a transient feature as consequence of the eruptive sequences. Subsequent measurements are required when the volcano system is quiescent.

Description: Artificial neural networks (ANNs) are a valuable and well-established inversion technique for the estimation of geophysical parameters from satellite images; once trained, they help generate very fast results. Furthermore, satellite remote sensing is a very effective and safe way to monitor volcanic eruptions in order to safeguard the environment and the people affected by those natural hazards. This paper describes an application of ANNs as an inverse model for the simultaneous estimation of columnar content and height of sulphur dioxide (SO 2 ) plumes from volcanic eruptions using hyperspectral data from remote sensing. In this study two ANNs were implemented in order to emulate a retrieval model and to estimate the SO 2 columnar content and plume height. ANNs were trained using all infrared atmospheric sounding interferometer (IASI) channels between 1000–1200 and 1300–1410 cm –1 as inputs, and the corresponding values of SO 2 content and height of plume, obtained from the same IASI channels using the SO 2 retrieval scheme by Carboni et al. , as target outputs. The retrieval is demonstrated for the eruption of the Eyjafjallajökull volcano (Iceland) for the months of 2010 April and May and for the Grimsvotn eruption during 2011 May. Both neural networks were trained with a time series consisting of 58 hyperspectral eruption images collected between 2010 April 14 and May 14 and 16 images from 2011 May 22 to 26, and were validated on three independent data sets of images of the Eyjafjallajökull eruption, one in April and the other two in May, and on three independent data sets of the Grímsvötn volcanic eruption that occurred in 2011 May. The root mean square error (RMSE) values between neural network outputs and targets were lower than 20 Dobson units (DU) for SO 2 total column and 200 millibar (mb) for plume height. The RMSE was lower than the standard deviation of targets for the Grímsvötn eruption. The neural network had a lower retrieval accuracy when the target value was outside the values used during the training phase.

Description: Due to cyclic fluctuations of reservoir water levels, bank slopes in drawdown areas are subjected to wetting–drying cycles. In order to reasonably evaluate the stabilities of sandstone slopes in the drawdown area of the Three Gorges Reservoir, it is a primary premise to obtain the strength and failure characteristics of the sandstones undergoing wetting–drying cycles. In this paper, the conventional triaxial compression tests, ultrasonic velocity and porosity measurements and microstructural observations were conducted on Jurassic Red-Bed sandstone (JRS) specimens undergoing wetting–drying cycles. The results from the triaxial experiments indicate that the peak strengths of the JRS are dramatically reduced after the first wetting–drying cycle, and then remain approximately constant with increasing number of wetting–drying cycles. The failure modes of the JRS samples undergoing different wetting–drying cycles are all brittle failures under low confining pressures ( ≤ 15 MPa). The decrease in P -wave velocity and increase in porosity with increasing number of wetting–drying cycles reveals the raise of damage level of the sandstone specimens, which is the main reason for the decline of peak strength. Detailed microstructural analysis has shown obvious argillization phenomena after undergoing wetting–drying cycles, which weakens the cements between grains in the sandstone and increases the damage of the sandstone.