Regional variation of Rayleigh wave attenuation coefficients in the Eastern Pacific

Seismograms recorded for five earthquakes on the east Pacific rise have been analyzed to obtain the attenuation coefficients of the fundamental Rayleigh mode for the eastern Pacific in the 15–110 second period range. The attenuation coefficients have been obtained using two new methods, a reference-station method, and an iterative method by which the seismic moment and regionalized attenuation coefficient values are obtained simultaneously after considering the effect of the source directivity and time-function. The reference-station method was applied to the entire eastern Pacific, excluding paths along the east Pacific rise. When using the iterative method we divided the eastern Pacific into three sub-regions, designated as the north-eastern Pacific, the Nazca plate and the east Pacific rise. Although much scatter is present, the data suggest that attenuation coefficients for the Nazca plate are higher than those for the northeastern Pacific, and both are substantially higher than average values obtained for the entire Pacific Ocean. Two paths that lie along or near the east Pacific rise are characterized by especially high attenuation coefficients. These values suggest that a low-Q zone exists beneath that narrow feature.     

Morphology and geotechnique of active-layer detachment failures in discontinuous and continuous permafrost, northern Canada

Fifty active-layer detachment failures triggered after forest fire in the discontinuous permafrost zone (central Mackenzie Valley, 65° N.) are compared to several hundred others caused by summer meteorological triggers in continuous permafrost (Fosheim Peninsula, Ellesmere Island, 80°N). Most failures fall into compact or elongated morphological categories. The compact type occur next to stream channels and have little internal disturbance of the displaced block, whereas the elongated types can develop on any part of the slope and exhibit greater internal deformation. Frequency distributions of length-to-width and length-to-depth ratios are similar at all sites. Positive pore pressures, expected theoretically, were measured in the field at the base of the thawing layer. Effective stress analysis could predict the instability of slopes in both areas, providing cohesion across the thaw plane was set to zero and/or residual strength parameters were employed. The location of the shear planes or zones in relation to the permafrost table and the degree of post-failure secondary movements (including headwall recession and thermokarst development within the failure track) differed between the localities, reflecting dissimilarity in the environmental triggers and in the degree of ground thermal disturbance.     

Frontal advance of turf-banked solifluction lobes, Kluane Range, Yukon Territory, Canada

Humus horizons associated with thirteen solifluction lobes on a single slope were radiocarbon dated at fixed distances behind the risers, as well as ahead of the lobes. The dated horizons, together with stratigraphic observations of planar discontinuities inside the lobes and shallow burial of vegetation ahead of the lobes, all suggest rapid advance of lobe fronts. We infer that lobe advance at this site results from slow accumulation of soliflucted material behind a relatively rigid riser, progressive steepening of the riser and build-up of stress, and finally rupture of the front and its collapse onto the slope. Rebuilding of the lobe usually then takes place. The time for a full cycle of development is estimated to be in the order of a few hundred years. Long-term frontal movement appears inversely related to lobe size, and is mainly controlled by soil moisture and sediment characteristics. These findings call into question the usefulness of solifluction lobe advance rates for paleoclimate reconstructions, at least at this site.     

Design criteria for roadway supports to resist dynamic loads

Summary Conditions under which dynamic loads occur in mines are briefly described and the special vulnerability of roadways in coal mines to fail under dynamic load is considered. A method for assessment of shock load energy anticipated is proposed, based on the volume of rock and the velocity of rock particles induced by rapid rock failure and/or seismic tremors. Case examples from Upper Silesian coal mines are given and the safety factor of steel supports against the shock energy from rockbursts is discussed. The shock energy damping ability of various parts of steel supports and support systems is calculated as a basis for rational support design. Results of six mine experiments where various types of roadway support were installed and then loaded dynamically by blasting within surrounding rocks are discussed.     

Hydrothermal alteration and fluid inclusion study of the Lower Cretaceous porphyry Cu–Au deposit of Tiámaro, Michoacán, Mexico

The Tiámaro deposit in Michoacán state has been dated as Lower Cretaceous (Valanginian), though most of the porphyry deposits in central Mexico were dated or have an attributed Eocene–Oligocene age. The host rocks belong to a volcanoplutonic complex overlain by red conglomerates. These rocks were intruded by pre-Valanginian plutonic and hypabissal rocks. Propylitic, phyllic, and argillic alteration assemblages developed, and their superimposition draws the evolution of the deposit. Stage I is represented by propylitic assemblages, stage II contains the main ore forming stockworks and both phyllic and argillic assemblages, and stage III contains late carbonatization assemblages. The obtained temperatures and salinities from inclusion fluids are low for a porphyry-type deposit, but we interpret that the known part of the deposit represents the shallow portion of a bigger deposit. The evolution of mineralizing fluids draws a dilution trend of brines from “porphyry-like” to “epithermal-like” stages. The richest ore zone is roughly located between the 300 and 350 °C isotherms, though unnoticed resources may occur at depth.     

Net primary production and decomposition of salt marshes of the Ebre delta (Catalonia,Spain)

Net primary production was measured in three characteristic salt marshes of the Ebre delta: anArthrocnemum macrostachyum salt marsh,A. macrostachyum-Sarcocornia fruticosa mixed salt marsh andS. fruticosa salt marsh. Above-ground and belowground biomass were harvested every 3 mo for 1 yr. Surface litter was also collected from each plot. Aboveground biomass was estimated from an indirect non-destructive method, based on the relationship between standing biomass and height of the vegetation. Decomposition of aboveground and belowground components was studied by the disappearance of plant material from litter bags in theS. fruticosa plot. Net primary production (aboveground and belowground) was calculated using the Smalley method. Standing biomass, litter, and primary production increased as soil salinity decreased. The annual average total aboveground plus belowground biomass was 872 g m⁻² in theA. macrostachyum marsh, 1,198 g m⁻² in theA. macrostachyum-S. fruticosa mixed marsh, and 3,766 g m⁻² in theS. fruticosa biomass (aboveground plus belowground) was 226, 445, and 1,094 g m⁻², respectively. Total aboveground plus below-ground net primary production was 240, 1,172, and 1,531 g m⁻² yr⁻¹. There was an exponential loss of weight during decomposition. Woody stems and roots, the most recalcitrant material, had 70% and 83% of the original material remaining after one year. Only 20–22% of leafy stem weight remained after one year. When results from the Mediterranean are compared to other salt marshes dominated by shrubbyChenopodiaceae in Mediterranean-type climates, a number of similarities emerge. There are similar zonation patterns, with elevation and maximum aboveground biomass and primary production occurring in the middle marsh. This is probably because of stress produced by waterlogging in the low marsh and by hypersalinity in the upper marsh.     

New OSL dating of UK loess: indications of two phases of Late Glacial dust accretion in SE England and climate implications

Windblown dust deposits or loess, locally known in the UK as brickearth, blanket many parts of southern England outside the limits of the British and Irish Ice Sheet. The best‐studied loess exposures occur in Kent, southeast England, where the deposits are believed to be Late Glacial in age. New sedimentological evidence is presented for phases of accretion with two distinct units, a calcareous brickearth exhibiting periglacial cryoturbation and a non‐calcareous, massive brickearth. Optically stimulated luminescence (OSL) dating has been applied to chemically isolated modal quartz silts to derive a chronology for sediment accretion and to better understand the palaeoclimatic significance of these deposits. © British Geological Survey/Natural Environment Research Council copyright 2007. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.     

A facies geological test of stable isotope interpretation of the Upper Permian depositional environment in West Spitsbergen

Lithofacies analysis of two transgressive sequences within the Kapp Starostin Formation (Upper Permian, West Spitsbergen), indicates considerable differences in seawater chemistry between their depositional environments. The onshore part of the sequence represented in the Vøringen and Svenskeegga Members is largely calcareous, with allochthonous and corroded siliceous materials. In contrast, in the equivalent sequence within the Hovtinden Member, silica is ubiquitous whereas carbonate materials include mainly corroded bioclasts. These differences are consistent with predictions of the oceanographic model of ‘overfed’ and ‘hungry’ oceans proposed previously on the basis of carbon and oxygen stable isotope analysis.