Field‐based and spectral indicators for soil erosion mapping in semi‐arid mediterranean environments (Coastal Cordillera of central Chile)

The Coastal Cordillera of central Chile is naturally sensitive to soil erosion due to moderate to steep slopes, intense winter rains when the vegetation cover is scarce, and deeply weathered granitic rocks. In 1965, 60 per cent of its surface was moderately to very severely eroded. Today this process is still largely active, but no data are currently available to evaluate the real extent, distribution and severity of soil degradation on a regional scale. This information is vital to support efficient soil conservation plans. A multi‐scale approach was implemented to produce regional land degradation maps based on remote sensing technologies. Fieldwork has shown that the surface colour or ‘redness’ and the density of coarse fragments are pertinent erosion indicators to describe a typical sequence of soil degradation in the context of mediterranean soil developed on granitic materials and micaschists. Field radiometric experiments concluded that both factors influence the reflectance of natural surfaces and can be modelled using radiometric indices accessible from most satellites operating in the optical domain, i.e. redness index and brightness index. Finally the radiometric indices were successfully applied to SPOT images to produce land degradation maps. Only broad classes of erosion status were discriminated and the detection of the degradation processes was only possible when most of the fertile layer had already been removed. This technology provides decision‐making information required to develop regional soil conservation plans and to prioritize actions between catchment areas, especially in vast inter‐tropical regions where spatialized data are not always readily available. Copyright © 2006 John Wiley & Sons, Ltd.     

Study of the erosion rates in the upper Maracujá Basin (Quadrilátero Ferrífero/MG,Brazil) by the in situ produced cosmogenic 10Be method

The present work quantifies the erosive processes in the two main substrates (schists–phyllites and granites–gneisses) of the upper Maracujá Basin in the Quadrilátero Ferrífero/MG, Brazil, a region of semi‐humid tropical climate. Two measuring methods of concentration were used: (i) in situ produced ¹⁰Be in quartz veins (surface erosion rates) and (ii) ¹⁰Be in fluvial sediments (basin erosion rates). The results confirm that (i) erosion tends to be more aggressive close to the headwaters than in the lower parts of the basin and (ii) the region is now affected by dissection. Copyright © 2006 John Wiley & Sons, Ltd.     

The November 2002 eruption at Piton de la Fournaise volcano,La Réunion Island: ground deformation,seismicity, and pit crater collapse

An eruption on the eastern flank of Piton de la Fournaise volcano started on 16 November, 2002 after 10 months of quiescence. After a relatively constant level of activity during the first 13 days of the eruption, lava discharge, volcanic tremor and seismicity increased from 29 November to 3 December. Lava effusion suddenly ceased on 3 December while shallow earthquakes beneath the Dolomieu summit crater were still recorded at a rate of about one per minute. This unusual activity continued and increased in intensity over the next three weeks, ending with the formation of a pit crater within Dolomieu. Based on ground deformation, measured by rapid-static and continuous GPS and an extensometer, seismic data, and lava effusion patterns, the eruptive period is divided into five stages: 1) slow summit inflation and sporadic seismicity; 2) rapid summit inflation and a short seismic crisis; 3) rapid flank inflation, onset of summit deflation, sporadic seismicity, accompanied by stable effusion; 4) flank inflation, coupled with summit deflation, intense seismicity, and increased lava effusion; and finally 5) little deflation, intense shallow seismicity, and the end of lava effusion. We propose a model in which the pre-intrusive inflation of Stage 1 in the months preceding the eruption was caused by a magma body located near sea level. The magma reservoir was the source of an intrusion rising under the summit during Stage 2. In Stage 3, the magma ponded at a shallow level in the edifice while the lateral injection of a radial dike reached the surface on the eastern flank of the basaltic volcano, causing lava effusion. Pressure decrease in the magmatic plumbing system followed, resulting in upward migration of a collapse front, forming a subterranean column of debris by faulting and stoping. This caused intense shallow seismicity, increase in discharge of lava and volcanic tremor at the lateral vent in Stage 4 and, eventually the formation of a pit crater in Stage 5.     

GPS network monitors the Western Alps' deformation over a five-year period: 1993–1998

  The Western Alps are among the best studied collisional belts with both detailed structural mapping and also crustal geophysical investigations such as the ECORS and EGT seismic profile. By contrast, the present-day kinematics of the belt is still largely unknown due to small relative motions and the insufficient accuracy of the triangulation data. As a consequence, several tectonic problems still remain to be solved, such as the amount of N–S convergence in the Occidental Alps, the repartition of the deformation between the Alpine tectonic units, and the relation between deformation and rotation across the Alpine arc. In order to address these problems, the GPS ALPES group, made up of French, Swiss and Italian research organizations, has achieved the first large-scale GPS surveys of the Western Alps. More than 60 sites were surveyed in 1993 and 1998 with a minimum observation of 3 days at each site. GPS data processing has been done by three independent teams using different software. The different solutions have horizontal repeatabilities (N–E) of 4–7 mm in 1993 and 2–3 mm in 1998 and compare at the 3–5-mm level in position and 2-mm/yr level in velocity. A comparison of 1993 and 1998 coordinates shows that residual velocities of the GPS marks are generally smaller than 2 mm/yr, precluding a detailed tectonic interpretation of the differential motions. However, these data seem to suggest that the N–S compression of the Western Alps is quite mild (less than 2 mm/yr) compared to the global convergence between the African and Eurasian plate (6 mm/yr). This implies that the shortening must be accomodated elsewhere by the deformation of the Maghrebids and/or by rotations of Mediterranean microplates. Also, E–W velocity components analysis supports the idea that E–W extension exists, as already suggested by recent structural and seismotectonic data interpretation. Received: 27 November 2000 / Accepted: 17 September 2001     

Progress in Carrier Phase Time Transfer

The progress of the joint Pilot Project for time transfer, formed by the International GPS Service (IGS) and the Bureal International des Poids et Mesures (BIPM), was recently reviewed. Three notable milestones were set. (1) The IGS will implement, at least in a test mode, an internally realized time scale based on an integration of combined frequency standards within the IGS network. This will eventually become the reference time scale for all IGS clock products (instead of the current GPS broadcast time). (2) A new procedure for combined receiver and satellite clock products will be implemented officially in November 2000. Receiver clocks are an entirely new product of the IGS. (3) The BIPM will coordinate an effort to calibrate all Ashtech Z12-T (and possibly other) receivers suitable for time transfer applications, either differentially or absolutely. Progress reports will be presented publicly in the spring 2001. ? 2001 John Wiley & Sons, Inc.     

Evolutionary properties of stochastic models of earthquake accelerograms: Their dependence on magnitude and distance

An approach to generate artificial earthquakeaccelerograms on hard soil sites is presented. Eachtime-history of accelerations is considered as arealization of a non-stationary gaussian stochasticprocess, with statistical parameters depending onmagnitude and source-to-site distance. In order tolink the values of these parameters for each groundmotion record with the corresponding magnitude andsource-to-site distance, semi-empirical functionalrelations called generalized attenuationfunctions are determined. The set of realground-motion time histories used to obtain thesefunctions correspond to shocks generated at differentsources and recorded at different sites in thevicinity of the southern coast of Mexico. The resultsshow significant dispersion in the parameters of themodel adopted, which reflect that associated with thereal earthquakes included in the sample employed.The problem of conditional simulation of artificialacceleration time histories for prescribed intensitiesis briefly presented, but its detailed study is leftfor a companion paper. The criteria and modelsproposed are applied to generate two families ofartificial acceleration records for recurrenceintervals of 100 and 200 years at a specific sitelocated in the region under study. The results shownin this article correspond to acceleration timehistories recorded on firm ground for earthquakesgenerated at the subduction zone that runs along thesouthern coast of Mexico, and cannot be generalized tocases of earthquakes generated at other sources orrecorded at other types of local conditions. Thismeans that the methods and functional forms presentedhere are applicable to these other cases, but thevalues of the parameters that characterize thosefunctions may differ from those presented here.     

An evaluation of methodologies for the generation of stochastic hydraulic conductivity fields in highly heterogeneous aquifers

 Stochastic techniques, such as Monte Carlo experiments, are more and more frequently used for the study of flow and transport in heterogeneous aquifers. When the aquifer is composed of distinct hydrofacies, a common way to model heterogeneity is to first generate equally-possible hydrofacies fields, and then convert these hydrofacies fields into hydraulic conductivity (K) fields by assigning a single K value to each facies. This technique assumes relative homogeneity of K within each facies but may not be appropriate for the most conductive facies that often exhibits substantial variability. In this paper, we assessed the impacts of assigning multiple random K, rather than a uniform K value, to the highly conductive facies on the results of a flow and transport model. A set of fifty stochastic hydrofacies maps depicting an environment similar to the Snake River Plain aquifer (SRPA) in south-east Idaho were generated. Simulations demonstrated that a uniform K value, if carefully chosen, can reasonably reproduce the specific discharges and early particle arrival times produced by multiple K values. Yet, the results obtained with a uniform K value are dramatically less variable than those obtained with multiple K values. It is therefore concluded that stochastic simulations with uniform K assigned to the most conductive and variable facies do not necessarily portray the entire uncertainty in the analyses.     

The aegean sea oil spill in the Galicia Coast (NW Spain). I. Distribution and fate of the crude oil and combustion products in subtidal sediments.

The spatial distribution and fate of petrogenic and pyrogenic hydrocarbons in coastal sediments following the Aegean Sea oil spill (Galicia, NW Spain) was investigated through a detailed study of chemical markers. Alkanes and acyclic isoprenoid hydrocarbons were degraded within six months of the accident, but triterpane and sterane distributions were still detectable and were useful in monitoring, respectively, the oil source and weathering. Aromatic steranes were also useful source indicators, and oil degradation was clear from the decrease of certain alkyl dibenzothiophene and phenanthrene isomers. The pyrogenic polycyclic aromatic hydrocarbons produced in the tanker wreck were found more persistent than the petrogenic ones. However, the presence of the oil in the sediments was recognized even one year after the accident and was found particularly preserved in the subsurface layers.