Characterization of regional and local scattering effects from small-aperture seismic array recordings

In 1998, four small-aperture arrays separated by 20 km have been deployed in the southern French Alps in order to record the natural seismicity during two consecutive months. One of the main objectives of this experiment was to characterize the heterogeneities that influence the wavefield propagation, by analysis of the coda characteristics recorded by each of the seismic arrays.The time-azimuth-velocity characteristics of the coda phases have been obtained using a high-resolution wavenumber decomposition method.A statistical analysis, using the coda characteristics of the whole data set (20 regional earthquakes) recorded by the four arrays has been performed and lead to the calculation of the density of scattered energy within the medium. Three regional heterogeneous areas (> 10 km) have been characterized, and are located in the N–NE, W and S directions from the four arrays. Scattered energy is also located at local distances (< 10 km) from the four arrays.The comparison of the waveforms recorded (i) with one of the array, for different groups of earthquakes, and (ii) with the four arrays, for one group of earthquakes, show that the geometrical configuration of the source–scattering area–recording site system can strongly influence the energetic distributions related to the secondary phases of the seismograms.In particular cases, we experimentally show that the interactions of the direct wavefield with the heterogeneous structures of the medium can be sufficiently energetic to induce secondary scattered phases that dominate the seismic motions recorded at a given site. In such case, these effects should be taken into account for the quantification of the expected ground motion recorded during an earthquake.     

Reaction of OH Radicals with Acetone: Determination of the Branching Ratio for the Abstraction Pathway at 298 K and 1 Torr

We have determined the 2-oxo-propyl CH₃C(O)CH₂ (sometimes called 1-methylvinoxy or acetonyl) radical yield for the reaction of acetone with OH radical relative to the 2-oxo-propyl yields for the reactions of F- and Cl atoms with acetone using the Discharge Flow technique. The 2-oxo-propyl radical has been monitored by Laser Induced Fluorescence LIF at short reaction times in the systems: OH + acetone (R1), F + acetone (R2), and Cl + acetone (R3). From these measurements we have deduced the branching ratio for the 2-oxo-propyl radical formation in the title reaction to be in the range 0.8 R 1.     

Analytical Procedures for the Measurement of Boron Isotope Compositions by Ion Microprobe in Meteorites and Mantle Rocks

An analytical procedure has been developed to measure in situ the ¹¹ B/¹⁰ B ratio in terrestrial basaltic rocks and meteoritic chondrules having B concentration of less than 1 μg g^?1 using a small radius ims3f ion microprobe. The central difficulties for these measurements are (i) the removal of the trace amount of B contamination introduced in the sample during polishing, (ii) the precise calibration of instrumental mass fractionation of B isotopes and (iii) the low count rates of ¹⁰ B and ¹¹ B. Contamination experiments conducted with isotopically labelled B enriched in ¹⁰ B showed that ultrasonic cleaning in bi-distilled water (< 1 ng g^?1 B) and pre-sputtering of the analysed area decrease B contamination to the level of 0.01 μg g^?1. Analyses of isotope standards spanning a range of ¹¹ B/¹⁰ B between 3.93 and 4.20 showed that instrumental mass fractionation was constant within? during one session of analyses. Repeated analyses of a standard glass showed a reproducibility of instrumental mass fractionation between February 1991 and October 1996 of 1.3. Taking into account all sources of error, boron isotope measurements are accurate to within 5 for meteoritic samples having B contents in the range 0.1 to 1 μg g^?1. A slightly better accuracy of 1.5 can be achieved for basaltic glasses which can be sputtered with very intense primary beams.     

Shear modulus and damping ratio of grouted sand

An experimental comparative study of three different grouted sands in terms of their effects on the values of two dynamic properties is presented. The dynamic properties studied are the shear modulus and the damping ratio which are determined with resonant column tests and cyclic triaxial tests. The behaviour of a pure Fontainebleau sand is compared with the behaviour of a Fontainebleau sand grouted with a silicate grout, a micro-fine cement grout and a mineral grout. The effects of the grouting treatment, the type of grout, the confining pressure, and the strains, on the shear modulus and the damping ratio are studied. The test results have shown that grouting improves the stiffness of the sand especially for small strains. Whatever the type of material, confining stress improves the shear modulus whereas it has a negligible effect on the damping ratio. When strain increases, the shear modulus decreases and the damping ratio increases.     

Titanium in phengite: a geobarometer for high temperature eclogites

Phengite chemistry has been investigated in experiments on a natural SiO₂–TiO₂-saturated greywacke and a natural SiO₂–TiO₂–Al₂SiO₅-saturated pelite, at 1.5–8.0 GPa and 800–1,050°C. High Ti-contents (0.3–3.7 wt %), Ti-enrichment with temperature, and a strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes in composition with pressure result from the Tschermak substitution (Si + R²⁺ = Al^IV + Al^VI) coupled with the substitution: Al^VI + Si = Ti + Al^IV. The latter exchange is best described using the end-member Ti-phengite (KMgTi[Si₃Al]O₁₀(OH)₂, TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous experimental studies are derived to use the equilibrium MgCel + Rt = TiP + Cs/Qz as a thermobarometer in felsic and basic rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression by entrapment in zircon or garnet, thus providing an opportunity to determine the T–P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects on phase relationships and melting rates with decreasing P or increasing T in the continental crust.     

Dendrogeomorphic reconstruction of past debris‐flow activity using injured broad‐leaved trees

Tree‐ring records from conifers have been regularly used over the last few decades to date debris‐flow events. The reconstruction of past debris‐flow activity was, in contrast, only very rarely based on growth anomalies in broad‐leaved trees. Consequently, this study aimed at dating the occurrence of former debris flows from growth series of broad‐leaved trees and at determining their suitability for dendrogeomorphic research. Results were obtained from gray alder (Alnus incana (L.) Moench), silver birch and pubescent birch (Betula pendula Roth and Betula pubescens Ehrh.), aspen (Populus tremula L.), white poplar, black poplar and gray poplar (Populus alba L., Populus nigra L. and Populus x canescens (Ait.) Sm.), goat willow (Salix caprea L.) and black elder (Sambucus nigra L.) injured by debris‐flow activity at Illgraben (Valais, Swiss Alps). Tree‐ring analysis of 104 increment cores, 118 wedges and 93 cross‐sections from 154 injured broad‐leaved trees allowed the reconstruction of 14 debris‐flow events between AD 1965 and 2007. These events were compared with archival records on debris‐flow activity at Illgraben. It appears that debris flows are very common at Illgraben, but only very rarely left the channel over the period AD 1965–2007. Furthermore, analysis of the spatial distribution of disturbed trees contributed to the identification of six patterns of debris‐flow routing and led to the determination of preferential breakout locations of events. The results of this study demonstrate the high potential of broad‐leaved trees for dendrogeomorphic research and for the assessment of the travel distance and lateral spread of debris‐flow surges. Copyright © 2010 John Wiley & Sons, Ltd.     

Stratigraphic simulations of the shelf of the Gulf of Lions: testing subsidence rates and sea‐level curves during the Pliocene and Quaternary

Determining the relative importance of factors interacting to control stratigraphic organization is a key issue in sedimentology. The Pliocene‐Quaternary chronostratigraphy on the Gulf of Lions platform is still poorly constrained, giving rise to different interpretations of the evolution of its subsidence through time. This paper examines the Pliocene‐Quaternary sedimentary filling of the Gulf of Lion's shelf with Dionisos, a numerical stratigraphic model. Our results show that a constant subsidence rate accurately reproduces the observed geometries, whereas a varying subsidence rate reproduces them only if the acceleration of subsidence is limited. At this time‐scale, a third‐order eustatic curve is also reappraised: a higher resolution curve (built using δ¹⁸O measurements) gives a more realistic restitution of our stratigraphic markers. Finally, the constant subsidence rate and sediment fluxes implied in these modellings are discussed relative to climate and local factors of sedimentation.     

Experimental evidence of decompression melting during exhumation of subducted continental crust

Experiments have been carried out on a metagreywacke at 800, 850 and 900°C, in the pressure range 0.5–5 GPa to locate the solidus and the eclogite/amphibolite facies transition in felsic rocks, identify the nature of the reactions responsible for major mineralogical changes, and determine the proportions of phases as a function of pressure. The mineral assemblage phengite + clinopyroxene + garnet + quartz/coesite is stable above 2.3 GPa while biotite + plagioclase + garnet + quartz is stable below 2 GPa. The model reaction for the eclogite/amphibolite facies transition in metagreywackes is:

Dating of snow avalanches by means of wound‐induced vessel anomalies in sub‐arctic Betula pubescens

Dendrogeomorphic research has long relied on scarred trees to reconstruct the frequency of mass‐movement processes. Injuries have mostly been dated macroscopically by counting the tree rings formed after wounding. Tree‐ring anatomical anomalies induced by cambial injury, in contrast, have only recently been recognized as proxy records of past events. We investigated 12 sub‐arctic downy birch (Betula pubescens Ehrh.) trees scarred by snow avalanches in Norway and Iceland. Earlywood vessel lumina were measured for each tree in the xylem tissue bordering the scars. Seven successive rings were examined, namely two control rings laid down prior to wounding and five rings in the wound xylem. We provide evidence that snow‐avalanche‐induced wounding resulted in atypically narrow earlywood vessels over at least two years. Our data demonstrate that wound‐associated vessel anomalies represent tangible markers of mass‐movement processes, and as such make a viable tool for reconstructing past events. Similar dendrogeomorphic studies based on tree‐ring anatomy can be readily conducted with other mass‐movement processes, as well as with other broad‐leaved tree species. Ultimately, this new approach will foster increment coring over more invasive sampling techniques.