Potential Flank-Collapse of Soufrière Volcano, Guadeloupe, Lesser Antilles? Numerical Simulation and Hazards     

Anne Le Friant  Georges Boudon  Jean-Christophe Komorowski  Philippe Heinrich  Michel P. Semet 《Natural Hazards》2006,39(3):381-393

The past history of recurrent flank collapses of la Soufrière volcano of Guadeloupe, its structure, its well-developed hydrothermal system and the current activity constitute factors that could promote a future flank collapse, particularly in the case of a significant increase of activity, with or without shallow magmatic input. To address the hazards associated with such a collapse, we model the emplacement of the debris avalanche generated by a flank-collapse event in 1,250 BC (3,100 years B.P.). We use a finite-difference grain-flow model solving mass and momentum conservation equations that are depth-averaged over the slide thickness, and a Coulomb-type friction law with a variable basal (minimum) friction angle. Using the parameter values determined from this simulation, we then simulate the debris avalanche which could be generated by a potential collapse of the present lava dome. We then discuss the region which could be affected by such a future collapse, and additional associated hazards of concern.  相似文献   

Modelling phase–assemblage diagrams for magnesian metapelites in the system K2O–FeO–MgO–Al2O3–SiO2–H2O: geodynamic consequences for the Monte Rosa nappe, Western Alps     

R. Le Bayon  C. de Capitani  M. Frey 《Contributions to Mineralogy and Petrology》2006,151(4):395-412

Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H₂O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H₂O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H₂O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X _A as well as P and T. The predicted mineral associations for the low- and high-X _A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X _A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO₂ and H₂O) for low- and high-X _A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X _A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X _A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X _A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H₂O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased  相似文献   

Water volume change in the lower Mekong from satellite altimetry and imagery data     

F. Frappart  K. Do Minh  J. L'Hermitte  A. Cazenave  G. Ramillien  T. Le Toan  N. Mognard-Campbell 《Geophysical Journal International》2006,167(2):570-584

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Origin and fluxes of atmospheric REE entering an ombrotrophic peat bog in Black Forest (SW Germany): Evidence from snow, lichens and mosses     

Dominique Aubert  Gaël Le Roux  Michael Krachler  Bernd Kober  Peter Stille 《Geochimica et cosmochimica acta》2006,70(11):2815-2826

The fate of the Rare Earth Elements (REE) were investigated in different types of archives of atmospheric deposition in the Black Forest, Southern Germany: (1) a 70 cm snow pack collected on the domed part of a raised bog and representing 2 months of snow accumulation, (2) a snow sample collected close to the road about 500 m from the peat bog, (3) two species of lichens and (4) a peat profile representing 400 years of peat accumulation as well as a “preanthropogenic” sample and the living moss layer from the top of the core. REE concentrations in peat are significantly correlated to Ti which is a lithogenic conservative element suggesting that REE are immobile in peat bog environments. Snow, lichens and peat samples show similar PAAS (Post Archean Australian Shale) normalized REE distributions suggesting that the complete atmospheric REE signal is preserved in the peat profile. However, the annual flux of REE accumulated by the peat is ca. 10 times greater than that of the bulk winter flux of REE. This difference probably indicates that the REE concentrations in the snowpack are not representative of the average REE flux over the whole year. Despite the pronounced geological differences between this site (granite host-rock) and a previously studied peat bog in Switzerland (limestone host-rock) similar REE distribution patterns and accumulation rates were found at both sites. Given that both sites confirm an Upper Continental Crust signature, the data suggests both sites are influenced by regional and not local, soil-derived lithogenic aerosols.  相似文献   

Parasequence development in the Ediacaran Shuram Formation (Nafun Group, Oman): high-resolution stratigraphic test for primary origin of negative carbon isotopic ratios     

Erwan Le Guerroué  Philip A. Allen  Andrea Cozzi 《Basin Research》2006,18(2):205-219

Neoproterozoic carbonates are known to show exceptional variations in their carbon isotopic ratios, and in the absence of biostratigraphy and a firm geochronological framework, these variations are used as a correlation tool. However, it is controversial whether the carbon isotope record reveals a primary oceanographic signal or secondary effects such as diagenesis. The Shuram Formation of the Nafun Group of Oman allows a stratigraphic test of this problem. The Nafun Group (Huqf Supergroup, Oman) in the Huqf area of east-central Oman consists of inner carbonate ramp facies of the Khufai Formation overlain by marine, storm-generated, red and brown siltstones of the Shuram Formation. Towards its top, the Shuram Formation is composed of distinctive shallowing-upward, 4–17-m-thick parasequences cropping out continuously over 35 km, which show recessive swaley cross-stratified siltstones capped by ledges comprising wave-rippled, intraclast-rich ooidal carbonate. These storm-dominated facies show a regional deepening in palaeobathymetry towards the south. The carbonates of the Shuram Formation are marked by an extreme depletion in ¹³C in bulk rock. δ¹³C values quickly reach a nadir of −12‰ just above the Khufai-Shuram boundary and steadily return to positive values in the overlying mainly dolomitic Buah Formation. The Shuram excursion is thought to be ca . 50 Myr in duration and extends over 600 m of stratigraphy. Carbon isotopic values show a systematic variation in the parasequence stack, with values varying both vertically through the stratigraphy (∼2‰ per 45 m) and laterally in the progradation distance (∼1‰ over 35 km). This supports a primary, oceanographic origin for these extremely negative carbon isotopic values and independently argues strongly against diagenetic resetting.  相似文献   

Preface     

Robin Spence  Benoit Le Brun 《Bulletin of Earthquake Engineering》2006,4(4):319-321

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Reply to the Comment on “Chronology of Vesuvius' activity from A.D. 79 to 1631 based on archeomagnetism of lavas and historical sources” by R. Lanza and E. Zanella     

Claudia Principe  Jean-Claude Tanguy  Simone Arrighi  Maxime Le Goff 《Bulletin of Volcanology》2006,68(4):397-398

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The International DORIS Service: genesis and early achievements     

G. Tavernier  H. Fagard  M. Feissel-Vernier  K. Le Bail  F. Lemoine  C. Noll  R. Noomen  J. C. Ries  L. Soudarin  J. J. Valette  P. Willis 《Journal of Geodesy》2006,80(8-11):403-417

All space-geodetic techniques are now organized as separate services of the International Association of Geodesy (IAG), supporting the first pilot project “Global Geodetic Observing System (GGOS)”. The International DORIS (Détermination d’Orbite et Radiopositionnement Intégrés par Satellite) Service (IDS) was created in mid-2003 to organize a DORIS contribution to this project and to foster a larger international cooperation on this topic. The goal of this paper is to summarize the key steps that were taken to create this structure and to present its current organization and recent results. At present, more than 50 groups from 35 different countries participate in the IDS at various levels, including 43 groups hosting DORIS stations in 32 countries all around the globe. Four Analysis Centres (ACs) provide results, such as estimates of weekly or monthly station coordinates, geocentre variations or Earth polar motion, that will soon be used to generate IDS-combined products for geodesy and geodynamics. As a first test, a preliminary combination was performed for all the 2004 data from these four ACs. Three of them show RMS of weighted station residuals with respect to this combination solution between 1 and 2 cm. The main topic under investigation is a discrepancy in the scale factor of the terrestrial reference frame (TRF) to map the individual solutions into the combination solution, which reaches 6 cm (multiplying the unit-less scale factor by the Earth radius to get convert scale to millimetre in vertical at the Earth’s surface). Finally, foreseen improvements of the DORIS technology are discussed as well as future improvements concerning the service organization itself and the accuracy and reliability of its scientific products.  相似文献   

Two-dimensional PP/PS-stereotomography: P- and S-waves velocities estimation from OBC data     

M. Alerini  G. Lambaré  R. Baina  P. Podvin  S. Le Bégat 《Geophysical Journal International》2007,170(2):725-736

We present the extension of stereotomography to P - and S -wave velocity estimation from PP - and PS -reflected/diffracted waves. In this new context, we greatly benefit from the use of locally coherent events by stereotomography. In particular, when applied to S -wave velocity estimation from PS -data, no pairing of PP - and PS -events is a priori required. In our procedure the P -wave velocity model is obtained first using stereotomography on PP -arrivals. Then the S -wave velocity model is obtained using PS -stereotomography on PS -arrivals fixing the P -wave velocity model. We present an application to an 'ideal' synthetic data set demonstrating the relevance of the approach, which allows us to recover depth consistent P - and S -waves velocity models even if no pairing of PP - and PS -events is introduced. Finally, results to a real data set from the Gulf of Mexico are presented demonstrating the potential of the method in a noisy data context.  相似文献   

一种基于频率域偏振分析提高三分量地震资料信噪比的方法          下载免费PDF全文

陈谮  高乐  赵烽帆 《地球物理学进展》2007,22(1):255-261

三分量地震资料包含着丰富的运动学和动力学信息，各种随机噪声的存在严重降低了资料的信噪比，给有效信息的提取带来了一定的困难.本文在引入多窗谱分析方法的基础上，通过对谱密度矩阵特征问题的求解提取三分量地震记录的偏振参数，实现频率域偏振分析；根据所求得的偏振参数以及随机噪声和有效信号的偏振特性差异，设计相应的自适应滤波器，通过该滤波器的滤波作用，最终实现压制随机噪声的目的.  相似文献