Ultrasonic Velocities, Acoustic Emission Characteristics and Crack Damage of Basalt and Granite   总被引：2，自引：0，他引：2  

Sergei Stanchits  Sergio Vinciguerra  Georg Dresen 《Pure and Applied Geophysics》2006,163(5-6):975-994

Acoustic emissions (AE), compressional (P), shear (S) wave velocities, and volumetric strain of Etna basalt and Aue granite were measured simultaneously during triaxial compression tests. Deformation-induced AE activity and velocity changes were monitored using twelve P-wave sensors and eight orthogonally polarized S-wave piezoelectric sensors; volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface. P-wave velocity in basalt is about 3 km/s at atmospheric pressure, but increases by > 50% when the hydrostatic pressure is increased to 120 MPa. In granite samples initial P-wave velocity is 5 km/s and increases with pressure by < 20%. The pressure-induced changes of elastic wave speed indicate dominantly compliant low-aspect ratio pores in both materials, in addition Etna basalt also contains high-aspect ratio voids. In triaxial loading, stress-induced anisotropy of P-wave velocities was significantly higher for basalt than for granite, with vertical velocity components being faster than horizontal velocities. However, with increasing axial load, horizontal velocities show a small increase for basalt but a significant decrease for granite. Using first motion polarity we determined AE source types generated during triaxial loading of the samples. With increasing differential stress AE activity in granite and basalt increased with a significant contribution of tensile events. Close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly. A concomitant increase of double-couple events indicating shear, suggests shear cracks linking previously formed tensile cracks.  相似文献   

The southernmost record of tropical pollen grains in the mid-Cretaceous of Patagonia, Argentina   总被引：1，自引：0，他引：1  

Viviana Barreda  Sergio Archangelsky   《Cretaceous Research》2006,27(6):778-787

Pollen grains characteristic of tropical Northern Gondwana (Schrankipollis, Brenneripollis and Pennipollis peroreticulatus) have been recorded from the Cretaceous of Patagonia. They were recovered from the Late Albian–Cenomanian Kachaike Formation in Santa Cruz Province, southern Argentina. The palynological assemblages are dominated by gymnosperm pollen and bryophyte-pteridophyte spores, whereas angiosperms are poorly represented. The angiospermoid type Schrankipollis has not been reported previously from Argentina, and a new species S. kachaikensis is described. Two species of Brenneripollis (of uncertain affinity) and Pennipollis peroreticulatus (related to the Alismatales) are also reported. The first appearances of Pennipollis peroreticulatus are clearly diachronous from north to south, ranging from the Barremian in tropical regions to the Coniacian–Santonian in Antarctica. S. kachaikensis and P. peroreticulatus show restricted stratigraphic distributions through the Kachaike Formation and may be stratigraphically useful markers. A tetrad of Walkeripollis (related to modern Winteraceae) is also recognized. This is the oldest record of Winteraceae in the southern temperate region where the family lives today. Evidence for migration of Winteraceae from tropical Gondwana to Antarctica and Australia throughout South America, is provided by this new finding. The presence of tropical elements in the austral margin of South America gives support to previous studies on the expansion of warm temperatures towards high latitudes during the mid Cretaceous.  相似文献   

Seismically induced rock slope failures resulting from topographic amplification of strong ground motions: The case of Pacoima Canyon, California   总被引：3，自引：0，他引：3  

Sergio A. Sepúlveda  William Murphy  Randall W. Jibson  David N. Petley 《Engineering Geology》2005,80(3-4):336-348

The 1994 Northridge earthquake (M_w = 6.7) triggered extensive rock slope failures in Pacoima Canyon, immediately north of Los Angeles, California. Pacoima Canyon is a narrow and steep canyon incised in gneissic and granitic rocks. Peak accelerations of nearly 1.6 g were recorded at a ridge that forms the left abutment of Pacoima Dam; peak accelerations at the bottom of the canyon were less than 0.5 g, suggesting the occurrence of topographic amplification. Topographic effects have been previously suggested to explain similarly high ground motions at the site during the 1971 (M_w = 6.7) San Fernando earthquake. Furthermore, high landslide concentrations observed in the area have been attributed to unusually strong ground motions rather than higher susceptibility to sliding compared with nearby zones. We conducted field investigations and slope stability back-analyses to confirm the impact of topographic amplification on the triggering of landslides during the 1994 earthquake. Our results suggest that the observed extensive rock sliding and falling would have not been possible under unamplified seismic conditions, which would have generated a significantly lower number of areas affected by landslides. In contrast, modelling slope stability using amplified ground shaking predicts slope failure distributions matching what occurred in 1994. This observation confirms a significant role for topographic amplification on the triggering of landslides at the site, and emphasises the need to select carefully the inputs for seismic slope stability analyses.  相似文献   

Hall effect on magnetic reconnection at the Earth's magnetopause     

Laura F. Morales  Sergio Dasso  Daniel O. Gmez  Pablo Mininni 《Journal of Atmospheric and Solar》2005,67(17-18):1821

We analyze in situ observations of magnetic reconnection at the Earth magnetopause to estimate the importance of the Hall current during the merging of interplanetary and magnetospheric magnetic field lines. The reconnection process is studied through numerical simulations, integrating the Hall MHD equations in 2.5 dimensions. A large influence of the Hall effect is found, which can be measured by a significant increase of the reconnection rate.  相似文献   

Multimode migration of scattered and converted waves for the structure of the Hikurangi slab interface, New Zealand     

John N. Louie  Sergio Chvez-Prez  Stuart Henrys  Stephen Bannister 《Tectonophysics》2002,355(1-4)

Reflectivity imaging of local earthquake seismograms has revealed the structure of the Hikurangi subduction interface at the location of two strong earthquakes that occurred in 1990. The earthquakes originated within the continental plate of the North Island of New Zealand and below in the subducting Pacific slab. We used seismograms from 500 well-located events in two earthquake sequences recorded by a small temporary seismograph deployment to directly image the structure and multiphase reflectivity of the plate interface. Synthetic tests of the imaging method show the effects of the poor 3-d geometric coverage afforded by the seismometer array. Kirchhoff summation image sections computed from synthetics show accurate depth imaging of backscattering interfaces. Phase-converting interfaces imaged with forward-scattered waves are smeared by poor ray coverage to 5-km depth inaccuracy and are only imaged over a small range of their horizontal extent. From the data, we computed image sections for P–P, P–S, S–P and S–S scattering. We mitigated imaging artifacts due to poor ray coverage with an obliquity factor, an antialiasing criterion and enhancement by resampling statistics. Imaging used a sharply layered velocity model. We tested for the effects of imaging with first-arriving headwaves by imaging through smoothly varying velocity models. For our ray geometry, early-arrival headwaves contribute little to the images.The plate interface appears as a 3–5-km thick P–P and possibly S–S backscatterer with 5° NW dip, offset 5 km down-to-the-NW above a normal fault in the slab. When illuminated from below, a wedge of the interface on the downdip side of the slab fault forms a very prominent P–P forward scatterer. The edges of the wedge forward-scatter some S–P and S–S energy, but an order of magnitude less than the P–P forward scattering. The imbalances between forward scattering of P and S energy suggest a wedge of subducted sediment retaining significant porosity but with rigidity close to that of surrounding rocks.  相似文献   

A teleseismic shear-wave splitting study to investigate mantle flow around South America and implications for plate-driving forces   总被引：2，自引：0，他引：2  

George Helffrich  Douglas A. Wiens  Emilio Vera  Sergio Barrientos  Patrick Shore  Stacey Robertson  Rodrigo Adaros 《Geophysical Journal International》2002,149(1):F1-F7

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Cadmium and manganese distributions in the Hudson River estuary: interannual and seasonal variability     

Min Yang  Sergio A. Sa  udo-Wilhelmy 《Earth and Planetary Science Letters》1998,160(3-4):403-418

Surface waters collected along the salinity gradient of the Hudson River estuary in four cruises between 1995 and 1997 were size-fractionated into particulate (>0.45 μm), ‘dissolved' (<0.45 μm), colloidal (10 kDa, 0.45 μm) and low molecular weight (<10 kDa) phases. Dissolved Cd concentrations (range: 0.11–1.19 nM) in surface waters of the estuary appear to have decreased fourfold (from an average of 2.36 to 0.61 nM) over a 23-year period, since the initial analysis of samples collected in the 1970s by Klinkhammer and Bender [Estuar. Coastal Shelf Sci. 12 (1981) 629–643]. This interannual decline reflects improvement in sewage treatment and the elimination of industrial Cd sources to the Hudson River estuary. In contrast, dissolved Mn levels (range: 0.033–1.46 μM) have remained relatively constant over the same period of time, suggesting that anthropogenic sources have very limited impact on Mn concentrations in the estuary. The concentrations of both Cd and Mn appeared to strongly depend on the season and/or river discharge. The highest concentrations were detected under low freshwater discharge, implying that limited hydraulic flushing allows a build-up of metals in the water column. Although the decline in Cd levels within the estuary reflects a reduction in the magnitude of anthropogenic inputs, mass balance estimates indicated that current sources of Cd to the estuary include sewage discharges (in the lower estuary around Manhattan) and diagenetic remobilization from industrial Cd deposited in sediments nearly 2 decades ago (in the upper estuary near Foundry Cove). Moreover, under low river discharge, the sources considered in our model (sewage, riverine input, atmospheric deposition, and benthic fluxes) could account for no more than 60% of the Cd exported from the lower estuary to the ocean. This suggests that undefined sources such as ground water and inputs from other watersheds (e.g., Long Island Sound and Newark Bay) may potentially influence the water quality of the New York Harbor. The size-fractionated metal concentrations indicated that most of the traditionally defined ‘dissolved' Cd and Mn consisted of <10 kDa molecular weight species. High molecular weight colloidal species of Mn accounted for about 50% of the dissolved fraction at the riverine end-member and <5% at intermediate and high salinities. Colloidal Cd accounted for <6% of the dissolved phase throughout the estuary. Unlike the non-conservative excess (relative to ideal dilution of river water and seawater) of dissolved Mn observed along the estuary, high molecular weight colloidal Mn appeared to be removed at the head of the estuary. The small contribution of colloidal Cd and Mn to the ‘dissolved' phase suggests that remobilization from suspended particulate phases and/or from sediments occurs through the formation of small molecular weight species.  相似文献   

A large marine ecosystem governance framework     

Lucia Fanning  Robin Mahon  Patrick McConney  Jorge Angulo  Felicity Burrows  Bisessar Chakalall  Diego Gil  Milton Haughton  Sherry Heileman  Sergio Martínez  L’ouverture Ostine  Adrian Oviedo  Scott Parsons  Terrence Phillips  Claudia Santizo Arroya  Bertha Simmons  Cesar Toro 《Marine Policy》2007

A large marine ecosystem (LME) governance framework, developed from a need to effectively address the sustainable management of the shared living marine resources of the Caribbean, is proposed. The framework is based on four propositions and focuses on a linked examination of two well-known components of LME-level governance: the policy cycle process by which decisions are made and the multi-level nature of LMEs. It accommodates the diversity of policy cycles at multiple levels and the linkages among them required for effective governance of LMEs. The framework takes into account of factors such as context, purpose, jurisdictional scale, capacity and complexity and provides a means to identify critical areas for intervention.  相似文献   

A seismic reflection study of the External Calabrian Arc in the northern Ionian Sea (eastern Mediterranean)     

Sergio Rossi  Renzo Sartori 《Marine Geophysical Researches》1981,4(4):403-426

The External Calabrian Arc is located off the convex side of the Calabro-Peloritanian Arc in the northern Ionian Sea. A systematic reflection seismic survey indicates that it is made of different structural elements whose characters seem consistent with an active accretionary margin. The main structures are the Crotone-Spartivento slope (comparable to an inner trench slope) and the intermediate depressions (comparable to a trench area). Internal to these elements, the Crotone-Spartivento basin may represent a fore-arc basin. This partly outcrops in Calabria and its structure suggests that the accretionary margin developed at least since middle-upper Miocene.Subduction processes do not affect a true oceanic crust, because of the great thickness of sediments covering the whole eastern Mediterranean. Hence some peculiar features occur in the system. as the cobblestone topography, or are lacking, as a typical and continuous trench zone.In the areas with cobblestone topography we distinguish a Calabrian Ridge sensu stricto from a Calabrian Ridge sensu lato. The former is a N-S trending swell, external to the supposed trench zone, interpreted as a sedimentary outer-arc ridge produced by rather surficial tectonic accumulation of sediments further chaoticized by gravitative mechanisms. The Ridge s.l. is a very wide area with low relief and little or no seismic penetration. Tectonization seems gentler than in the Ridge s.s. and structural axes seem to possess different orientations. These areas are interpreted as due to a widespread surficial chaoticization above presumed decollement layers occurring within the sedimentary column of the Ionian bathyal plain.The pattern of deformations of the Calabrian Ridge seems consistent with the Calabro-Peloritanian Arc actively overriding the eastern Mediterranean, with a resultant direction of movement essentially towards the East.  相似文献   

Salt marsh geomorphology: Physical and ecological effects on landform   总被引：1，自引：0，他引：1  

Raymond Torres  Sergio Fagherazzi  Danika vanProosdij  Charles Hopkinson   《Estuarine, Coastal and Shelf Science》2006,69(3-4):309

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