196,075 research outputs found
Tsunami hazard scenarios in the Adriatic Sea domain
No full textThe tsunami phenomenon is mainly detected in oceanic domains but it can also occur in small basins as the Adriatic Sea. The presence of great waves has been recorded a few times in the past centuries on the Adriatic shorelines, therefore this suggests the idea to evaluate which could be the maximum amplitude reached by a possible future tsunami event. In this framework we calculate several synthetic mareograms applying to the shallow water basin case both the theory of modal summation by Panza et al. (2000) and the theory of the Green's function by Yanovskaya et al. (2003). The first is applied to the case of tsunamis generated by an offshore source, the second to the case of tsunamis generated by an inland source. Both kinds of tsunamigenic events did already occur in the Adriatic domain, as witnessed in many catalogues (Caputo and Faita, 1984; Bedosti and Caputo, 1986; Tinti et al., 2004) and also pointed out in the recent "Catalogue of reported tsunami events in the Adriatic Sea" (see Appendix).
We calculate synthetic mareograms varying those parameters which are the most influencing in tsunami generation, such as magnitude, focal depth, water layer thickness, etc., in order to estimate the expected values of tsunami maximum amplitude and arrival time, in the whole Adriatic basin, for the selected scenarios
Constraints on an intrusive system beneath the Soufriere Hills Volcano, Montserrat, from finite difference modeling of a controlled source seismic experiment
No full textThe SEA-CALIPSO wide-angle seismic tomography experiment revealed a high velocity region beneath the island of Montserrat. Field recordings show a decrease in the amplitude of seismic signals crossing this high velocity region. We constrain the geometry and nature of this attenuating body, by forward modeling of the seismic wave field with a viscoelastic finite-difference method. We interpret the attenuation observed as caused by a scattering region, which we model as a stochastic perturbation of the velocity field with characteristic length scale of 0.4 km. The scattering region approximately coincides with the top part of the high velocity region identified in the tomographic model and is estimated to have a volume of about 800 km3 . We argue that the scattering is caused by geological heterogeneities corresponding to a system of dikes and sills and to the complex structure of the volcanic edifice
Three-dimensional seismic velocity tomography of Montserrat from the SEA-CALIPSO offshore/onshore experiment
No full textThe SEA-CALIPSO experiment in December 2007 incorporated a sea-based airgun source, and seismic recorders both on Montserrat and on the adjacent sea floor. A high quality subset of the data was used for a first arrival P-wave velocity tomographic study. A total of more than 115,000 traveltime data from 4413 airgun shots, and 58 recording stations, were used in this highresolution tomographic inversion. The experiment geometry limited the depth of well resolved structures to about 5 km. The most striking features of the tomography are three relatively high velocity zones below each of the main volcanic centers on Montserrat, and three low velocity zones flanking Centre Hills. We suggest that the high velocity zones represent the solid andesitic cores of the volcano complexes, characterized by wave speeds faster than adjacent volcaniclastic material. The low velocity zones may reflect porous volcaniclastic material and/or alteration by formerly active hydrothermal systems. Copyright © 2010 by the American Geophysical Union
Upper crustal structure of an active volcano from refraction/reflection tomography, Montserrat, Lesser Antilles
No full textTo better understand the volcanic phenomena acting on Montserrat, the SEA-CALIPSO seis-mic experiment (Seismic Experiment with Airgun-source – Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory) was conducted in 2007 December with the aim of imaging the upper crust and the magmatic system feeding the active Soufri ?ere Hills Volcano. The 3-D survey covered an area of about 50
× 40 km and involved the deployment of 247 land stations and ocean-bottom seismometers (OBSs). A subset of the data, recorded by four OBSs and four land stations on a southeast to northwest line, has been analysed, and traveltimes have been inverted to obtain a 2-D seismic velocity model through the island. Inverted phases include crustal and sediment P waves and wide-angle reflections. The resulting velocity model reveals the presence of a high velocity body (3.5–5.5 km s?1 ) beneath the island, with highest velocities beneath the Soufri ?ere and Centre Hills, cor responding primarily to the cores of these volcanic edifices, built of a pile of andesite lava domes and subsequent intrusions. In the off-shore region, velocities in the surficial sediment layer vary from 1.5 to 3.0 km s?1 , consistent with a mainly calcareous and volcaniclastic composition. A wide-angle reflector is observed at a depth of ?1200 m below the seabed, and appears to deepen beneath the island. The upper crust beneath this reflector has velocities of 4.0–6.0 km s?1 and is infer red to cor respond to plutonic and hypabyssal rocks and sedimentary material of the old arc. The high velocity region beneath the island, extends into the crust to a depth of at least 5 km, and is believed to be caused by an intrusive complex, possibly of intermediate composition. A low velocity zone, as would be expected in the presence of an active magma chamber, was not observed perhaps due to the limited resolution beneath ?5 km depth. Our results so far provide the first wide-angle seismic constraints on the upper crustal structure of the island to a depth of 10 km, and will help understanding the processes that drive volcanism at Montserrat and other island arc volcanoes
Avoiding Pitfalls in the Interpretation of Gadoxetic Acid–Enhanced Magnetic Resonance Imaging
No full textGadoxetic acid is extensively used in the following 3 main clinical situations: characterization of small nodules in patients with cirrhosis, preoperative staging of liver metastases, and characterization of incidentally discovered focal liver lesions. Owing to the rapid entry of gadoxetic acid into hepatocytes, the traditional features of liver tumors are modified on magnetic resonance (MR) imaging, especially during delayed phase sequences. Thus, although the added value of gadoxetic acid for the detection and characterization of focal liver lesions is now clear, its unique pharmacokinetics as well as the presence of mimicking and atypical lesions may lead to misdiagnoses. The goal of this article is to illustrate common and uncommon pitfalls associated with the use of gadoxetic acid–enhanced MR imaging. Moreover, additional MR imaging features that can help establish a correct diagnosis are described
Quantification of the Intrusive Magma Fluxes during Magma Chamber Growth at Soufriere Hills Volcano (Montserrat, Lesser Antilles)
No full textMagma fluxes in the crust control the thermal viability and mechanical stability of magma chambers. We estimated the magma fluxes required to generate the negative seismic velocity anomaly observed below Soufrière Hills volcano, Montserrat. Growth of a magma body by accretion of andesitic sills was simulated numerically and the resulting temperatures and melt fractions were used to calculate a synthetic anomaly of seismic wave velocity, which was filtered to be comparable with the velocity anomaly obtained from a tomographic experiment. Petrology indicates that before it was reheated, remobilized and erupted, the temperature of the magma residing in the chamber was about 850°C. We ran simulations where convection is assumed to be low and heat transfer is mostly by conduction and simulations where convection is assumed to be vigorous enough to rapidly cool the magma chamber to 850°C. In both cases, magma chamber growth over the last 350 years results in tomography anomalies that are too strong, unless the magma was emplaced at an unlikely low melt fraction (<0·5). Good fits between the modelled and the observed velocity anomaly were obtained with sills 2–5 km in radius emplaced over 6000–150 000 years, depending on the temperature and melt fraction of the emplaced magma. Because of a trade-off between intrusion dimensions and emplacement durations, the volumetric magma fluxes are restricted to 7 × 10?4 and 5 × 10?3 km3 a?1. The velocity anomaly can be reproduced with a chamber containing high melt-fraction magma or with a mush of crystals and melt. The range of magma ages in the modelled magma chamber is much wider than the crystal residence time of the erupted andesite. This suggests that the eruption taps small pockets of recently assembled magma and that the velocity anomaly is mostly due to a non-eruptible mush
Dr. Duane M. Jackson, Morehouse College, July 2011
No full textThis video is a conversation with Dr. Duane M. Jackson. Dr. Jackson talks about his paper, "Recall and the Serial Position Effect: The Role of Primacy and Recency on Accounting Students' Performance." Jackie Daniel, AUC Woodruff Library, is the interviewer
"Reflections on the subject of Emigration from Europe with a view to Settlement in the United States" By M. Carey.
No full text"Reflections on the subject of Emigration from Europe with a view to Settlement in the United States: containing bried sketches of the moral and political character of those states.
By M. Carey, member of the American philosophical, and of the American Antiquarian Society, and author of The Olive Branch, Cindiciae Hibernicae, essays on banking, on political economy, and on internal improvement.
To which are now added the English editor's comments on the subject; together with Important Advice to Emigrants, and Cautions Against Impositions Practiced in the Outports
Contrasts in morphology and deformation offshore Montserrat: New insights from the SEA-CALIPSO marine cruise data
No full textDuring the December 2007, SEA-CALIPSO experiment we collected seismic reflection profiles offshore of Montserrat. Off the east coast, we imaged deep fans of volcaniclastic debris from three volcanoes progressively active from ∼2 Ma to present. Near-shelf sedimentation rates of 8-9 cm/ka are approximated following cessation of local volcanic activity. The fans were deposited on sediments with apparent dips towards the ESE-trending Montserrat-Havers fault system (MHFS) in southern Montserrat. The MHFS encloses the Soufrire Hills Volcano, has elevated crustal blocks at Roche's Bluff, St. Georges Hill, and Garibaldi Hill, and extends off the west coast. Off the west coast, the N-dip of two faults supports a N-dip interpretation for a major component of MHFS, the Belham Valley fault. We propose that local deformation is affected by stress redistributions consistent with a right-stepping, sinistral en-echelon fault system, but the interplay of transtension and magmatism has resulted in complex and evolving stress regimes. Copyright 2010 by the American Geophysical Union
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
