1,721,041 research outputs found
Ancient tsunami records in the viscous remanent magnetization of reworked boulders in the Kingdom of Tonga
No full textThe movement history of boulders is crucial for the reconstruction of paleo-tsunamis. We report findings from viscous remanent magnetization studies of the boulders on Tongatapu Island, aiming to reconstruct their reworkings. Two boulders exhibited viscous remanence, whereas two larger boulders lacked viscous components but exhibited stable remanence. Both the viscous and stable components deviated from the geomagnetic field direction. These observations indicate that: (a) the boulders with a viscous component were reworked before the latest event, which could have reworked all boulders, and (b) the magnitude of the latest event was larger than that of an earlier event. The reworked timing indicated that the event occurred between 3,000 years ago and the fifteenth century. The difference in the wave height required to move boulders on the eastern and western coasts suggests that the source of the earlier tsunami was likely an eruption due to volcanoes along the Tonga Ridge.</p
Seismotectonics of northern Japan as inferred from analysis of large earthquakes.
No full textThe northern Japan (southern Kurile-Hokkaido-Honshu) region has a large history of large shallow earthquake occurrence. In addition to underthrust earthquakes associated with Pacific plate subduction, back-arc earthquakes, outer-rise earthquakes, and unusual tsunami earthquakes have occurred. In this study, three recent large earthquakes in this region, the 1993 Hokkaido Nansei-oki, 1994 Kurile, and 1994 Sanriku earthquakes have been analyzed using seismological, geodetic, and tsunami data. The 1896 Sanriku tsunami earthquake was also studied using tsunami data. The 1993 Hokkaido Nansei-oki earthquake was a back-arc event with a complex fault geometry, displaying a shallowly dipping fault in the north (except the northernmost part) and steeply dipping fault in the south. The complex fault geometry may be due to the earthquake location on a young immature plate boundary. The 1994 Kurile earthquake was an intra-slab event which tore the slab with left-lateral strike slip motion in the slab. The 1994 Sanriku earthquake was an underthrust event but did not rupture the dominant asperity that had been ruptured by the 1968 Tokachi-oki earthquake, a previous great underthrust event in the region, although the aftershock areas overlapped. The 1994 Sanriku event did not rupture the aseismic zone near the Japan trench. The 1896 Sanriku tsunami earthquake occurred very close to the trench, and did rupture the aseismic zone near the trench. I propose that the lateral variation of large earthquake occurrence along the Japan trench is controlled by the surface roughness of the plate interface. In the smooth plate interface zone, the coherent sediments form a homogeneous, large and strong contact zone between the plates. The rupture of this large contact causes the great underthrust earthquakes. In the rough surface region, large outer-rise earthquakes enhance the horst and graben structure. As this structure is subducted with sediments in the grabens, the horsts create enough contact with the overriding plate to cause a tsunami earthquake at shallow depths. When this structure is further subducted, many small strong contacts between the plates are formed but cause only small underthrust earthquakes.PhDGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105035/1/9624742.pdfDescription of 9624742.pdf : Restricted to UM users only
Asperity distribution of Alaskan-Aleutian earthquakes from inversion of tsunami waveforms.
No full textThe Alaskan-Aleutian arc has a history of rupturing in large and great earthquakes. The 20th century events include five great earthquakes: the 1938 Alaskan, 1946 Aleutian, 1957 Aleutian, 1964 Prince William Sound, and 1965 Rat Islands earthquakes. Due to the lack of seismic data for these earthquakes, a technique to invert tsunami waveforms is used to determine the source parameters of the Alaskan-Aleutian earthquakes. From the study of the 1965 earthquake, the results of tsunami waveform inversion are shown to be compatible with results of seismic wave studies. The areas of highest slip correspond to the tectonic blocks of the western Aleutians. The rupture area, slip distribution and seismic moment of the 1957 earthquake have been determined. The Unalaska Island area did not rupture in the 1957 event, making this area a seismic gap. The moment magnitude M\sb{\rm w} of this earthquake is 8.6, and the slip was concentrated in the western half of the aftershock zone. The 1938 earthquake did not rupture into the Shumagin Islands seismic gap. The slip in this earthquake was concentrated in the eastern third of the aftershock area. The 1946 earthquake is a very unusual seismic event, but the tsunami data can be modeled using a simple underthrusting mechanism. The slip was highest near the trench. The moment magnitude M\sb{\rm w} is 8.3. Tsunami data alone cannot resolve the entire slip distribution of the 1964 earthquake. Joint inversion of tsunami and geodetic data gives the complete slip distribution. The highest slip occurred in Prince William Sound and in the Gulf of Alaska where the Yakutat terrane is being subducted. High slip also occurred in the Kodiak Island region. Large near-trench slip was a major source of the transPacific tsunami. Areas of high slip correspond to seismologically determined areas of high moment release. The factors controlling asperity size and location are variable. This work can be used as a basis for future hypothesis testing concerning future earthquakes, asperities and seismic gaps.PhDEarth SciencesGeophysicsPhysical oceanographyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129701/2/9610154.pd
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Etude des supercycles sismiques de la zone de subduction des Ryukyus : apport des terrasses récifales et des microatolls coralliens
No full textCes dernières années, l'occurrence des séismes majeurs de subduction a souligné la nécessité d'approfondir nos connaissances des zones de subduction et de leur potentiel sismogénique. L'aléa sismique est lié au couplage sismique, ainsi qu'aux caractéristiques structurales de la zone de subduction qui sont finalement peu contraints dans la majorité des zones de subduction. Situé entre le Japon et Taiwan, l'arc insulaire des Ryukyus qui longe la fosse de subduction, ou la plaque Philippines plonge en dessous de la plaque Eurasienne avec une vitesse de 8 cm/an. Le couplage sismique des Ryukyus est incertain, car si la zone présente des points communs avec des zones de subduction ayant hébergé des séismes majeurs, les données de GPS et de sismicité décrivent une convergence asismique. Les Ryukyus sont une zone d'étude remarquable car elle contient des terrasses marines et les microatolls coralliens, témoignant des déformations verticales de la plaque à long et court-terme respectivement. Ici, nous proposons d'améliorer notre compréhension de cette zone, par une approche multi temporelle. Les terrasses marines se forment par interaction entre mouvement tectonique vertical et variations eustatiques sur plusieurs centaines de milliers d'années. Grâce aux modèles numériques de surface issus des images satellite Pléiades très haute résolution obtenus sur 13 îles, nous avons identifié 13 terrasses marines dans la morphologie côtière des Ryukyus centrales et du sud. Nous avons corrélé ces terrasses avec les variations de niveau de la mer, permettant d'estimer un âge et une surrection que nous avons confirmés par la modélisation en deux dimensions de la croissance récifale des îles. Les terrasses marines, datées entre les stades isotopiques marins 3 et 21, se sont soulevées à des vitesses entre 0.13±0.05 et 1.62±0.64 m/ka au cours du Pléistocène supérieur. Les variations spatiales de ces surrections à l'échelle de l'arc témoignent d'une segmentation de l'arc à long-terme, qui serait liée à la courbure de l'arc mais également à la subduction de reliefs bathymétriques. A l'échelle décennale, voire millénaire, nous avons également étudié les déformations verticales des îles à l'aide des microatolls coralliens. Nous avons échantillonné et analysé 12 coraux dans les Ryukyus du sud dont 4 fossiles. Nous avons reconstruit les variations relatives du niveau marin issues de mouvements tectoniques et de variations du niveau marin d'origine climatique jusqu'à -3000 ans. A long-terme, l'analyse de sites de coraux fossiles indiquerait un enfoncement. Nous avons pu mettre en évidence plusieurs séismes passés, dont deux pouvant être associés à des tsunamis. La corrélation de ces séismes avec les mouvements de l'interface de subduction nous mène à estimer un cycle sismique des Ryukyus du sud sur les derniers millénaires, qui serait en réalité composé de plusieurs supercycles sismiques. Ces supercycles sont définis par des séismes ayant une récurrence allant de quelques décennies à quelques siècles. Le mouvement tectonique millénaire enregistré par les microatolls n'est pas linéaire au cours du temps. Ces 200 dernières années, les microatolls ont enregistré une surrection jusqu'à 3 mm/an, variable le long des Ryukyus du Sud et également observée à plus court-terme par le GPS ou dans les microatolls fossiles. Cette variabilité entre les îles et dans le temps indique des différences de couplage et de structure de l'interface de subduction. L'origine de ces variations est discutée au regard des caractéristiques de la subduction. Cette approche multi-temporelle de la zone de subduction des Ryukyus permet donc de mieux caractériser l'aléa sismique estimé au préalable par le GPS pour le très court-terme, puisqu'elle établit le lien entre structures déterminées à long-terme, comportement tectonique à moyen-terme, et potentiel sismique à court-terme.Those past decades, the occurrence of major megathrust earthquakes highlighted the need to improve our knowledge of the behavior and seismogenic potential of subduction zones. Seismic hazard is related to seismic coupling and structural features of the subduction zone, which are poorly documented in most of subduction zones. Located between Japan main island and Taiwan, the Ryukyus island arc follows the Ryukyus trench, where the Philippines Sea plate subducts below the Eurasian plate at a rate of 8 cm/yr. Seismic coupling has been questioned, since the subduction zone present similarities with subduction zone where large Mw9-class megathrust earthquakes occurred, and since recent GPS and seismic data rather correspond to that of an aseismic convergence. The Ryukyus allows for an unprecented study of the vertical deformation of the overriding plate at long and short-term time scales since they present marine terraces and coral microatolls, respectively. IThrough this multi time-scale study, we aim to improve our knowledge on the Ryukyus subduction zone, using long-term marine terraces and coral microatolls at short-term. Marine terraces result from the interplay between vertical tectonic motion and sea-level variations over the past few hundreds of thousand years. Using digital surface model computed from very high-resolution Pleiades satellite images over 13 islands, we documented up to 13 terraces between central and southern Ryukyus. We correlated their formation with MiS 3 to 21, and estimated Pleistocene uplift rates between 0.13±0.05 et 1.62±0.64 m/ka that we constrained with reef growth modelling. Spatial distribution of those uplift along the arc revealed the along-arc segmentation, that we related to curvature of the arc, as well as subduction of bathymetric reliefs. Over several decades to several millenia, we documented vertical deformation of the arc using coral microatolls. We sampled and analyzed up to 12 microatolls in the southern Ryukyus, including 4 fossil ones. We reconstructed the relative sea-level history that result from combination of vertical tectonic motion and climatic sea-level variations over 5ka. Over a long time-scale, the analysis of two fossil corals sites would rather describe a subsidence motion. Combining the study fossil microatolls sampling sites and slices permitted to evidence up to 19 past earthquakes, inlcuding at least two that may be related to past tsunamis previously documented in the islands geology. Correlating those earthquakes with motions on the interplate thanks to elastic modelling led us to infer seismic supercycles along the Ryukyus islands over the last 5ka. Those supercycles would last several centuries and would be defined by recurrence time for subduction eathquakes of several decades to few centuries. Although the tectonic motion documented at the millenia scale with fossil microatolls may represent that of the southern Ryukyus islands, it is not constant and show variations in the stress accumulation and in the coupling of the plate interface with time. Those past two centuries, modern microatolls recorded uplift motions of up to 3 mm/yr. Variability in the uplift motion highlights differences between islands, linked with differences in structure and coupling of the plate interface. Those motions have also been observed at shorter time scale through GPS data or in the records of fossil microatolls. Processes promoting such variations are discussed together with features of the subduction zone, such as the seismogenic segments size in the megathrust. Our multi time-sclae study of the Ryukyus subduction zone helped improve the seismic hazard previously estimated with GPS data over very short-term, since it documents the relationship between long-term structures of the plate interface, its tectonic behavior at the mid- term time scale, and its seismic potential at short-term
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
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