1,721,077 research outputs found
Towards the construction of a lacustrine paleoseismic record in south-central Alaska : a trembling tale of landslides and turbidites
No full textTsunamis in south central Chile : evidence from coastal lakes
Full text linkThe south central Chilean coast lies on the Peru-Chile subduction zone. As a result, the area is prone to megathrust earthquakes. The convergent motion of the oceanic Nazca Plate and the South American Plate during these earthquakes causes co-seismic vertical deformation on the seafloor, which in turn generates tsunamis. The tsunamis propagate across the Pacific Ocean and eventually inundate the South American and other coasts, where they sometimes cause catastrophic damage. Alongside the destruction of infrastructure and the loss of life, tsunamis deposit distinct sediments. This thesis focuses on tsunami deposits in two coastal lakes, Lake Cucao and Lake Huelde, on the west coast of Isla de Chiloé, south central Chile (42.6° S), and their immediate surroundings.
The thesis establishes the sedimentary characteristics of the most recent tsunami deposit in the lakes' sedimentary records. The tsunami in question was caused by the AD 1960 Great Chilean Earthquake (M\textsubscript{W} 9.5). The AD 1960 tsunami inundated both lakes, which are located \textasciitilde{}2 km apart and \textasciitilde{}1.2 km behind the present coastline. The stratigraphy of both lakes features gyttja, interrupted by the abrupt emplacement of a sandy layer with mud rip-up clasts and a mud cap. This sandy layer reflects a sudden change in the sedimentary environment. Using grain size analysis and comparisons with samples from modern environments, it is demonstrated that the proximal (seaward) part of the tsunami deposit consists of a mixture of sand derived from subaerial sources and reworked lacustrine gyttja. In the distal (landward) part of Lake Cucao, the tsunami deposit lacks the sand component and consists entirely of remobilised lake sediments. The repetition of tsunami deposit sequences (or subsets thereof) in Lake Huelde suggests a minimum of three inundating waves. Sub-bottom profiles and side scan sonar mosaics reveal tsunami inundation over the barrier and more prominently through the outlet river channel. The dominant role of the river channel as a pathway for sediment transport is also described in core samples by tsunami deposits that are fining away from the channel mouth. The identification and description of the AD 1960 deposit provide important insights into tsunami sedimentation in coastal lakes and set a baseline as to what tsunami deposits should look like in the deeper sedimentary record of the two coastal lakes.
In Lake Huelde, the established expectation of the sedimentary characteristics of tsunami deposits is used to investigate the exceptionally long and continuous sedimentary record. Lake Huelde's sedimentary record contains 17 distinct detrital layers from the last 5500 yrs, which are interpreted as tsunami deposits. With the unusually high number of tsunami deposits in a single sedimentary record it is possible to analyse the recurrence pattern. Recurrence rates and pattern are an integral part to tsunami hazard assessment. In probabilistic tsunami hazard assessment tsunami recurrences are typically modelled with a Poisson distribution, which, as I demonstrate, does not necessarily represent the hazard correctly. Results of \textsuperscript{137}Cs- and \textsuperscript{210}Pb-dating, infrared stimulated luminescence (IRSL) dating and radiocarbon dating are used as input for Bayesian age-depth modelling. The resulting age-depth relationship is used to estimate the age of tsunami deposits. The inferred mean recurrence time of \textasciitilde{}325 yrs broadly agrees with the existing regional paleoseismic and paleotsunami records. However, our record reveals a large temporal variability between tsunami events, which forms a bimodal probability density function for the recurrence pattern with modes at \textasciitilde{}115 and \textasciitilde{}490 yrs. As the mean recurrence time coincides with the lowest probability between the two modes, I conclude that the sense of safety in coastal communities in south central Chile, 55 yrs after the last catastrophe, may be deceiving. A tsunamigenic earthquake in the near future would not redefine the extremes of the presented record.
Lake Cucao's sedimentary history is more complex than Lake Huelde's, however, its sedimentary record holds valuable paleotsunami information, too. The area of the lake basin has been submerged since the postglacial sea-level rise and may have recorded tsunami inundations in its sedimentary record since then. A radiocarbon date from a sediment core and internal acoustic reflections, which converge towards a tidal channel, indicate that tidal currents were active at least sporadically over the last 3700 yrs. Little vertical displacement over the last 3700 yrs maintained the ability of Lake Cucao to record tsunamis in its sedimentary record. In total 15 detrital layers are interpreted as tsunami deposits with a varying level of confidence. The level of confidence depends on five criteria; there are site-specific criteria, i.e. i) high magnetic susceptibility of the sediment indicating high detrital content in contrast to the organic-rich gyttja, ii) core-to-core correlation, and iii) acoustic reflection correlation to the sedimentary record (the latter two indicate traceability of the detrital layer in the sedimentary record), and general criteria, i.e. iv) presence of mud clasts indicating an episode of extreme flow conditions in an otherwise calm environment and v) age correlation to known paleotsunamis in the area. The neighbouring Lake Huelde contains 14 or 15 tsunami deposits in the same time interval as Lake Cucao, suggesting that both lakes may have been equally reliable in recording tsunami inundation. This study adds a long paleotsunami record on a coastline, where extreme tsunamis occur relatively frequently and where long paleotsunami records are still sparse. Many important sedimentary features were revealed by side scan sonar data and sub-bottom profiles, which demonstrates how indispensible acoustic imagery can be in understanding the depositional environment, especially in lakes as dynamic as the coastal Lake Cucao. Imaging tsunami deposits in coastal lakes with sidescan sonars and sub-bottom profiles can now be added to the growing list of tools in the toolkit to observe, describe and recognise tsunami deposits.
The amount of data collected about the numerous tsunami deposits in both lakes allows a ranking of tsunami deposits according to multiple criteria, which may represent the size of the associated tsunami. These criteria are i) maximum magnetic susceptibility, ii) average magnetic susceptibility, iii) traceability (lateral extent), iv) average tsunami deposit thickness and v) percentage of mud rip-up clast intervals. Integrating the two coastal lake records, it is evident that the AD 1960 tsunami was one of the major events in both lakes in the last \textasciitilde{}4000 yrs. Two older tsunami deposits of similar characteristics as the AD 1960 tsunami deposit are tentatively correlated across both lakes. Their ages are \textasciitilde{}3740 and \textasciitilde{}3890 cal. yrs BP. However, the relative ranking of tsunamis only compares the sedimentary effect in both lakes and cannot be translated directly to relative tsunami size, because environmental changes are not accounted for. For example, landscape evolution, horizontal and vertical shoreline displacement all affect how and where an environment records a tsunami inundation. Models that could quantify the tsunami size in absolute terms need this information. These models can be physical or numerical, however, physical models of tsunamis focus on subaerial landslide-induced tsunamis. Of the numerical models, there are forward models and inverse models. Forward models are extremely complex dynamic models and can include all aspects of tsunamis from fault slip simulation during the causative earthquake to vertical seafloor deformation, tsunami generation, propagation and inundation, and eventually erosion and deposition of coastal sediments. Inverse models try to infer tsunami inundation parameters, e.g. flow depth, from characteristics of tsunami deposits. However, none of the existing inverse tsunami models are applicable to tsunami sediments in coastal lakes. Furthermore, only little is known of how the environment changed around Lake Cucao and Lake Huelde, e.g. relative sea-level change, vegetation changes, horizontal shoreline displacement and more. Further investigation towards the environmental history and application of numerical forward models could facilitate quantitative paleotsunami research
Sublacustrine landslide processes and their paleoseismological significance: revealing the recurrence rate of giant earthquakes in South-Central Chile
No full textHuman society can be severely impacted by several kinds of natural phenomena such as earthquakes, climate changes, volcanic eruptions, landslides, etc. Therefore, understanding the nature and frequency of such natural processes is fundamental for making correct assessments and mitigations of their associated hazards. Such understanding can be gained by studying lake sedimentary records as these can be generally regarded as high-sensitive archives of natural environmental events. Sublacustrine landslide deposits are important components in these sedimentary records, but their significance has not been studied often.
This PhD thesis evaluates the wide range of information that can be extracted from sublacustrine landslide records by analyzing high-resolution reflection seismic profiles and sediment cores. In this way, new insights are gained regarding following topics:
i) The processes governing the frontal development and mobility of subaqueous landslides.
ii) The hydrological variability over the last 140 000 years in equatorial East Africa.
iii) The use of subaqueous sediment volcanoes as an additional paleoseismological proxy.
iv) The recurrence pattern and mode of megathrust earthquakes in South-Central Chile
Earth-Science Reviews / Time-dependent recurrence of strong earthquake shaking near plate boundaries: a lake sediment perspective
No full textAccurate probabilistic seismic hazard analysis requires a good knowledge of the recurrence parameters of the strongest earthquakes in a region. Due to the typical short temporal span of instrumental and historical data, it is often unclear whether one should adopt a time-dependent or time-independent (Poissonian) recurrence model, the choice of which has large repercussions on the probability estimates for new strong events. The rapidly-growing discipline of lacustrine paleoseismology aims at producing long continuous records of strong seismic shaking, which integrate the activity of all significant seismic sources in a region and allow a reliable determination of recurrence patterns. The typical continuous sedimentation regime in lakes can lead to complete, sensitive paleoseismic records and a reduced temporal uncertainty in recurrence intervals. Here, I present a worldwide compilation of published long lacustrine paleoseismic records grouped per tectonic domain, and statistically explore the variability of their recurrence intervals expressed by the coefficient of variation (CoV). A CoV 1 indicates a time-independent process, whereas CoV 1) may be related to changes in the sensitivity of the lacustrine paleoseismograph or to real earthquake clustering due to e.g. stress transfer between neighboring faults. The most useful lacustrine paleoseismic records can be retrieved in high-seismicity settings where many paleoseismic events can be recorded in a relatively short time span (i.e. <10 kyr). Such records yield sufficient intervals to reach a stable CoV and to allow probability distribution fitting, but avoid large changes in seismicity and sediment dynamics, which can be caused by e.g. the direct and indirect effects of regional deglaciation.Submitted versio
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
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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