1,721,092 research outputs found
Probabilistic seismic hazard assessment: Combining Cornell-like approaches and data at sites through Bayesian inference
No full textThe societal importance and implications of seismic-hazard assessment forces the scientific community to pay increasing attention to the evaluation of uncertainty in order to provide accurate assessments. Probabilistic seismic hazard assessment (PSHA) formally accounts for the natural variability of the involved phenomena, from seismic sources to wave propagation. Recently, increased attention has been paid to the consequences of alternative modeling procedures on hazard results. This uncertainty, essentially of epistemic nature, has been shown to have major impacts on PSHA results, leading to extensive applications of techniques like the logic tree. Here, we develop a formal Bayesian inference scheme for PSHA that allows us, on the one hand, to explicitly account for all uncertainties and, on the other hand, to consider a larger set of sources of information, from heterogeneous models to past data. This process decreases the chance of undesirable biases and leads to a controlled increase of the precision of the probabilistic assessment. In addition, the proposed Bayesian scheme allows (1) the assignment of a subjective reliability to single models, without requirement of completeness or homogeneity, and (2) a transparent and uniform evaluation of the strength of each piece of information used on the final results. The applicability of the method is demonstrated through the assessment of seismic hazard in the Emilia-Romagna region of northern Italy. In this application the results of a traditional Cornell-McGuire hazard model based on a logic tree are updated with the historical macroseismic records to provide a unified assessment that accounts for both sources of information
Some insights on the flank eruptive activity of Mount Etna volcano (Sicily, Italy)
No full textIn the first part of this work, we make use of two non-parametric statistical pattern recognition algorithms and a multiple regression analysis to analyse seismic clusters occurring around Mount Etna, Italy. The aim is to determine if the onset of flank eruptions at Mount Etna is linked to variations in the regional seismicity at a timescale of few weeks. From the analysis, we find that the discrimination between clusters preceding flank eruptions and clusters not related in time to flank activity is mainly linked to the volume output of the previous flank eruption, in some cases together with the time elapsed from its end. Instead, we do not find any difference in the seismicity features characterizing different types of clusters, except for a very small contribution of the number of seismic events in the clusters. This result does not confirm the existence, suggested in the past, of a direct link between the regional state of stress at a timescale of few weeks and the occurrence of flank eruptions on Mount Etna volcano. On the contrary, the result suggests that a prominent role in the flank eruption occurrence is played by the re-charging of the feeding system. In the second part of this study we analyse the relationship between the magma volume erupted in an eruption and the interevent time following it, finding that a ‘time-predictable model’ satisfactorily describes the occurrence of eruptions at Mount Etna in the last decades. The latter analysis is carried out both on the flank eruption catalogue only, and on the complete catalogue of flank and summit eruptions, with comparable results
A review and new insights on the estimation of the b-valueand its uncertainty
Full text linkThe estimation of the b-value of the Gutenberg-Richter Law and its uncertainty is crucial in seismic hazard studies, as well as in verifying theoretical assertions, such as, for example, the universality of the Gutenberg-Richter Law. In spite of the importance of this issue, many scientific papers still adopt formulas that lead to different estimations. The aim of this paper is to review the main concepts relative to the estimation of the b-value and its uncertainty, and to provide some new analytical and numerical insights on the biases introduced by the unavoidable use of binned magnitudes, and by the measurement errors on the magnitude. It is remarked that, although corrections for binned magnitudes were suggested in the past, they are still very often neglected in the estimation of the b-value, implicitly by assuming that the magnitude is a continuous random variable. In particular, we show that: i) the assumption of continuous magnitude can lead to strong bias in the b-value estimation, and to a significant underestimation of its uncertainty, also for binning of ?M = 0.1; ii) a simple correction applied to the continuous formula causes a drastic reduction of both biases; iii) very simple formulas, until now mostly ignored, provide estimations without significant biases; iv) the effect on the bias due to the measurement errors is negligible compared to the use of binned magnitudes
A technical note on the bias in the estimation of the b-value and its uncertainty through the Least Squares technique
Full text linkWe investigate conceptually, analytically, and numerically the biases in the estimation of the b-value of the Gutenberg-Richter Law and of its uncertainty made through the least squares technique. The biases are introduced by the cumulation operation for the cumulative form of the Gutenberg-Richter Law, by the logarithmic transformation, and by the measurement errors on the magnitude. We find that the least squares technique, applied to the cumulative and binned form of the Gutenberg-Richter Law, produces strong bias in the b-value and its uncertainty, whose amplitudes depend on the size of the sample. Furthermore, the logarithmic transformation produces two different endemic bends in the Log(N) versus M curve. This means that this plot might produce fake significant departures from the Gutenberg-Richter Law. The effect of the measurement errors is negligible compared to those of cumulation operation and logarithmic transformation. The results obtained show that the least squares technique should never be used to determine the slope of the Gutenberg-Richter Law and its uncertainty
On the validation of earthquake-forecasting models: The case of pattern recognition algorithms
No full textEarthquake forecasting is one of the geophysical issues with a potentially large social and political impact. Besides the purely scientific interest, the loss of lives and the huge damage caused by seismic events in many regions of the world have led many research groups to work in this field. Until now, however, the results obtained have not been convincing and they often have been a matter of intense debates. In part, these debates are due to the ambiguous definition of key concepts, such as precursor and forecast/prediction, as well as to the lack of a clear strategy to set up and check an earthquake-forecasting model. In this article, we provide insights that might contribute to better formally defining the earthquake-forecasting problem, both in setting up and in testing the validity of the forecasting model. As an illustration, we have applied these insights to forecasting models M8 and CN based on a pattern recognition approach. We found that the forecasting capability of these algorithms is very likely significantly overestimated
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
Testing the performance of some nonparametric pattern recognition algorithms in realistic cases
No full textThe success obtained by Statistical Pattern Recognition in many disciplines is certainly related to the quality and availability of many data, normally distributed. However, in other disciplines, the data sets consist of few measurements, often binned, correlated, and not normally distributed. Usually, we do not even know which features have an influence on the process. The main goal of this paper is to evaluate the performance of some nonparametric Pattern Recognition algorithms when applied to such data. Finally we show the results of the application of the four nonparametric statistical pattern recognition techniques to real volcanological data. © 2003 Pattern Recognition Society. Published by Elsevier Ltd. All rights reserved
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
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