1,721,327 research outputs found
A statistical analysis of seismicity in Italy: the clustering properties
No full textThe clustering properties of Italian seismicity are analyzed statistically on the basis of the Istituto Nazionale di Geofisica seismic catalog. The traditional generalized Poisson model is found to be inefficient for a quantitative analysis. Based on the influence region, a new model is developed that allows to assess some spatial features of clustering as well. The results are the following: 1) there is no evidence of migration; 2) clustering is mostly originated by main shock-aftershock sequences; 3) cluster parameters are function of the region studied; 4) several different parameterizations are possible; and 5) the best fit is provided by an influence region of respectively 14 days to 60 days and 80 to 140km. -Author
Boxer - a Fortran program to compute source parameters of earthquakes from macroseismic data
No full textBoxer program is a revised and simplified version of several computer codes which have been written in the last 10 years to compute focal parameters of earthquakes from macroseismic data and have also been actually used to compute synthetic earthquake parameters for the “Catalogo dei Forti Terremoti in Italia dal 461 a.C. al 1990” [Boschi et al. 1995, 1997], the Catalogo Parametrico dei terremoti Italiani (CPTI) [CPTI Working Group, 1999] and to make the computation of the paper by Gasperini et al. [1999].
Release 3.3 implements an improved method to compute magnitude using a weighting scheme also including the reliability of empirical regressions for each intensity class as well as a procedure to compute coefficients of magnitude intensity relations. This new method has been actually adopted for revaluing the magnitudes of the historical seismic catalog [CPTI Working Group, 2004] used by the recent INGV initiative for the computation of the new seismic hazard map of Italy [Seismic-Hazard Map Working Group, 2004]. In this version are also included a previously undocumented option allowing to write import files for the MapInfo Graphic Information System (GIS) and a new option generating input files to plot epicenters and boxes by the General Mapping Tool [Wessel and Smith, 1991]. Other minor changes concerned the formats of input-output files.
A description of the procedure to compute the azimuth and to draw the “boxes” representing the seismogenic structures can be found in Gasperini et al. [1999]. More details on the computation of epicenter and magnitude are reported by Gasperini and Ferrari [1995, 1997, 2000].
Acknowledgements
The program took advantage of the contribution of Filippo Bernardini, Graziano Ferrari and Gianluca Valensise (who also suggested the name) and by important improvements made by Gianfranco Vannucci (particularly regarding the code to generate MapInfo and GMT files).
References
Boschi, E., G. Ferrari, P. Gasperini, E. Guidoboni, G. Smriglio and G. Valensise (1995). Catalogo dei forti terremoti in Italia dal 461 a.C. al 1980, ING-SGA, Bologna, 973 pp. and a CD-ROM.
Boschi, E., E. Guidoboni, G. Ferrari, G. Valensise and P. Gasperini (1997). Catalogo dei forti terremoti in Italia dal 461 a.C. al 1990, ING-SGA, Bologna, 644 pp. and a CD-ROM.
Gasperini, P., and G. Ferrari (1995). Stima dei parametri sintetici, in: E. Boschi et al. (Eds.),Catalogo dei Forti Terremoti in Italia dal 461 a.C. al 1980, ING-SGA publ., 96-111.
Gasperini, P., and G. Ferrari (1997). Stima dei parametri sintetici: nuove elaborazioni, in: E. Boschiet al. (Eds.), Catalogo dei Forti Terremoti in Italia dal 461 a.C. al 1990, ING- SGA publ., 56-64.
Gasperini, P., and G. Ferrari (2000). Deriving numerical estimates from descriptive information:thecomputation of earthquake parameters. Annali di Geofisica, 43, 729-746.
Gasperini P., Bernardini F., Valensise G. and Boschi E. (1999). Defining Seismogenic Sourcesfrom Historical Earthquake Felt Reports, Bull. Seism. Soc. Am., 89, 94-110.
CPTI Working Group (1999), Catalogo Parametrico dei Terremoti Italiani. (Boschi et al. Eds.), Editrice Compositori, Bologna, Italy, pp.88, (available at http://emidius.mi.ingv.it/CPTI/)
CPTI Working Group (2004), Catalogo Parametrico dei Terremoti Italiani, versione 2004 (CPTI04). (Gasperini P., Camassi R., Mirto C. and Stucchi M. Eds.) INGV, Bologna, Italy, http://emidius.mi.ingv.it/CPTI/
Seismic-Hazard Map Working Group (Stucchi M., Akinci A., Faccioli E., Gasperini P.,Malagnini L., Meletti C., Montaldo V. e Valensise G.), (2004) Redazione della mappa dipericolosità sismica prevista dell’ordinanza PCM del 20 marzo 2003, n 3274, All. 1, INGV, Milano. disponibile a: http://zonesismiche.mi.ingv.it/documenti/rapporto_conclusivo.pdf
Wessel P. and W.H.F Smith (1991). Free software helps map and display data, EOS Trans. Am.Geophys. Un., 72, 44
Local magnitude revaluation for recent Italian earthquakes (1981-1996)
No full textIn the framework of a joint project of the Italian Gruppo Nazionale per la Difesa dai Terremoti (GNDT) and the Istituto Nazionale di Geofisica (ING), devoted to the reorganization of the Italian earthquake instrumental database from 1981 to 1996, we approached the problem of the homogeneous determination of local magnitude in Italy. As a first step, we built a database of real Wood-Anderson amplitudes deduced from the available data of the two instruments that were in operation in Italy up to 1989 as well as of simulated Wood Anderson amplitudes computed by Very Broad Band recordings, available since 1990. The simultaneous computation of the attenuation function together with the magnitudes and the station residuals allows us to verify that, for the Italian area, this function does not significantly differ from the original one given by Richter. In the second step, the so obtained Ml magnitudes are used as a reference data set to estimate a new empirical relation for the duration magnitude Md using the unified phases database of the Italian National Seismic Network (RSNC) of ING and of the most important local seismic networks operating in Italy. The same data set was also employed to calibrate magnitude Ma based on amplitudes coming from short period vertical seismograms recorded by the RSNC automatic acquisition system. At last a set of reasonable criteria to choose the most reliable among Ml, Md and Ma has been formulated. The resulting set of magnitudes proved to be definitely better calibrated than the one obtained by procedures in use at RSNC
The attenuation of seismic intensity in Italy: A bilinear shape indicates the dominance of deep phases at epicentral distances longer than 45 km
No full textThe attenuation of seismic intensity with distance in Italy is analyzed by using felt intensity report data obtained from two comprehensive historical data-bases recently made available. The observed attenuation pattern that in the past was interpreted as a logarithmic or root (square or cubic) attenuation law shows quite clearly two different linear trends in the near and in the far field. At distances shorter than 45 km, the decrease of the intensity with distance is about one degree per 20 km, while at longer distances the slope is about one degree per 50 km. This is in agreement with some recent findings of realistic modeling of seismic ground motion that has been explained as the transition from upper-crust direct Sg phases to waves reflected at the Moho controlling the energy main release. The slope of the curve in the far field shows a regional dependence in agreement with recent works on the attenuation of Pn and Sn phases in Italy. If effective, this correlation might allow us to discriminate the contribution of crustal and subcrustal paths in seismic intensity attenuation studies
Deep continental roots: the effects of lateral variations of viscosity on post-glacial rebound
No full textThe existence of lateral viscosity variations in the earth mantle could be inferred by recent tomographic results. This can potentially cause substantial changes in the interpretation of the results from usual postglacial uplift modelling which assumes a uniform mantle. In this work we study the impact of a high viscosity craton located below the lithosphere in Fennoscandia. The solution is obtained using a finite element code which treats the surface rebound of an axisymmetrical viscoelastic half-space. Our findings are potentially very important in the interpretation of GPS data. -from Author
Evaluating the statistical validity beyond chance of ‘VAN’ earthquake precursors
No full textAll predictions of the future can be to some extent successful by chance. This is a crucial issue mostly overlooked in assessing the validity of earthquake precursors. We analyse statistically the effectiveness of VAN predictions beyond chance by studying the complete list of predictions for the period 1987 January 1–1989 November 30 recently published by Varotsos & Lazaridou (1991) using any possible combination of the ‘rules of the game’ that they consider. We find that the apparent success of VAN predictions can be confidently ascribed to chance; conversely, we find that the occurrence of earthquakes with Ms≥ 5.8 is followed by VAN predictions (with identical epicentre and magnitude) with a probability too large to be ascribed to chance. Copyright © 1992, Wiley Blackwell. All rights reserve
Glacial isostasy and the interplay between upper and lower mantle lateral viscosity heterogeneities
No full textFrom the analysis of sea‐level data near the centre of the former Fennoscandian ice sheet and the theoretical predictions of vertically and laterally stratified Earth models, we explore the extent to which lateral viscosity variations could have influenced the estimate of long term mantle viscosity. We follow a finite element scheme in cylindrical symmetry, focusing on the effects of lateral viscosity contrasts of different magnitudes and wavelengths. Sea‐levels are consistent with long wavelength lateral viscosity contrasts of at most 2 orders of magnitude; short wavelength variations, comparable with the lateral extension of the surface load, should be ruled out. Differences in the average viscosities inferred from laterally stratified and uniform mantle models, may be as high as 1 order of magnitude. If we allow for different lateral viscosity patterns in the upper and lower mantle, we find that sea‐levels are consistent with short wavelength variations in the upper mantle of around 2 orders of magnitude, coupled with smoother lateral viscosity contrasts in the lower mantle. Copyright 1989 by the American Geophysical Union
Contamination of frequency-magnitude slope (b-value) by quarry blasts: An example for italy
Full text linkArtifacts often affect seismic catalogs. Among them, the presence of man-made contaminations such as quarry blasts and explosions is a well-known problem. Using a contaminated dataset reduces the statistical significance of results and can lead to erroneous conclusions, thus the removal of such nonnatural events should be the first step for a data analyst. Blasts misclassified as natural earthquakes, indeed, may artificially alter the seismicity rates and then the b-value of the Gutenberg and Richter relationship, an essential ingredient of several forecasting models. At present, datasets collect useful information beyond the parameters to locate the earthquakes in space and time, allowing the users to discriminate between natural and nonnatural events. However, selecting them from webservices queries is neither easy nor clear, and part of such supplementary but fundamental information can be lost during downloading. As a consequence, most of statistical seismologists ignore the presence in seismic catalog of explosions and quarry blasts and assume that they were not located by seismic networks or in case they were eliminated. We here show the example of the Italian Seismological Instrumental and Parametric Database. What happens when artificial seismicity is mixed with natural one?
Precursor candidacy and validation: The VAN case so far
No full textA reliable identification of seismic precursors requires a two-step statistical approach: a retrospective "learning" step to establish the candidate precursors on the basis of ad hoc chosen laws of the game, and a "validation" step which evaluates candidate precursors on an independent data set according to the previously established laws of the game. No published work has so far provided a sound support to the candidacy of the VAN signals as earthquake precursors. The problem is complicated by a chronic fuzziness in the text of VAN's predictions, which often allows different interpretations, and by the absence of any clear statement by VAN of the laws of the game. Copyright 1996 by the American Geophysical Union
Aftershocks hazard in Italy part I: Estimation of time-magnitude distribution model parameters and computation of probabilities of occurrence
No full textWe analyzed the available instrumental data on Italian earthquakes from 1960 to 1996 to compute the parameters of the time-magnitude distribution model proposed by Reasenberg and Jones (1989) and currently used to make aftershock forecasting in California. From 1981 to 1996 we used the recently released Catalogo Strumentale dei Terremoti 'Italiani' (CSTI) (Instrumental Catalog Working Group, 2001) joining the data of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and of the Italian major local seismic network, with magnitude revalued according to Gasperini (2001). From 1960 to 1980 we used instead the Progetto Finalizzato Geodinamica (PFG) catalog (Postpischl, 1985) with magnitude corrected to be homogeneous with the following period. About 40 sequences are detected using two different algorithms and the results of the modeling for the corresponding ones are compared. The average values of distribution parameters (p = 0.93±0.21, Log10(c) = 1.53±0.54, b = 0.96±0.18 and a = -1.66±0.72) are in fair agreement with similar computations performed in other regions of the World. We also analyzed the spatial variation of model parameters that can be used to predict the sequence behavior in the first days of future Italian seismic crisis, before a reliable modeling of the ongoing sequence is available. Moreover some nomograms to expeditiously estimate probabilities and rates of aftershock in Italy are also computed
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