1,721,105 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
Patterns of hot spot volcanism
No full textThe aim of this paper is to identify the characteristic features, in terms of geophysical measured parameters, of the hot spot sites over the Earth's surface. The hot spots, generally defined as volcanoes not obviously related to plate boundaries, are found to occur in sites characterized by either one of two main significant patterns: (1) positive values of geoid anomalies with harmonic degree between 2 and 10, (2) clustered volcanoes in slow moving plates. A third pattern, representative of isolated volcanoes anomalously close to mid-ocean ridges (distance from the ridge less than ~600 km) can also be tentatively identified. This result implies the existence of two main types of hot spot volcanism, the first one of sublithospheric origin, and the second one mainly due to a favorably "soft' lithosphere. -from Author
Statistical analysis of the geomagnetic reversal sequences
No full textThe main difficulty in assessing the statistical properties of the mechanism responsible for the inversion of the Earth's magnetic field is caused by the existence of several geomagnetic time-scales which have been proposed on the basis of different assumptions and approximations. We have used the 11 scales proposed earlier by other workers to study the symmetry of the distributions of the normal and reversed polarity intervals, the presence of a trend and/or different regimes in the rate of polarity change, and the type of statistical distribution that rules the occurrence of reversals. This comparative analysis gives the following results common to all geomagnetic scales: 1. (1)the normal and reversed polarity intervals are symmetrically distributed; 2. (2)the rate of reversal occurrence increases exponentially with no significant periodicity; 3. (3)a time-dependent gamma distribution provides an appropriate fit for the series of reversals. © 1990
Feasibility of a synchronized correlation between Hawaiian hot spot volcanism and geomagnetiC polarity
No full textIf a correlation existed between the volcanic eruptive activity of the Hawaiian–Emperor chain and the Earth's magnetic field it would be a very important evidence of a link between outer core convection to Earth surface phenomena. In the quantitative definition of the problem, the main difficulty is the scarcity of reliable data. A careful scrutiny of the available data shows that a statistical study is possible only for the last 7.2 million years. In order to obtain a meaningful answer we apply two independent statistical tests related to the validity of the following hypotheses: a) the probability of occurrence of volcanic events is independent of polarity; b) the volume of the erupted material is independent of polarity. We find no evidence supporting the existence of a correlation between eruptive activity and geomagnetic polarity and hence there is no evidence for a direct link between the core and surface phenomena. © 1990 by the Chinese Geophysical Societ
VAN: Candidacy and validation with the latest laws of the game
No full textWe reassess the set of VAN predictions in the period 1987-1989 with the latest laws of the game [Varotsos et al., 1996]. This reassessment does not modify our previous conclusions: VAN predictions have alarm and success rates too low to accept them as candidate precursors, and a degree of time association with earthquakes which suggests rejection of any precursor nomination. At the same time, this reassessment confirms the tendency of predictions in the period 1987-1989 to significantly follow earthquakes, with 13 earthquakes tracked by a spatially close prediction within 11 days. A validation study of the independent set of VAN predictions in the period 1990-1992, leaving the laws of the game unchanged while restricting the operative region to account for network reduction, denies any significant association of VAN predictions with earthquakes, but confirms the tendency of VAN predictions to follow rather than precede earthquakes. Copyright 1996 by the American Geophysical Union
What criticality in cellular automata models of earthquakes?
No full textSix different 2-D prototype cellular automata models are developed to analyse the main variants of the massless automata proposed so far to reproduce earthquake physics. The analysis aims at identifying the existence of features common to these models, if any. The different model variants were studied with regard to: (1) initial grid configuration, homogeneous or random heterogenous; (2) loading function, random or uniform; (3) local dissipation; (4) local redistribution. As a first general result, it is found that the model exhibit critically over a very restricted range of spatial scales, much smaller than that imposed by the geometrical dimensions of the grid alone. The latter, in contrast, governs the initial transient dynamics, which exhibits much larger events. As a second general result, the foreshocks are found to increase systematically in both rate of occurrence and size prior to main shocks, with a simultaneous progressive deficit of small events. This, in turn, implies an increase in correlation length and a 'precursor' decrease in the b-value. As a third general result, the presence of foreshocks in cellular automata and the difficulties in detecting them in real earthquakes still only give a limited applicability of the present cellular automata models to the real world. As a final general result, periodic recurrence of main shocks is found only for locally dissipative models
Passive imaging in nondiffuse acoustic wavefields
No full textA main property of diffuse acoustic wavefields is that, taken any two points, each of them can be seen as the source of waves and the other as the recording station. This property is shown to follow simply from array azimuthal selectivity and Huygens principle in a locally isotropic wavefield. Without time reversal, this property holds approximately also in anisotropic azimuthally uniform wavefields, implying much looser constraints for undistorted passive imaging than those required by a diffuse field. A notable example is the seismic noise field, which is generally nondiffuse, but is found to be compatible with a finite aperture anisotropic uniform wavefield. The theoretical predictions were confirmed by an experiment on seismic noise in the mainland of Venice, Italy. © 2008 The American Physical Society
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
The periodicity of geomagnetic reversals
No full textSeveral periodicities have been claimed in the time series of geomagnetic reversals, but none has survived subsequent methodological objections. We apply the established tool of periodogram analysis to the most accurate geomagnetic scale available and obtain the following results: a trend, that is well fitted by an exponential curve, is present in the rate of reversals occurrence and there is a 15 Ma periodicity that is significant at approximately the 0.05 level; no significant period is present in the series of the intervals between reversals. These results indicate that geomagnetic reversals since 85 Ma BP occur according to a generalized Poisson process with a mean which increases exponentially, and which may have a superimposed periodic modulation. In physical terms, this would imply a mechanism of reversal generation which is stochastic and has an energy input increasing in time modulated by a 15 Ma periodicity. This is consistent with reversals originated by cyclones in the outer core, a long-term change in the lower mantle temperature distribution, and a pulsating 8-10 km thick layer at the base of the mantle. © 1992
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