88,030 research outputs found
Extending the Usefulness of Seismic Hazard Studies
No full textA seismic hazard study was performed and published in the Bulletin of the Seismological Society of America by Boschi, Gasperini, and Mulargia in 1995. The application of statistical modeling to the independently defined 54 Italian seismotectonic regions led to the result that two regions had an estimated probability of occurrence for an M ≥ 5.9 event close to 1.0 in the following five years. These were the l’Aquila region and southeastern Sicily.
This forecast was dramatically verified on April 2009 at l'Aquila.
in spite of the fact that first author has since then been in charge of the leading responsibility for seismic risk in Italy, and of the persistence of an earthquake swarm, no practical action was taken. This appears as a solid proof of the lack practical utility of seismic hazard studies, which can never reach a reliability sufficient to order a disruption of everyday life. Hazard studies must therefore be retargeted to guide seismic vulnerability studies at the level of single buildings, the only ones really capable to reduce seismic destruction
Estimates of Vs30 Based on Constrained H/V Ratio Measurements Alone
No full text"One more use of H/V of microtremors is described by S. Castellaro and F. Mulargia. They propose a method to infer the ever-needed Vs 30 by fitting the observed H/V spectrum to the theoretical one computed under assumption taht noise wavefield is composed of the fundamental mode Rayleigh waves. The authors find their technique superior and more informative than all popular array methods, allowing detection of deviations from 1-D subsoil geometry over lengths of a few meters!" (CASSIDY J., MUCCIARELLI M., HERAK M. 8eds.), Increasing Seismic Safety by Combining Engineering Technologies and Seismological Data, p.2
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
Time and space clustering of Etna volcano earthquakes during the period May 1983-February 1987
No full textParticularly well located earthquakes exist at Etna volcano for the period May 1983-February 1987, since an 18-stations seismic network was in operation. The space-time clustering of the seismic activity is analyzed statistically in this period by using the model of Gasperini and Mulargia (1989), which is based on the influence region. The following clustering features are identified: (1) clustering is mainly originated by swarm-type sequences; (2) two magnitude-dependent best fits are obtained by an influence region of respectively 1.5 to 6 days and 15 to 20 km; (3) no evident space and time relation between clusters and eruptions is apparent. © 1992
Statistical analysis of seismic and eruptive activities at Mt. Etna during 1978-1987
No full textSeismic and eruptive activities that occurred at Etna volcano during the decade 1978-1987 have been analyzed statistically. The seismic activity consists of about 7500 events. This catalog has been found complete above the magnitude threshold 2.8. On the basis of the complete catalog (1458 earthquakes), the clustering features of seismicity have been investigated. The hypothesis of a Simple Poisson process is rejected. Applying a Generalized Poisson process of the Shlien and Toksoz (1970) type, the "E" parameter of cluster size appears to be strongly dependent on the chosen time interval. The application of Gasperini and Mulargia (1989) algorithm for identifying the single earthquake sequences indicates that the whole period is composed of only three sequences. Etnean seismicity appears therefore characterized by a "diffuse" low-magnitude (less than about 3.0) earthquake occurrence. From the volcanological point of view, two time series of eruptions (flank and flank + summit) have been analyzed in order to identify different regimes in both magma output and inter-event time. No change-points are apparent in the magma output series, while both inter-event time series of flank and flank + summit eruptions are characterized by one change-point each. No evident relation between the series of eruptions and the identified earthquake sequences is apparent. © 1990
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
The seismic noise wavefield is not diffuse
No full textPassive seismology is burgeoning under the apparent theoretical support
of diffuse Acoustics. However, basic physical arguments suggest
that this theory may not be applicable to seismic noise. A procedure is
developed to establish the applicability of the diffuse field paradigm to
a wavefield, based on testing the latter for azimuthal isotropy and spatial
homogeneity. This procedure is then applied to the seismic noise
recorded at 65 sites covering a wide variety of environmental and subsoil
conditions. Considering the instantaneous oscillation vector measured
at single triaxial stations, the hypothesis of azimuthal isotropy
is rejected in all cases with high confidence, which makes the spatial
homogeneity test unnecessary and leads directly to conclude that the
seismic noise wavefield is not diffuse. However, such a conclusion has no
practical effect on passive imaging, which is possible also in non-diffuse
wavefields
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
- …
