144 research outputs found
Application of SASHA to seismic hazard assessment for Portugal mainland
No full textIn the frame of the UPStrat-MAFA âUrban Disaster Prevention Strategies Using MAcroseismic Fields and FAult Sourcesâ project, seismic hazard has been assessed in Portugal in terms of macroseismic intensity. Assessment has been performed by using a probabilistic approach based on the statistical analysis of local seismic histories (i.e., the record of seismic effects at each locality) performed by a new version of the SASHA code (DâAmico and Albarello in Res Lett 79(5):663â671, 2008) on purpose modified to account for this specific area of study. Local seismic histories are reconstructed by considering documented effects or indirect estimates deduced from epicentral information or numerical simulations. All these pieces of evidence are combined taking into account relevant uncertainty and statistical completeness of information locally available. Distribution of expected maximum intensity (i.e., the maximum intensity characterized by a fixed exceedance probability for a exposure time of 50Â years) has been obtained and compared with the one deduced from alternative approaches
Statistical analysis of Horizontal to Vertical Spectral Ratios (HVSR)
No full textStatistical properties of the horizontal to vertical spectral ratios (HVSR) applied to noise recording are analyzed in order to define optimal strategies for numerical processing and identification of possible artifacts. To this purpose, two time series have been analyzed: one constituted by environmental seismic noise in the presence of a genuine physical signal and one relative to pure instrumental noise, both obtained with the same experimental apparatus. By means of suitable statistics, some guidelines for the HVSR analysis are provided. A statistical test proposed by Albarello (2001) for the identification of artifacts in the HVSR function has been analyzed and invalidated
Detection of spurious maxima in the site amplification characteristics estimated by the HVSR technique
No full textSpurious peaks in the site amplification characteristics estimated by horizontal-to-vertical spectral ratios (HVSR) of microtremors can be generated when ground motion is characterized by negligible spectral amplitudes and instrumental/numerical noise is dominant. In order to individuate HVSR maxima that cannot be safely considered as an effect of actual ground-motion characteristics, a new statistical procedure is proposed. In particular, statistics can be computed with the assumption that random noise is Gaussian. In this case, it results that the proposed statistics are independent from the level of noise and only depends on the procedures adopted for the computations of spectral amplitudes from the monitored signal. In order to use such statistics to discriminate suspect HVSR peaks, its major sampling properties in the case of signal dominated by random noise have been deduced by numerical simulations. An application of this methodology to HVSR measurements carried out at a test site in northern Italy is described and discussed
Structure of ambient vibration wavefield in the frequency range of engineering interest ([0.5,20] Hz): Insights from numerical modeling
No full textThe expected structure of an ambient vibration wavefield at the top of a shallow soft layer overlying a rigid bedrock is explored by applying a full wavefield physical model, under the hypothesis that ambient vibrations are the effect of a uniform distribution of random independent point-like harmonic sources at the surface of a flat, weakly dissipative Earth. The comparison of the results provided by this model with those deduced on the assumption that surface waves dominate the wavefield allows evaluation of the respective roles of body and surface waves (Love and Rayleigh) in their fundamental and higher modes. This analysis reveals that the structure of the ambient vibration wavefield strongly depends on the subsoil structure (P- and S-wave velocity profiles and thickness of the uppermost soft sedimentary layer) and on the distribution of ambient vibration sources around the receiver. This dependence also changes along with the frequency range of interest. In this regard, three frequency domains are identified, each showing a different sensitivity to the relevant parameters: below the fundamental resonance frequency for S-waves f(S), above the frequency f(u) min {2f(S), f(P)}, where f(P) is the resonance frequency for P-waves and in-between. A consequence that emerges is that a number of possible combinations of body and surface waves are possible, which could account for the heterogeneous results obtained from experimental studies. These findings also indicate constraints on the use of simplified models based on the assumption that surface waves dominate the ambient vibration wavefield, as is currently the case in most engineering applications
Power spectral density function and spatial autocorrelation of the ambient vibration full-wavefield generated by a distribution of spatially correlated surface sources
No full textSynthetic dispersion curves are here computed in the frame of an ambient-vibration fullwavefield model, which relies on the description of both ambient-vibration ground displacement and its sources as stochastic fields defined on the Earth's surface, stationary in time and homogeneous in space. In this model, previously developed for computing synthetic Horizontal-to-Vertical Spectral Ratio curves, the power spectral density function and the spatial autocorrelation of the displacement are naturally described as functions of the power spectral density function of the generating forces and of the subsoil properties (via the relevant Green's function), by also accounting for spatial correlation of these forces. Dispersion curves are computed from the displacement power spectral density function and from the spatial autocorrelation according with the well-known f-k and SPAC techniques, respectively. Two examples illustrate the way this new ambient-vibration model works, showing its possible use in better understanding the role of the surface waves in forming the dispersion curves, as well as its capability to capture some remarkable experimental findings
The Difficult Balance among Scientific, Technical, and Political Issues in Seismic Hazard Assessment
No full textScientific and technical communities are in charge for providing political authorities, based on sound information and authoritative considerations, plausible seismic hazard estimates to be translated into prescriptions and guidelines to be adopted by citizens and stakeholders. However, this ideal flow presents several problems, hampering the development of such a simple and rational pathway. As a matter of fact, because seismic hazard assessment is just a part of a risk‐oriented procedure, the implementation of its outcomes should be mediated by further considerations dealing with the possible impact on the risk mitigation practice. As an example, this occurred in the U.S. seismic..
Validation of insensity attenuation relationships
No full textA new approach is proposed for the empirical validation of intensity attenuation relationships to be implemented in standard procedure devoted to probabilistic seismic hazard assessment (PSHA). To this purpose, the overall number of documented intensities above a given threshold in the area of interest is compared with the one expected on the basis of the considered attenuation relationship. By using this methodology, the reliability of empirical relationships can be analyzed by also taking into account the uncertainty affecting ill-defined intensity attributions both at the epicenter and at the site. To assess the feasibility of this procedure, an attenuation relationship recently proposed for the Italian region has been evaluated by considering felt intensities documented in the area during the past two centuries. Although the macroseismic database considered for validation is the same used for the parameterization of the investigated relationship, important discrepancies have been detected between observed and computed intensities in the range of values significant for seismic hazard. This result indicates that a careful parameterization of attenuation relationships in their complete probabilistic form is mandatory when such relationships have to be implemented in PSHA procedures
The use of historical data in earthquake prediction: an example from water-level variations and seismicity
No full textHorizontal-to-vertical spectral ratios from a full-wavefield model of ambient vibrations generated by a distribution of spatially correlated surface sources
No full textA new effective model is presented to compute horizontal-to-vertical spectral ratios (HVSR) relative to ambient vibrations, under the assumption that these are originated by a distribution of spatially correlated random surface sources. The major novelty of this model lies in the description of both ground displacement and sources as stochastic fields defined on the Earth's surface, stationary in time and homogeneous in space. In this frame, the power spectral density of the displacement stochastic field can be written as a function of the power spectral density of the force stochastic field and of the subsoil properties, through the relevant Green's function. Spatial correlation between ambient vibration sources is shown to be a necessary condition to warrant convergence of the integrals defining the frequency power spectra of the displacement field that make up the HVSR curve. Furthermore, it is shown that this HVSR curve may be significantly affected by the effective range of the force-field correlation on the Earth's surface. This formalization reduces computational efforts with respect to the previous version of the model based on distributed surface sources and may provide synthetic HVSR-curve patterns that are in line with those given by that computationally more troublesome version, as well as with those deduced under the assumption that the ambient vibrations constitute a diffuse wavefield
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