Averaged number of visits

International audienceWe introduce a new indicator for dynamical systems, namely the averaged number of visits, to estimate the frequency of visits in small regions when a map is iterated up to the inverse of the measure of this region. We compute this quantity analytically and numerically for various systems and we show that it depends on the ergodic properties of the systems and on their topological properties, such as the presence of periodic points

Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project

Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the VS,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data.Published2367-24205T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

HVSR curve by a full-wavefield model of ambient vibrations generated by a distributin of correlated surface sources

A 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

Structure of ambient vibration wavefield in the frequency range of engineering interest ([0.5,20] Hz): Insights from numerical modeling

2nononeThe 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.noneD. ALBARELLO; LUNEDEI E.Albarello, Dario; Lunedei, E

Application of the principal component analysis (PCA) to HVSR data aimed at the seismic characterization of earthquake prone areas

In this work, we propose a procedure based on principal component analysis on data sets consisting of many horizontal to vertical spectral ratio (HVSR or H/V) curves obtained by single-station ambient vibration acquisitions. This kind of analysis aimed at the seismic characterization of the investigated area by identifying sites characterized by similar HVSR curves. It also allows to extract the typical HVSR patterns of the explored area and to establish their relative importance, providing an estimate of the level of heterogeneity under the seismic point of view. In this way, an automatic explorative seismic characterization of the area becomes possible by only considering ambient vibration data. This also implies that the relevant outcomes can be safely compared with other available information (geological data, borehole measurements, etc.) without any conceptual trade-off. The whole algorithm is remarkably fast: on a common personal computer, the processing time takes few seconds for a data set including 100-200 HVSR measurements. The procedure has been tested in three study areas in the Central-Northern Italy characterized by different geological settings. Outcomes demonstrate that this technique is effective and well correlates with most significant seismostratigraphical heterogeneities present in each of the study areas

Power spectral density function and spatial autocorrelation of the ambient vibration full-wavefield generated by a distribution of spatially correlated surface sources

Synthetic 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

A large scale ambient vibration survey in the area demaged by May-June 2012 seismic sequence in Emilia Romagna, Italy

Results of an extensive ambient vibration surveys carried out by different research teams in the area damaged by May–June 2012 seismic sequence in Emilia Romagna (Northern Italy) are summarized and analysed. In particular, ambient vibrations were acquired by both single station and seismic array configurations. Average horizontal to vertical spectral ratios (HVSR) at single station measurements were considered to evaluate local resonance phenomena. Despite the fact that general trends that can be detected are the effect of the subsoil configuration, H/V spectral ratios show a significant dependence on meteo-climatic conditions: in particular, HVSR amplitudes in the low frequency range (.5 Hz) correlate significantly with the sea wave activity in the Central Mediterranean. Anyway, resonance frequencies estimated from HVSR peaks appear persistent and have been used to estimate the local depth of impedance contrasts responsible for seismic resonance phenomena. To this purpose, average VS values up to any depth h(VSh) were assessed in the form of a standard power law constrained by Rayleigh dispersion curves deduced from the seismic arrays, and borehole data. In the whole area (except in the Mirandola area) two significant interfaces have been identified corresponding to two main resonance frequencies around 0.8–0.9 and 0.25–0.3 Hz respectively. The first one is attributed to a seismic impedance contrast located in the depth range 60–110 m. The second one corresponds to a deeper interface, possibly located at several hundreds of meters of depth. As concerns the Mirandola area, a single interface has been identified with depths varying in the range 50–115 m corresponding to resonant frequencies in the range 0.8–1.4 Hz. Finally a tentative geological interpretation of the above resonant interfaces is presented

Seismic hazard assessment at municipality scale: the Unesco cultural heritage site of San Gimignano (Italy)

In the frame of the project Rischio Sismico negli Edifici Monumentali-RiSEM (Seismic Risk in Monumental Buildings), a seismogeological reference model was developed for the historical center of the town of San Gimignano in central Italy in order to evaluate possible small-scale lateral variation of seismic hazard. To this aim, an on-purpose geological and geomorphological survey was performed along with ambient vibration measurements (both in a single station and array configurations) to characterize seismic response in the study area. Expectedseismic amplificationeffectswere quantifiedby considering asimplifiedapproach developedinItaly forseismic microzoning at the municipality scale. This study provided preliminary information supporting site-specific analyses of the local seismic response and makes it possible the identify most critical situations in which eventual seismic retrofitting interventions are more urgent to reduce seismic ris

Seismic characterization and reconstruction of reference ground motion at accelerometric sites of the Italian national accelerometric network (RAN)

We present a field procedure that has been extensively used in Italy to characterize local seismic response at accelerometric sites and to retrieve ground motion at reference soil conditions by deconvolution analysis. To allow a generalized application to large areas where borehole data are generally lacking or inadequate for the seismic characterization for soils down to the reference seismic bedrock, cost-effectiveness of the considered procedures is a main issue. Thus, major efforts have been devoted to optimize available information and exploit fast and cheap surface geophysical prospecting. In particular, geological/geomorphological survey and passive seismic prospecting (both in single- and multi-station configurations) were jointly considered to reconstruct seismo-stratigraphical site conditions. This information was then used to feed numerical modeling aiming at computing the local seismic response and performing a deconvolution analysis to reconstruct ground motion at reference soil conditions. Major attention was devoted to evaluate and manage uncertainty involved in the procedure and to quantify its effect on final outcomes. An application of this procedure to a set of sites included in the Italian Accelerometric Network is presented