Di Giacomo, Domenico

A temporary network of 33 seismic stations was deployed in the area struck by the 6th April 2009, Mw 6.3, L’Aquila earthquake (central Italy), with the aim to investigate the site amplification within the Aterno river Valley. The seismograms of 18 earthquakes recorded by 14 of the 33 stations were used to evaluate the average horizontal to vertical spectral ratio (HVSR) for each site and the standard horizontal spectral ratio (SSR) between a site and a reference station. The obtained results have been compared to the geological and geophysical information in order to explain the resonance frequencies and the amplification levels with respect to surface geology of the valley. The result indicate that there is no uniform pattern of amplification, due to the complex geologic setting, as the thickness and degree of cementation of the deposits is highly variable. As consequence, a large number of the local site response is observed, therefore it is very difficult to elaborate a unique model that can explain such a variability of the amplification.

In this work, we investigate the site amplification effects observed in the Norcia plain, Central Italy. Data from 30 selected local earthquakes (2 ≤ Ml ≤ 4.1) recorded by a temporary seismic network composed by 15 stations, are analyzed to determine the spa- tial variability of site effects. Both the Horizontal-to-Vertical spectral ratio and the Standard Spectral Ratio techniques are applied to estimate the site amplification effects. The results show that most of the sites in the valley are affected by strong amplifications (up to a factor of 20) in the frequency range 0.5–5 Hz. The value of the fundamental frequency of resonance is strictly dependent on the location within the basin and on the sediment thickness. Strong amplifications also affect the vertical components. The time-frequency analysis performed on a station located inside the basin shows the presence of a large spectral amplitudes after the S-wave phase, not observed on a station located on the bedrock, suggesting the pres- ence of locally generated wave trains. Then, in agreement with earlier observations for other alluvial basins in Central Italy, 2D–3D effects play an important role in determining the site amplification effects in Norcia.

Computing the magnitude of an earthquake requires correcting for the propagation effects from the source to the receivers. This is often accomplished by performing numerical simulations using a suitable Earth model. In this work, the energy magnitude Me is considered and its determination is performed using theoretical spectral amplitude decay functions over teleseismic distances based on the global Earth model AK135Q. Since the high frequency part (above the corner frequency) of the source spectrum has to be considered in computing Me, the influence of propagation and site effects may not be negligible and they could bias the single station Me estimations. Therefore, in this study we assess the inter- and intrastation distributions of errors by considering the Me residuals computed for a large data set of earthquakes recorded at teleseismic distances by seismic stations deployed worldwide. To separate the inter- and intrastation contribution of errors, we apply a maximum likelihood approach to the Me residuals. We show that the interstation errors (describing a sort of site effect for a station) are within ±0.2 magnitude units for most stations and their spatial distribution reflects the expected lateral variation affecting the velocity and attenuation of the Earth's structure in the uppermost layers, not accounted for by the 1-D AK135Q model. The variance of the intrastation error distribution (describing the record-to-record component of variability) is larger than the interstation one (0.240 against 0.159), and the spatial distribution of the errors is not random but shows specific patterns depending on the source-to-station paths. The set of coefficients empirically determined may be used in the future to account for the heterogeneities of the real Earth not considered in the theoretical calculations of the spectral amplitude decay functions used to correct the recorded data for propagation effects.

Kyrgyzstan, which is located in the collision zone between the Eurasian and Indo-Australian lithosphere plates, is prone to large earthquakes as shown by its historical seismicity. Hence, an increase in the knowledge and awareness by local authorities and decision makers of the possible consequence of a large earthquake, based on improved seismic hazard assessments and realistic earthquake risk scenarios, is mandatory to mitigate the effects of an earthquake. To this regard, the Central Asia Cross-Border Natural Disaster Prevention (CASCADE) project aims to install a cross- border seismological and strong motion network in Central Asia and to support microzonation activities for the capitals of Kyrgyzstan, Uzbekistan, Kazakhstan, Tajikistan, and Turkmenistan. During the first phase of the project, a temporary seismological network of 19 stations was installed in the city of Bishkek, the capital of Kyrgyzstan. Moreover, single-station noise recordings were collected at nearly 200 sites. In this study, the site amplifications occurring in Bishkek are assessed by analyzing 56 earthquakes extracted from the data streams continuously acquired by the network, as well as from the single-station noise measurements. A broadband amplification (starting at ∼0:1 and 0.2 Hz), is shown by the standard spectral ratio (SSR) results of the stations located within the basin. The reliability of the observed low-frequency amplification was validated through a time–frequency analysis of denoised seismograms. Discrepancies between horizontal-to-vertical spectral ratio and SSR results are due to the large amplification of the vertical component of ground motion, probably due to the effect of converted waves. The single-station noise results, once their reliability was assessed by their comparison with the earthquake data, have been used to produce the first fundamental resonance frequency map for Bishkek, whose spatial variation shows a good agreement with the presence of an impedance contrast within the Tertiary sedimentary cover.

Geological, geophysical and geotechnical investigations, for the characterization of the strong-motion recording sitesmanaged by the ItalianCivil Protection, have been carried out in the framework of the project “Italian strong-motion database in the period 1972–2004”. The project aimed at creating an updated database of strong-motion data acquired in Italy by different institutions in the time span 1972–2004, and at improving the quality of disseminated data. This article illustrates the state of the recording site characterization before the beginning of the project, explains the criteria adopted to select the sites where geophysical/ geotechnical investigation have been performed and describes the results of the promoted field surveys.

In this work, the results of a quick microtremor survey performed in the municipalities situated in the epicentre area of the Ml 5.2 2004 Salo` earthquake (North Italy) are presented. The aim of this study is to understand if the large amount of damage caused by the event (about 215 million euros only in the areas near to the epicenter) is correlated more to the local surface geology conditions or to the vulnerability of ancient Italian historical centers. A preliminary seismic zonation was carried out in 5 villages including about 30 measurements of microtremors analyzed by the Nakamura technique (hereinafter HVNR). The points of measurement were carefully selected considering sites located both near damaged buildings and over different local geology conditions (alluvium deposits, fluvial-glacial deposits, debris fans and rock). In order to strengthen the HVNR results and to evaluate the reliability of the Nakamura analysis, a comparison with spectral ratios calculated on earthquakes (hereinafter HVSR) recorded at the strong motion station of Vobarno was made. In general, the outcomes of the survey highlight a possible correlation between local geology conditions and ground motion amplification for different frequency bands. In order to check if this evidence is linked with the damage, a series of macroseismic intensity values were collected for different zones of the investigated area, and a nonparametric correlation approach was used to establish a possible correlation between damage and ground motion amplification for selected frequency bands. The results show, from a statistical point of view, that in the area surrounding the epicenter of the November 24, 2004 mainshock, the damage pattern is not strongly dependent upon the local surface geology but more correlated to the low quality of the civil structures present in the area, including old buildings of the last century.

Il terremoto di magnitudo ML=5.2 che nel 2004 ha colpito il lato occidentale del Lago di Garda (Nord Italia) viene modellato sia nell’approssimazione di sorgente puntiforme sia mediante simulazioni da sorgente estesa, al fine di investigare l’anisotropia del campo di risentimento osservata tra 5 e 15 km di distanza epicentrale. Il confronto con le osservazioni viene effettuato in termini intensità macrosismica IMCS osservata, considerando i soli comuni localizzati su roccia (o formazione «rigida») fino a 40 km dall’epicentro; un ulteriore confronto fra simulazioni e osservazioni viene effettuato utilizzando i dati registrati dalla stazione accelerometrica di Vallio Terme (GVD), localizzata a 13.3 km dall’epicentro, appartenente alla rete nazionale (RAN). Le simulazioni più semplici utilizzano una sorgente puntiforme ed equazioni predittive empiriche in termini di intensità. Le modellazioni a faglia estesa riproducono il campo di scuotimento in termini di valori di picco dello scuotimento, successivamente convertiti in intensità attraverso relazioni empiriche. Nonostante la moderata magnitudo dell’evento, le simulazioni a faglia estesa, che richiedono l’assunzione di ipotesi plausibili sia sulla geometria della sorgente che sulle proprietà di attenuazione del mezzo di propagazione, sono in grado di confermare l’anisotropia osservata della distribuzione di intensità a scala regionale (30×30 km2). Inoltre, poiché la distribuzione delle intensità presenta anche eterogeneità a scala locale (di dimensione inferiore a 3 km), probabilmente dovute ad effetti geologici e geomorfologici, oppure ad effetti di interazione terreno-struttura, nella seconda parte del lavoro vengono presentati alcuni esempi di misure di rumore sismico e di misure di frequenza propria degli edifici.

Defining a procedure to calculate the H/V spectral ratio of seismic noise has recently been the focus of several studies. Although some issues have been addressed and a general procedure to follow in choosing the optimal seismological equipment, carrying out the measurements and analyzing the data has been agreed upon by the majority of specialists in this field, some points still remain open. One open question is whether transients should be avoided when H/V spectral ratios are calculated. Starting from contradictory results of previous papers, an ad-hoc experiment at the Nauen test site (Germany) was conducted to verify the influence of transients on the H/V spectral ratio of seismic noise. We show, consistent with some previous studies, that transients generated by high frequency content sources can amplify spectral ratio peaks at low frequencies. After careful polarization analysis of the data and estimation of the spectral amplitude decay with distance, we provide an explanation in terms of the composition of the wavefield over certain distance ranges. Our results highlight the importance of a better understanding of the wavefield composition for better exploiting the potential of H/V spectral ratio method.

Nell’area del Garda, gli effetti di sito sono stati valutati in modo speditivo, utilizzando le informazioni geologiche e geomorfologiche di superficie insieme a misure di rumore ambientale. Dove possibile, sono stati utilizzati anche dati e informazioni disponibili dagli studi di rischio sismico della Regione Lombardia-CNR (1996). Le misure di rumore sono state effettuate nei quattro comuni prescelti: Vobarno, Salò, Toscolano Maderno e Gardone Riviera. Inoltre è stata condotta una campagna di misure lungo la Val Sabbia, dove sono stati rilevati i maggiori danni. Nel comune di Gardone Riviera è stata selezionata un’area rappresentativa dell’intero versante che va da questo comune fino a Gargnano, caratterizzato da depositi eluviali sovrastanti calcari stratificati. Come area rappresentativa dei conoidi detritici, presenti in vari comuni del lago, deposti lungo il fronte del sovrascorrimento e immergenti nel lago di Garda, è stato selezionato l’abitato di Toscolano Maderno. L’area di Vobarno si trova su depositi alluvionali recenti e terrazzati del fiume Chiese, mentre Salò è ubicatosu depositi morenici di spessore significativo (massimo un centinaio di metri) su cui poggiano depositi alluvioniali recenti. Lo scopo di questo studio è la classificazione dei suoli secondo la normativa italiana (Ordinanza PCM 3274; 2003 e Norme tecniche per le costruzioni; D.M. 14/09/05) attraverso l’utilizzo congiunto delle informazioni geologiche e della frequenza di risonanza dei depositi, calcolata attraverso i rapporti spettrali della componente orizzontale e verticale del rumore ambientale. Inoltre si è verificato se esistesse una correlazione fra dati di intensità risentita durante l’evento del 24 Novembre 2004 e le condizioni di sito, individuate utlizzando informazioni geologiche.