Seismic Ground Motion Amplifications Estimated by Means of Spectral Ratio Techniques: Examples for Different Geological and Morphological Settings

One of the most important issue in seismic hazard and microzonation studies is the evaluation of local site response (i.e. the tendency of a site to experience during an earthquake greater or lower levels of ground shacking with respect to another). In general site effects reflect all modifications (in amplitude, frequency content and duration) of a wave-field produced by a seismic source during the propagation near the surface, due to particular geologic (stratigraphy and morphology), geotechnical (mechanical properties of deposits) and physical (e.g. coupling of incident, diffracted and reflected seismic waves) conditions of a particular site. Actually local seismic amplification represents one of the main factors responsible for building damage during earthquakes: this statement is supported by well documented evidences of structural damages during past moderate to high energy events occurred both in Italy (e.g. 23th November 1980, Mw 6.9, Irpinia earthquake, Faccioli, 1986; 26th September 1997, Mw 6.0, Umbria-Marche earthquake, Caserta et al., 2000; the 31th October 2002, Mw 5.7, Molise earthquake, Strollo et al., 2007; 6th April 2009, Mw 6.3, L’Aquila earthquake, Cultrera et al., 2009) and in other worldwide countries (e.g. 3rd March 1985, Mw 7.8, Chile earthquake, Celebi, 1987; 17th August 1999, Mw 7.6, Izmit earthquake, Sadik Bakir et al., 2002). For this reason the site effects evaluation, performed by experimental methods but also through numerical simulations, has attracted the attention of engineering seismology and earthquake engineering communities. Of consequence, in the last decade many experiments were performed in correspondence of different setting such as alluvial basins (Parolai et al., 2001 and 2004; Ferretti et al., 2007; Massa et al., 2009; Bindi et al., 2009) or topographies (e.g. Pischiutta et al., 2010; Massa et al., 2010; Buech et al., 2010; Marzorati et al., 2011; Lovati et al., 2011). The present work has the aim to evaluate the capabilities of the most common passive methods at present used in seismology to evaluate the site response: HVSR (Horizontal to Vertical Spectral Ratio technique on seismic noise, Nakamura, 1989, or earthquakes, Lermo & Chavez Garcia, 1993) and SSR (Standard Spectral Ratio, Borcherdt, 1970).Published195-2184.1. Metodologie sismologiche per l'ingegneria sismicaope

Site Classification Assessment for Estimating Empirical Attenuation Relationships for Central-Northern Italy Earthquakes

The aim of this article is to investigate the ground motion attenuation of the most industrialized and populated regions of Italy, evaluating the capability of different approaches to estimate site dependent models. The 5.2 magnitude earthquake on November 24, 2004 shocked the areas of Northern Italy producing damage of about 215 million euros. The data set, including 243 earthquakes of local magnitude up to 5.2, has been collected in the period December 2002–October 2005 by 30 threecomponent seismic stations managed by Istituto Nazionale di Geofisica e Vulcanologia, Sezione of Milano (INGV-MI). Empirical attenuation relationships have been estimated for horizontal peak ground velocity (PGHV), acceleration (PGHA), displacement (PGHD), and for response spectral acceleration (SA) for periods between 0.1 and 1.5 s. To estimate suitable attenuation models, in particular for sites characterized by thick sedimentary geological formations, a soil discrimination based on EU8 code can lead to wrong evaluations. On the contrary, a classification based on H/V spectral ratios of seismic ambient noise (NHV) allows the models to fit better real and predicted data and to reduce the uncertainties of the process. For each receiver, NHV have been strengthened by additional H/V spectral ratio of earthquake data (EHV), calculated considering different portions of the analysed signals. In order to validate the PGHA attenuation relationship for greater magnitudes, accelerometric records, relative to Central-Northern Italy strong motions occurring in the last 30 years, have been collected and superimposed to our attenuation curves

Livelli di detezione da stazioni sismiche in pozzo

Nell’ambito di un progetto pilota per l’iniezione e lo stoccaggio di CO2 da effettuarsi nell’area di Cortemaggiore (PC), la Sezione di Milano-Pavia dell’INGV è stata incaricata dell’attività di monitoraggio sismico di superficie. A tale scopo è stata realizzata una rete costituita da 7 stazioni. I sensori sono posizionati in un’area di dimensioni di circa 6x2 km. Tutte le stazioni sono dotate di acquisitore digitale a 24 bit tipo Lennartz M24/NET con ricevitore del segnale di tempo GPS. L’area è caratterizzata da un forte disturbo antropico ed industriale. Per migliorare la qualità del segnale, 4 sensori sono installati in pozzo alla profondità di 100 metri. Per le stazioni in pozzo il sensore usato è il Lennartz LE-3D/BH mentre in superficie viene utilizzato LE-3Dlite MKII. I 2 sensori presentano caratteristiche tecniche simili, con frequenza propria pari ad 1 Hz, limite superiore in frequenza di 80 Hz e range dinamico di 136 dB. In questo lavoro sono stimati i livelli di microtremore sismico e viene valutata la soglia di detezione, ovvero il valore minimo di magnitudo registrabile in funzione della distanza dalla sorgente sismica

A microtremor survey in the area shocked by the ml 5.2 Salò earthquake (north Italy): an empirical approach to determine the effects of ground motions

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 Salò 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 millions of 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 analysed 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 intensities values were collected for different zones of the investigated area, and a non parametric 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 24 November 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

Pan-European ground-motion prediction equations for the average horizontal component of PGA, PGV, and 5 %-damped PSA at spectral periods up to 3.0 s using the RESORCE dataset

This article presents a set of Ground-Motion Prediction Equations (GMPEs) for Europe and the Middle East, derived from the RESORCE strong motion data bank, following a standard regression approach. The parametric GMPEs are derived for the peak ground acceleration, peak ground velocity, and 5 %-damped pseudo-absolute acceleration response spectra computed over 23 periods between 0.02 and 3 s, considering the average horizontal-component ground-motions. The GMPEs are valid for distances less than 300 km, hypocentral depth up to 35 km and over the magnitude range 4–7.6. Two metrics for the source-to-station distance (i.e. Joyner-Boore and hypocentral) are considered. The selected dataset is composed by 2,126 recordings (at a period of 0.1 s) related to 365 earthquakes, that includes strong-motion data from 697 stations.The EC8 soil classification (four classes from A to D) discriminates recording sites and four classes (normal, reverse, strike-slip, and unspecified) describe the style of faulting. A subset which contains only stations with measured Vs30 and earthquakes with specified focal mechanism (1,224 records from 345 stations and 255 earthquakes) is used to test of the accuracy of the median prediction and the variability associated to the broader data set. A random effect regression scheme is applied and bootstrap analyses are performed to estimate the 95 % confidence levels for the parameters. The total standard deviation sigma is decomposed into between-events and within-event components, and the site-to-site component is evaluated as well. The results show that the largest contribution to the total sigma is coming from the within-event component. When analyzing the residual distributions, no significant trends are observed that can be ascribed to the earthquake type (mainshock-aftershock classification) or to the non-linear site effects. The proposed GMPEs have lower median values than global models at short periods and large distances, while are consistent with global models at long periods (T>1) s. Consistency is found with two regional models developed for Turkey and Italy, as the considered dataset is dominated by waveforms recorded in these regions

A microseismic study in a low seismicityarea: the 2001 site-response experimentin the Città di Castello Basin (Italy)

A site response experiment was performed in the basin of Città di Castello (a small town in Central Italy) in May 2001. This study is part of a project on the evaluation of seismic hazard in seismogenic areas funded by the Gruppo Nazionale Difesa dai Terremoti (GNDT). The experiment consisted of a dense fixed transect configuration with most of the stations recording in continuous mode, and several ambient noise measurements both in single station and in array configuration spread over the investigated area. The dense transect was composed of 26 seismic stations in a crosswise configuration with a maximum inter-station distance of 250 m. The stations were deployed in the southern part of the basin, from the eastern bedrock outcrop to the western edge, across the town. About 70 earthquakes were recorded during 10 days of deployment, generally low magnitude or regional events. We located 23 earthquakes and 17 of them were located using the waveform similarity approach at 4 stations outside the target area. These 4 stations were part of a dense temporary seismic network involved in a previous experiment of the same project, aimed at performing a high-resolution picture of the local seismicity. Delay analysis on the recorded waveforms allowed us to infer the basin geometry at depth and estimate the S-wave velocity of sediments. Moreover, we evaluated relative site response along the E-W transect by performing a standard spectral ratio. Amplification factors up to 9 are found inside the basin; at frequencies above 5 Hz stations closer to the edges show higher amplification, whereas stations located in the middle of the basin, where the alluvial sediments are thicker (CD11-CD14), show higher amplification below 5 Hz. We considered the average amplification in two frequency bands (1-5 Hz and 5-10 Hz), representative of the resonance frequency for 2-3 storey buildings and 1 storey houses,respectively. Our results suggest that the potential hazard for 2-3 storey buildings is higher in the center of the basin (amplification factor up to 6), and for 1 storey houses is higher at the edges (amplification factor up to 5)