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

Estimation of topographical effects at Narni ridge (Central Italy): comparisons between experimental results and numerical modelling

In the present work the seismic site response of Narni ridge (central Italy) is evaluated by comparing experimental results and numerical simulations. The inhabited village of Narni is located in the central Italian Apennines at the top of a steep massive limestone ridge. From March to September 2009 the site was instrumented with 10 weak-motion stations, 3 of which located at the base of the ridge and 7 at the top. The velocimetric network recorded 642 events of ML up to 5.3 and hypocentral distance up to about 100 km. The great amount of data are related to the April 2009 L’Aquila sequence. The site response was analyzed using both reference (SSR, Standard Spectral Ratio) and non reference spectral techniques (HVSR, Horizontal to Vertical Spectral Ratio). Moreover directional analyses were performed in order to evaluate the influence of the ridge orientation with respect to the selected source-site paths. In general the experimental results show amplification factors for frequencies between 4 and 5 Hz for almost all stations installed along the crest. The SSR technique provides amplification factors up to 4.5 detected considering directions perpendicular to the main elongation of the ridge. The results obtained from the monitoring activity were used as a target for bidimensional and tridimensional numerical simulations, performed using a hybrid finite-boundary element method for 2D and a boundary element method for 3D analyses respectively. In general, the results obtained through numerical simulation fit well the experimental data in terms of range of amplified frequencies, but they underestimate by a factor of about 2 the related amplification factors with respect to the observations

Strong-motion parameters of the Mw=6.3 Abruzzo (Central Italy) earthquake

INGVPublished1.1. TTC - Monitoraggio sismico del territorio nazionaleope

INGV strong-motion data web-portal: a focus on the Emilia seismic sequence of May-June 2012

In Italy, strong-motion monitoring was started in 1972 by different Institutions, although mainly through Ente Nazionale per l'Energia Elettrica (ENEL; Italian National Electricity Company) and Dipartimento della Protezione Civile (DPC; Italian Department of Civil Protection), with different purposes. These included permanent acceleromet- ric monitoring and temporary monitoring during seismic se- quences or before permanent installation. Today, the National Accelerometric Network (RAN; Rete Accelero- metrica Nazionale) [Gorini et al. 2010, Zambonelli et al. 2011] is operated by the DPC and consists of 464 digital sta- tions. These are distributed throughout the whole national territory, with a prevalence for areas of major seismicity. In 2006, the INGV began strong-motion monitoring, by installing 22 accelerometric stations in northern Italy (RAIS; Rete Accelerometrica Italia Settentrionale; Accelerometric Network of Northern Italy; http://rais.mi.ingv.it/). In 2008, the monitoring was extended to a national scale: this effort led to the installation of 105 accelerometers, collocated with the velocimetric sensors, in selected Rete Sismica Nazionale (RSN; National Seismic Network) sites [Amato and Mele 2008] that are managed by the Centro Nazionale Terremoti (CNT; National Earthquake Centre). Overall, the 127 strong- motion stations that form the INGV Italian strong-motion network homogeneously cover the whole Italian territory. The progress achieved in Italy in the field of strong-mo- tion monitoring and strong-motion data archiving and dis- semination was illustrated in a recently published special issue of the Bulletin of Earthquake Engineering [Luzi et al. 2010]. The strong-motion data recorded by the RAN have been distributed and are available on request to the DPC and to the Italian Accelerometric Archive (ITACA), as the Italian strong-motion database (http://itaca.mi.ingv.it/) [Pacor et al. 2011a], which has been updated with records to 2009. The INGV strong-motion data are archived in real-time and dis- tributed through the European Integrated Data Archive (EIDA; http://eida.rm.ingv.it/) web portal. Recently, an INGV working group developed the first version of a web portal with the aim of archiving, processing and distributing accelerometric data recorded by permanent and temporary INGV stations. This web portal (www.mi. ingv.it/ISMD/; Figure 1, top panel) is composed of two main modules: the former is known as the INGV Strong Motion Data (ISMD, www.mi.ingv.it/ISMD/ismd.h tml/; Figure 1, bottom left panel) and has as its main scope the analyse and distribution in quasi-real time (a few hours after event oc- currence) of the uncorrected accelerometric data, and the related metadata obtained after an automatic processing pro- cedure. This latter, known as the Dynamic Archive (DYNA, http://dyna.mi.ingv.it/DYNA-archive/; Figure 1, bottom right panel) is a dynamic database where manually post- processed accelerometric waveforms are provided, together with their metadata. Both of these archives are designed and structured in such a way that their compilations and updat- ing will be almost completely automatic. At the end of May 2012, a first prototype of the ISMD module was published, providing the uncorrected strong- motion data recorded by the INGV stations for the main events of the Emilia seismic sequence [Massa et al. 2012]

OMBRA: OBSERVING MONTELLO BROAD ACTIVITY DEPLOYMENT OF A TEMPORARY SEISMIC NETWORK TO STUDY THE DEFORMATION PROCESS ACROSS MONTELLO FAULT (EASTERN ALPS)

Istituto Nazionale di GeofisicaPublishedope

OMBRA: Observing Montello BRoad Activity. Una rete temporanea per lo studio dei processi di deformazione attraverso la faglia del Montello (Alpi orientali).

L’area veneta delle Alpi orientali è caratterizzata da una debole sismicità di background. In particolare, l’attività sismica registrata negli ultimi 30 anni [Castello et al., 2006; Bollettino Sismico INGV1] mostra eventi di bassa energia (ML<3) lungo l’arco alpino in corrispondenza dell’anticlinale del Montello (situato a NW di Treviso). Sono noti però alcuni eventi di magnitudo medio-alta che hanno storicamente interessato la regione: l’episodio più significativo è il terremoto di Asolo del 1695 (Imax 10 e MaW 6.61), affiancato da tre ulteriori eventi sismici di intensità Imax≥VIII (magnitudo equivalente 6.0) avvenuti nel 778, 1286 e 1836 [CPTI Working group 2004] (Figura 1). Il Montello è catalogato tra i segmenti sismogeneticamente attivi del fronte alpino [Valensise and Pantosti, 2001; Galadini et al., 2005; Poli et al., 2008], originato dall’uplift di una struttura di thrust S-vergente, con slip rate di deformazione stimato tra 1.5 mm/yr [Burrato et al., 2009] e 1.8-2.0 mm/yr [Benedetti et al., 2000]. Scopo del progetto OMBRA è quello di studiare alcune questioni ancora aperte e scientificamente controverse. Ci si chiede come questi eventi storici forti possano integrarsi nel contesto della debole sismicità di fondo osservata recentemente. Inoltre è interessante capire come una velocità di placca relativamente alta possa accomodarsi nel pattern regionale e inoltre quali strutture tra l’anticlinale e il fronte alpino possano essere potenzialmente attive