ALBERT

Description: During three moderate-magnitude earthquakes occurred in September–October 1997 in the central Apennines, Italy, accelerations larger than 0.5 g were recorded in the town of Nocera Umbra, 10 to 15 km N-NW of the epicenters. The accelerograph is sited in a fault zone, close to a N30 E tectonic contact. Six temporary seismological stations installed across the fault recorded 82 aftershocks occurred in two seismogenic zones: the Colfiorito-Sellano area, S-SE of the array, and the Gualdo Tadino area, to the north. The array data reveal large variations in terms of both peak ground motions and spectral amplitudes. Within the fault zone, amplifications show a strong dependence on the source azimuth. At the accelerograph site, the effects are particularly large for events from S-SE: peak ground motions are a factor of 14 larger than those of a reference site and conventional spectral ratios attain amplitudes as large as 50 at 7 Hz along the N30 E direction of motion, parallel to the strike of the fault. Nineteen strong motion accelerograms were then used to compare ground motion properties between weak and strong events up to M0 = 1.2 1025 dyn cm. A particle motion analysis shows that the directional effect is also present in the strongest motions, even though the amplification of peak ground motion decreases when M0 increases. Results from stochastic simulations indicate that such a behavior is not due to nonlinearity: applying the empirical weak motion transfer functions in a purely linear model the observed peak ground motions of the largest events are fit satisfactorily.

Description: Published

Description: 2156

Description: JCR Journal

Description: reserved

Description: During the two mainshocks of September 26, 1997 in the Umbria-Marche border a strong-motion accelerograph recorded peak ground accelerations as large as 0.6 g, approximately, in the town of Nocera Umbra, at distances of 10 to 15 km from the epicentres. This value is significantly larger than expected on the basis of the usual regressions with magnitude and distance. A broad-band amplification up to a factor of 10 was consistently estimated in previous papers, using both weak and strong motion data recorded at the accelerograph site during local moderate earthquakes. To study the cause of this amplification we deployed six seismologic stations across the tectonic contact between the Ceno-Mesozoic limestone and the Mesozoic marly sandstone where the accelerograph is installed. Seismograms of 21 shallow aftershocks in the magnitude range from 2.2 to 4.0 and a subcrustal Mw = 5.3 event are analysed. Regardless of epicentre location, waveforms show a large complexity in an approximately 200 m wide band adjacent to the tectonic contact. This is interpreted as the effect of trapped waves in the highly fractured, lower velocity materials within the fault zone.

Description: Published

Description: 543-554

Description: JCR Journal

Description: reserved

Description: The fragmentation of the collisional border between the African and European plates has also originated the Apulian (Adriatic) microplate. Recent studies show the possibility of a non-unitary geodynamic evolution of this microplate: palaeomagnetic data from North-Western Greece and Southern Apulia indicate a different rotational behaviour. Between 41' and 43' latitude North, regional strike-slip fault systems cut crosswise the Adriatic basin, breaking the Adriatic block in at least two minor elements. The intense seismicity points out an active defonnational area. In the same region also other geophysical data identify a transitional zone.

Description: Published

Description: 71-80

Description: open

Description: La convergenza Africa-Eurasia, che ha dato origine alle catene montuose circum-mediterranee, ha provocato la frammentazione della zona di contatto tra le due placche in blocchi minori. Tra questi la microplacca adriatica ha giocato un ruolo determinante nell'evoluzione del Mediterraneo centrale e in particolare delle catene terziarie periadriatiche. I margini della microplacca sono stati definiti da diversi autori sulla base dell'attività sismica, con ipotesi anche recenti di un comportamento dinamico unitario del blocco adriatico, relativamente indeformato e asismico rispetto alle fasce orogeniche circostanti. Tale modello risulta tuttavia insufficiente a giustificare quanto emerge dall'analisi della sismicità recente dell'area: negli ultimi anni l'istituto Nazionale di Geofisica (ING) ha infatti notevolmente migliorato il sistema di acquisizione e di elaborazione dei dati sismici rilevati dalla Rete Sismica Nazionale Centralizzata (RSNC), che attualmente dispone di una copertura strumentale più omogenea del territorio. Ciò ha permesso di definire spazialmente con maggior precisione l'attività sismica italiana, compresa quella che ha interessato negli ultimi anni il bacino adriatico: dall'elaborazione dei nuovi dati si è infatti evidenziata una maggiore concentrazione degli ipocentri al di sotto del 43° parallelo, in una fascia a est del promontorio del Gargano. Questa zona ha un notevole significato geodinamico per la presenza di un'ampia fascia di deformazione, in corrispondenza dei due sistemi di faglie trascorrenti, noti in letteratura come faglie delle Tremiti e di Mattinata. Il verificarsi di terremoti con notevole rilascio di energia individua dunque un'importante zona sismogenetica nel basso Adriatico, finora poco studiato da questo punto di vista, che potrebbe modificare il quadro geodinamico del Mediterraneo centro-orientale. ENGLISH ABSTRACT: The convergence between the two plates Africa- Eurasia, which originated circum-Mediterranean mountain belts, determined the fragmentation of the collisional border in minor blocks. Among these the Adriatic microplate has played an important role in the evolution of the Centra1 Mediterranean region and, in particular, of the peri-Adriatic belts during Tertiary. The Adriatic microplate margins have been defined by severa1 authors on the basis of seismic activity, assuming also recently a unitary dynamical behaviour of the Adriatic block, considered relatively undeformed and aseismic compared with the peri- Adriatic belt. Nevertheless such a mode1 is inadeguate to justify the recent seismicity (1986-1990). During the last years the Istituto Nazionale di Geofisica (ING) has greatly improved the acquisition system and the elaboration of the seismic data, detected by the Telemetered National Seismic Network, providing a more homogeneous instrumental covering of territory. This fact has allowed a more precise spatial definition of the Italian seismic activity, including the one occurred in the last years within the Adriatic basin. In such a way a major hypocentral concentration has been possible to point out below 43' N latitude, on east of the Gargano headland. This area has a considerable geodynamical meaning due to a wide kinematic deformation belt, in coincidence of the two Tremiti and Mattinata regional strike-slip fault systems. The occurrence of strong earthquakes defines an important seismogenic structure in the Southern Adriatic, which would modify the geodynamical framework of the Central-Eastem Mediterranean.

Description: Published

Description: 233-241

Description: open

Description: During the ML = 5.6 and 5.8 earthquakes occurred in central Italy on 26 September 1997 the historic centre of Nocera Umbra, lying on top of a 120 m high hill, was diffusely damaged (VII-VIII degrees of MCS intensity). Some recently built houses in the modern part of the town suffered an even higher level of damage. A temporary seismic array was deployed to investigate a possible correlation between local amplifications of ground motion in this area and the observed pattern of damage. After a geologic and macroseismic survey, eight sites were selected as representative of different local conditions, such as topographic irregularities, sharp hard-to-soft lithology transitions, alluvium-filled valleys, and both undisturbed and deformed rocks. Horizontal-to-vertical spectral ratios for both microtremor and earthquake recordings, as well as spectral ratios referred to undisturbed rock sites, were used to quantify local variations of ground motion. In spite of the diffuse damage in the historic centre of Nocera Umbra, a small amplification is observed at the stations on the hill’s top. This suggests that the higher vulnerability of the ancient buildings mainly accounts for the diffuse damage in that part of the town. In the frequency band of engineering interest (1 to 10 Hz) the largest amplifications of ground motion are found at soft sites: in the Topino river valley, where many episodes of severe structural damage occurred, spectral amplification is significant over a broad frequency band ranging from 2 Hz to more than 20 Hz. In particular, in the central part of the valley high amplification (〉 4) is found from 3 to 10 Hz, reaching a maximum of 20 around 4 Hz. At the edge of the valley, close to the soil-to-rock transition, amplification is as large as 10 in a frequency band ranging from 4 to more than 20 Hz. A significant amplification (by a factor of 10 around 10 Hz) is observed also at one of the rock sites, possibly due to the presence of a cataclastic zone related to the activity of a regional fault that altered the mechanical properties of the rock.

Description: Published

Description: 555-565

Description: JCR Journal

Description: reserved

Description: Moderate-magnitude shallow earthquakes in the Atlantic Ocean, hundreds of kilometres southwest of Lisbon, can generate efficient suboceanic Rayleigh waves (SRW) that are well recorded in Portugal. Here we compare moderate-size earthquakes recorded by seismic stations in Portugal with the Tyrrhenian Sea earthquakes recorded in peninsular Italy where SRW were recently observed. In spite of a different behaviour of high frequencies due to the different tectonic setting of the two areas, similar results are found in the intermediate-period range, suggesting that this effect, if extrapolated to a magnitude larger than 8, could be devastating at regional distance in terms of ground motion amplitude and duration. Through 1D models, we explore the hypothesis that the high level of destruction and the long duration of shaking felt during the Great 1755 Lisbon earthquake were caused by SRW. In this preliminary study, we check the role of critical model parameters. We find that duration and amplitude are largest when the average thickness of the water layer is 2 km and shear-wave velocity of the ocean floor is close to the speed of sound in the water. Both conditions are realistic for a source in the Atlantic Ocean, few hundreds of kilometres southwest of Lisbon. Moreover, the propagation of SRW at regional distances accounts for durations of more than ten minutes as the effect of a single large earthquake.

Description: Published

Description: 283-295

Description: 4.1. Metodologie sismologiche per l'ingegneria sismica

Description: open