ALBERT

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: 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