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ABSTRACT: Nell’ambito della convenzione vigente tra l’Istituto Nazionale di Geofisica e Vulcanologia (INGV) e l’Agenzia di Protezione Civile della Regione Emilia Romagna è stata realizzata a fine settembre del 2011 una esercitazione sul rischio sismico con l’obiettivo di valutare il livello raggiunto nelle procedure che entrambi gli Enti attivano in occasione di una emergenza a seguito di un forte terremoto. La simulazione ha interessato oltre 50 unità di personale INGV, sia in sede che in area epicentrale, appartenenti a diverse Sezioni e sedi INGV (Ancona, Arezzo, Bologna, Irpinia, Milano, Pisa e Roma). La preparazione dell’evento si è fondata sulle esperienze del passato, in primis la lunga emergenza aquilana del 2009 [Margheriti et al., 2010; 2011; Moretti et al., 2011c], con uno sguardo alle nuove esigenze sia interne che esterne (ad esempio le istanze della Protezione Civile). Nonostante gli imprevisti e gli inevitabili errori commessi a cui si aggiunga lo sforzo per mettere insieme tante differenti professionalità e per rispettare sempre al meglio il programma e gli impegni presi con i responsabili dell’Agenzia di Protezione Civile della Regione Emilia Romagna, è stato ampiamente ripagato dall’aver vissuto un’esperienza positiva non solo da un punto di vista professionale ma anche umano. Questa esercitazione si è infatti rivelata un’importante occasione per relazionarsi e confrontarsi con i colleghi solitamente lontani rendendosi conto che da ciascuno di loro è sempre possibile imparare qualcosa. Questa esperienza si è mostrata di fondamentale importanza nella gestione dell’emergenza verificatasi a maggio 2012 in Pianura Padana emiliana [Moretti et al., 2012].
Quaderno di Geofisica. 02/2013; 108:25.
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Selvaggi G
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ABSTRACT: Seismic strain is analysed along the slab face of the Southern Tyrrhenian subduction zone using focal mechanisms of deep earthquakes which occurred in the period 1960-1998. Results show that the slab is mostly affected by down-dip shortening which strongly increases below 250 km depth. Extensional strain is mainly confined to the direction perpendicular to the slab face. The dominance of down-dip shortening and the minor along strike inplane extension implies thickening of the slab below 250 km depth. Assuming constant seismic efficiency along the slab, this strain pattern also implies a decrease of the down-dip velocity below 250 km depth. We also locate lower magnitude intermediate-depth and deep earthquakes using arrival times since 1985 available from the Italian seismic national network. These data show that the slab reaches the deeper part of the upper mantle, as suggested by the occurrence of a few ??600 km depth earthquakes, and that a large portion of the Tyrrhenian slab, between 100 and 250 km depth beneath the offshore of the Calabrian arc, is aseismic. Only a short part of the Tyrrhenian slab is seismically continuous from the top to the bottom. The lack of seismicity may indicate either that aseismic subduction is occurring or that the slab is detached from its upper part. Although the data are still inconclusive, they suggest that an aseismic subduction is the most appropriate interpretation, considering recent tomographic images of the slab and the results of this study, which agree well with the presence of a neutral down-dip stress zone, as also observed worldwide in deep slabs.
Annals of Geophysics. 01/2009;
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Selvaggi G
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ABSTRACT: Horizontal principal seismic strain rate axes have been calculated within a regular mesh of triangles covering the Italian peninsula in a time interval of 700 years. I have used both the method of Kostrov (1974), that requires knowledge of the seismic moment tensor of earthquakes, and the modified version provided by England and Molnar (1997) that makes use of length and kinematics of the activated faults. Seismic moment tensor of historical earthquakes can be inferred from recent literature, while length of faults has been obtained from the observation that strain drop is almost constant for large Apenninic earthquakes. Spatial strain distribution from historical earthquakes shows that the Apennines can be divided into three homogeneous structural arcs (Northern Apenninic, Southern Apenninic and Calabrian arcs) within which strain is roughly constant. Although NE-SW extension is the main deformation process along the two Apenninic arcs it involves a velocity more than five times greater in the Southern Apennines. Along the Calabrian arc, I tested the effect on the strain field of the contemporaneous ~WNW-ESE and ~NNE-SSW extension due to the longitudinal dilatation of the arc during its still ESE migration.
Annals of Geophysics. 01/1998;
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ABSTRACT: We analysed the one-year-long seismic swarm at the Alban Hills volcano which occurred during 1989-1990. We portray spatial distribution of seismic moment release, better delineating the activated volume during the swarm. The seismic structure is imaged as a 7-km long, 3-km wide, and 3-km thick volume, located between 2 and 5 km depth, and NW-SE striking. Fault plane solutions and scalar seismic moments for the largest earthquakes provide the description of the average strain rate tensor. The principal strain rate axes show a dominant extension in NE-SW direction, a SE-NW direction of compression and a negligible thickening rate. P and T axes direction of the smaller earthquakes suggests that the same mode of deformation is distributed all over the activated volume. These results are discussed in terms of seismic deforming processes active at the Alban Hills volcano, in the frame of magmatic inflation recently invoked to explain the rapid vertical uplift affecting part of the volcano. The observed average deformation is consistent with shear failures occurring on faults connecting stress-oriented dykes in response to an increasing fluid pressure.
Annals of Geophysics. 01/1998;
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ABSTRACT: In this paper we analyse the records of the two Potenza seismic sequences (Southern Apennines, Italy) which occurred in 1990 and 1991, in order to obtain information on the upper crustal structure in this area. The hypocentral depths are mainly concentrated below 10 km which is the supposed upper limit of the crystalline basement. The seismograms recorded at temporary arrays deployed during the two sequences clearly show on the vertical component, an intermediary phase between the P and S waves. For the investigated epicentral distances (less than 30 km) the delay between this secondary phase and the direct S wave arrival is almost constant at each station, suggesting that the observed intermediary phase might be an S to P conversion at a discontinuity shallower than the hypocentral depth. This interpretation has been supported by polarisation analysis and numerical modelling results. Considering the regional geological structure, these latter have also shown that the interface generating strong converted waves could be the top of the Apulian carbonate platform overlaid with recent clay deposits and flysch sediments. 1D inversion of the travel-time data was performed in order to evaluate a local vertical upper crustal profile.
Annals of Geophysics. 01/1998;
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ABSTRACT: The first modern studies of seismicity in Italy date back to the late 60's and early 70's. Although with a sparse seismic network available and only a few telemetered short-period stations, significant studies were carried out that outlined the main features of Italian seismicity (see, e.g., Boschi et al., 1969). Among these studies, one of the most important achievements was the reconnaissance of a Wadati-Benioff zone in Southern Tyrrhenian, described for the first time in detail in the papers of Caputo et al.(1970, 1973). Today, after three decades of more and more detailed seismological monitoring of the Italian region and tens of thousands earthquakes located since then, the knowledge of the earthquake generation processes in our country is much improved, although some of the conclusions reached in these early papers still hold. These improvements were made possible by the efforts of many institutions and seismologists who have been working hard to bring seismological research in Italy to standards of absolute quality, under the pivoting role of the Istituto Nazionale di Geofisica (ING). From the relocation of about 30000 crustal earthquakes and detailed studies on intermediate and deep shocks carried out in the last few years, we show that seismic release in peninsular Italy is only weakly related to the Africa-Eurasia convergence, but rather is best explained by the existence of two separate subduction/collision arcs (Northern Apennines and Southern Apennines-Calabria-Sicily). The width of the deforming belt running along peninsular Italy is 30 to 60 km, it is broader in the north than in the south, and the two arcs are separated by a region of more distributed deformation and stress rotations in the Central Apennines. Along the belt, the reconnaissance of regions of continuous and weak release of seismic energy, adjacent to fault areas which are currently «locked» (and therefore are the best candidates for future earthquakes) is another recent important achievement of the prolonged detailed seismic monitoring of our territory, which will provide in the future more and more precise indications of where earthquakes will strike. In addition, the accurate location of hundreds of intermediate and deep earthquakes beneath the two arcs has recently provided (together with seismic tomography results) new hints on the tectonic setting of Italy and its evolution over time, on the relations between deep processes and crustal stress, and ultimately on the mechanisms of earthquake generation in our country.
Annals of Geophysics. 01/1997;
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ABSTRACT: We analyzed the distribution of seismic moment in Italy, computed from instrumental seismicity recorded by the Italian National Seismic Network in the past 14 years, to map the areas where seismic deformation processes have been active in this time interval. Seismic moment is the most suitable parameter to quantify earthquake size. It is related to the geometric characteristics of faults, to seismic energy and it is a quantity that can be summed and represented in its cumulative value. The maps of seismic moment distribution display more information than epicentral maps, better showing actively deforming regions. They provide further and original evidence for the existence, within the Apenninic belt, of two regions (north and south) characterized by different seismic energy release. The seismic moment is almost continuously and homogeneously distributed all along the belt in the Northern Apennines, whereas in the Southern Apennines it is concentrated in the zones recently activated by mainshock-aftershock seismic sequences. Seismic deformation takes place in a 30 km narrow belt along the Apennines, while in the transition zone between the Northern and Southern Apennines this belt is about 100 km wide. Comparing instrumental seismic moment release with the areas struck by the largest historical earthquakes which have occurred in the past six centuries, we qualitatively extended back to the past the information on where the seismic deformation occurs. In the Southern Apennines background energy release from instrumental seismicity is very low in the several areas hit by large historical earthquakes, suggesting that these seismogenic zones are currently quiescent.
Annals of Geophysics. 01/1997;
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ABSTRACT: The earthquake of June 12, 1995 has been located using local and regional data (41°48.8'N, 12°30.8°6E at a depth of about 11.5 km) a few kilometers inside the city limit of Rome, in its southernmost part. This is the first event that occurred in Rome for which instrumental data are available. The local magnitude estimated from digital recordings is ML 3.6 and it was largely felt reaching intensity VI MCS. We constrained the focal mechanism by analyzing the S-wave polarization and it agrees well with the distribution of P-wave polarities. The fault plane solution shows a dominant strike slip mechanism (strike 275°, dip 70°, rake - 140°). Seismic moment, M0 = 2.3 ± 0.6 1021dyne × , was computed from S-wave displacement spectra of horizontal components of ground motion digital waveforms. The corresponding source radius ranges between 200 and 500 m, depending on the assumed stress drop (100 bars or 10 bars, respectively). The earthquake was preceded by a ML 2.6 foreshock. The seismic sequence lasted a few days during which 38 aftershocks were recorded. The seismicity pattern shows the characteristics of a mainshock-aftershock sequence, rather than swarm behavior which seems to characterize the activity of the neighboring seismogenic areas of the Alban Hills. We used a master event algorithm to locate some of the aftershocks. Results show that the relocated aftershocks are clustered in a small volume in proximity of the mainshock hypocenter.
Annals of Geophysics. 01/1996;
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ABSTRACT: n this paper we describe the location and the fault plane solution of the December 13, 1990, Eastern Sicily earthquake (ML = 5.4), and of its aftershock sequence. Because the main shock location is not well constrained due to the geometry of the permanent National Seismic Network in this area, we used a "master event" algorithm to locate it in relation to a well located aftershock. The revised location is slightly offshore Eastern Sicily, 4.8 km north of the largest aftershock (ML = 4.6) that occurred on December 16, 1990. The main shock has a strike-slip mechanism, indicating SE-NW compression with either left lateral motion on a NS plane, or right lateral on an EW plane. Two days after the main event we deployed a local network of eight digital stations, that provided accurate locations of the aftershocks, and the estimate of source parameters for the strongest earthquake. We observed an unusual quiescence after the ML = 5.4 event, that lasted until December 16, when a ML = 4.6 earthquake occurred. The fault plane solution of this aftershock shows normal faulting on E-W trending planes. Between December 16 and January 6, 1991, a sequence of at least 300 aftershock" was recorded by the local network. The well located earthquakes define a small source region of approximately 5 x 2 x 5 km3, with hypocentral depths ranging between 15 and 20 km. The paucity of large aftershocks, the time gap between the main shock occurrence and the beginning of the aftershock sequence (3.5 days), their different focal mechanisms (strike-slip vs. normal), and the different stress drop between main shock and after- shock suggest that the ML = 5.4 earthquake is an isolated event. The sequence of aftershocks began with the ML = 4.6 event, which is probably linked to the main shock with a complex mechanism of stress redistribution after the main faulting episode.
Annals of Geophysics. 01/1995;
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Annals of Geophysics. 01/1993;
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Annals of Geophysics. 01/1993;
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Annals of Geophysics. 01/1993;
