Description: The Tyrrhenian basin has been formed by extension of overriding continental lithosphere driven by roll back of the Ionian slab across the mantle. The basin is not actively extending but the tectonic structure provides information of the processes that controlled rifting and formation of conjugate margins. The basin opened from west to east, with rifting stopping after progressively larger stretching factors from north to south. The northern region stopped opening at extension factors about 1.8. Towards the south extension continued until full crustal separation that produced first intense magmatism that subsequently was followed by mantle exhumation. The final structure displays two conjugate margins with structures that evolved from symmetric to asymmetric as extension rates increase and a complex tectonic structure in between. The basin provides a natural laboratory to investigate a full rift system with variable amounts of extension. We present observations from a two-ship wide-angle (WAS) and multichannel reflection seismic (MCS) experiment that took place in spring 2010. The experiment took place on two legs: The first leg with Spanish R/V Sarmiento de Gamboa and Italian R/V Urania collected five WAS profiles striking E-W across the entire basin recorded on ocean bottom seismic stations and land stations with a 4800 c.i. G-II gun array as source. The second leg with R/V Sarmiento de Gamboa collected 16 MCS profiles (about 1500 km) using a 3.75 km-long streamer and a 3100 c.i. G-II gun array as source. MCS profiles were shot coincident with WAS profiles. WAS – MCS transects were located in regions with different amount of extension the study the full structure including the two conjugate margins. Additional MCS lines were shot concentrated in the region where mantle exhumation has taken place. The seismic information is placed in a 3D context with the integration of the multibeam bathymetry that covers the entire basin. We present the interpretation of the tectonic structure from MCS images and bathymetry and the calibrated stratigraphy of the basin that gives information of timing, duration, and amount of the tectonic extension for the different transects. We compare those results with the final P-wave velocity models from the five WAS profiles that supply information on the nature of the crust. Each transect provides information of the relationships among extension rates, crustal thickness, nature of the crust, and style of deformation. This information allows to interpret mechanisms of deformation, to infer the importance of magmatism in the rifting process, and to interpret the changes leading of mantle exhumation. Furthermore, the data provide insight in the process of formation of the structure conjugated margins.

Description: We use coincident wide-angle (WAS), multichannel seismic reflection (MCS) images and gravity data acquired with the MEDOC cruise in 2010 to characterize the crustal domains and tectonic structure across the Tyrrhenian basin. We present a ~450 km-long, E-W-trending transect, which crosses the entire basin, from Sardinia (40N), across Sardina basin, the Cornaglia Terrace and the deep Magnaghi and Vavilov basins, to the Campanian margin (Italy). The joint interpretation of the WAS model and time-migrated MCS profile give information to understand the rifting phases leading to continental break-up and mantle exhumation. The WAS data , recorded on 26 OBH/S (Ocean bottom hydrophones/ seismometers) and 5 land stations, were modelled to obtain a P-wave velocity model of the basin and the geometry of the crust-mantle boundary by joint refraction and reflection travel-time tomography. The statistical uncertainty of the model parameters has been estimated following a Monte Carlo-like approach. Subsequently, a velocity-derived density model using existing relationships for different rocks was used to infer the composition of domains that fit gravity data being consistent with the velocity model. The model display abrupt lateral heterogeneity, showing four crustal domains based on velocity gradients. From West to East, the first domain consist of a ~23 ±2 km-thick continental crust beneath Sardinia and its shelf, with a mean velocity of 6.5 ±0.3 km/s. Eastwards, the crust thins from 22 ±2 km to 12 ±1 km in ~140 km below the Sardinia basin. This second domain is interpreted as a highly extended continental crust, containing numerous faults imaged in the coincident MCS profile. The third domain, in the central part of the profile, includes basins under the deepest water depth, and is interpreted as floored by exhumed mantle. In this domain, no crust-mantle reflections are identified, neither in the WAS data nor in the MCS images. Here, the velocity increases rapidly from 2.6 ±0,1 km/s at the sea-floor to ~7.8 ±0,15 km/s at ~5 km below. The vertical velocity gradient is twice larger than typical for oceanic Layer 2, and consistent with that observed in regions of mantle exhumation like the West Iberian Margin. In this third domain, we find three conspicuous velocity anomalies located under large volcanic seamount, formed by Upper Pliocene and Middle Pleistocene extension-related magmatism of the Magnaghi seamount, D’Ancona Ridge and Vavilov seamount, respectively. In the Eastern segment of the profile, beneath the Campanian margin, there are well-defined crust-mantle reflections in both WAS data and MCS profile, displaying a progressive thickening of continental crust towards mainland. The velocity gradient in this fourth domain is similar to that of the highly extended continental crust of the second domain, which approximately corresponds to its conjugate margin. Based on these seismic observations we conclude that, in this part of the Tyrrhenian basin, extension occurred slowly enough to exhume mantle rocks without producing significant synchronous magmatism that generated well-defined oceanic crust.

Description: Overview on the ongoing works in the LOBSTER project and first results of travel time modelling in the Ligurian Sea.

Description: This paper presents an overview of the geophysical activities for the seismic microzonation of 138 municipalities belonging to four Italian regions (Abruzzo, Lazio, Marche and Umbria) that were severely damaged by the seismic sequence of Central Italy (August 2016–January 2017). This study is the result of a collaborative effort between research Institutions and professional geologists with the support of local Administrations and the Italian Civil Protection Department and sets an unprecedented large-scale example of geophysical investigations supporting detailed seismic microzonation studies. This manuscript presents the methodological approach adopted for the geophysical activities, including the technical protocols and procedures, the best practices, the final products and the results supporting a detailed microzonation study of III level. The first step of the study was the collection and critical review of all available geophysical and geological information for planning the new geophysical surveys (specifically their type and location), in order to assess the subsoil geometry and the seismic characterization of the areas under investigation. Integration with the newly acquired geophysical data allowed the identification of zones with homogeneous local seismic hazard as well as the reference seismo-stratigraphy for each area, defining for each geological unit the ranges of the relevant properties in seismic amplification studies: layering and thicknesses, density, P-wave and S-wave seismic velocity. We also present a few representative case studies illustrating the geophysical investigation for different geomorphological situations. These examples, together with the findings of the entire project, are discussed to point out the strength points and the criticalities, as well as the necessary requirements in the application of geophysical methods to detailed microzonation studies.

Description: Published

Description: 5475–5502

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: In August 2016, a magnitude 6.0 earthquake struck Central Italy, starting a devastating seismic sequence, aggravated by other two events of magnitude 5.9 and 6.5, respectively. After the first mainshock, four Italian institutions installed a dense temporary network of 50 seismic stations in an area of 260 km2. The network was registered in the International Federation of Digital Seismograph Networks with the code 3A and quoted with a Digital Object Identifier ( https://doi.org/10.13127/SD/ku7Xm12Yy9 ). Raw data were converted into the standard binary miniSEED format, and organized in a structured archive. Then, data quality and completeness were checked, and all the relevant information was used for creating the metadata volumes. Finally, the 99 Gb of continuous seismic data and metadata were uploaded into the INGV node of the European Integrated Data Archive repository. Their use was regulated by a Memorandum of Understanding between the institutions. After an embargo period, the data are now available for many different seismological studies.

Description: Published

Description: id 182

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal