Giovanni Bortoluzzi

Initial burst of oceanic crust accretion in the Red Sea due to edge-driven mantle convection

The 500 m.y. cycle whereby continents assemble in a single supercontinent and then fragment and disperse again involves the rupturing of a continent and the birth of a new ocean, with the formation of passive plate margins. This process is well displayed today in the Red Sea, where Arabia is separating from Africa. We carried out geophysical surveys and bottom rock sampling in the two Red Sea northernmost axial segments of initial oceanic crust accretion, Thetis and Nereus. Areal variations of crustal thickness, magnetic intensity, and degree of melting of the subaxial upwelling mantle reveal an initial burst of active oceanic crust generation and rapid seafloor spreading below each cell, occurring as soon as the lid of continental lithosphere breaks. This initial pulse may be caused by edge-driven subrift mantle convection, triggered by a strong horizontal thermal gradient between the cold continental lithosphere and the hot ascending asthenosphere. The thermal gradient weakens as the oceanic rift widens; therefore the initial active pulse fades into steady, more passive crustal accretion, with slower spreading and along axis rift propagation.

Exploring submarine earthquake geology in the Marmara Sea

The disastrous 1999 earthquakes in Turkey have spurred the international community to study the geometry and behavior of the North Anatolian Fault (NAF) beneath the Marmara Sea. While the area is considered mature for a large earthquake, the detailed fault geometry below the Marmara Sea is uncertain, and this prevents a realistic assessment of seismic hazards in the highly-populated region close to Istanbul. Two geological/geophysical surveys were recently conducted in the Marmara Sea: the first in November 2000 with the R/V Odin Finder, and the second in June 2001 with the R/V CNR-Urania. Both were sponsored and organized by the Institute of Marine Geology of the Italian National Research Council (CNR), in cooperation with the Turkish Council for Scientific and Technical Research (TUBITAK) and the Lamont-Doherty Earth Observatory of Columbia University Multi-beam bathymetry, multi-channel seismic reflection profiling, magnetometry high-resolution CHIRP sub-bottom profiling, and bottom imaging were carried out with a remotely operated vehicle (ROV). Over 60 gravity and piston cores were collected.

The tectonic puzzle of the Messina area (Southern Italy): Insights from new seismic reflection data

The Messina Strait, that separates peninsular Italy from Sicily, is one of the most seismically active areas of the Mediterranean. The structure and seismotectonic setting of the region are poorly understood, although the area is highly populated and important infrastructures are planned there. New seismic reflection data have identified a number of faults, as well as a crustal scale NE-trending anticline few km north of the strait. These features are interpreted as due to active right-lateral transpression along the north-eastern Sicilian offshore, coexisting with extensional and right-lateral transtensional tectonics in the southern Messina Strait. This complex tectonic network appears to be controlled by independent and overlapping tectonic settings, due to the presence of a diffuse transfer zone between the SE-ward retreating Calabria subduction zone relative to slab advance in the western Sicilian side.

How far did the surface rupture of the 1999 İzmit earthquake reach in Sea of Marmara

An open problem concerning the Mw 7.4, 1999 Izmit earthquake along the North Anatolian Fault (NAF) system is the apparent conflict between estimates of strike-slip deformation based on field and remote sensing data. This is due to the fact that the main strand of the NAF west of the epicenter lies below the Sea of Marmara. Seismological evidence and models based on synthetic aperture radar interferometry suggest that coseismic and early postseismic displacement accumulated after the earthquake could have reached the western end of the Izmit Gulf and possibly the southern edge of the Cinarcik Basin, tapering off along the northern coast of the Armutlu Peninsula, more than 60 km from the epicenter. This scenario is not confirmed by onshore field observations that point toward a termination of the surface rupture around 30 km to the east. These discrepancies convey high uncertainties in the estimate of the tectonic load produced by the Izmit earthquake on the adjacent fault segment toward Istanbul. We analyzed data from different sources, including high-resolution marine geophysical surveys and two Nautile dives along the fault-controlled canyon that connects Izmit Cinarcik basins. Our observations suggest that the surface rupture of the 1999 Izmit earthquake propagated through the shallow Gulf but did not reach the deep Marmara basins. In fact, along the slope between Cinarcik and the western end of the Izmit Gulf, we do not observe fault-related ruptures affecting the sea-floor but rather a series of active gas seeps and black patches that mark the presence of known active faults. Our findings have implications for seismic risk assessment in the highly populated region of Istanbul, both for the estimate of tectonic load transferred to the next fault segments and the location of the next earthquake. Citation: Gasperini, L., A. Polonia, G. Bortoluzzi, P. Henry, X. Le Pichon, M. Tryon, N. Cagatay, and L...

Seismic Tomography Experiment at Italy's Stromboli Volcano

Stromboli Island, located in the southern Tyrrhenian Sea, is the emerged part (about 900 meters above sea level) of an approximately 3-kilometer-high stratovolcano. Its persistent Strombolian activity, documented for more than 2000 years, is sometimes interrupted by lava effusions or major explosions. Despite the number of recently published geophysical studies aimed at clarifying the volcanos eruption dynamics, the spatial extent and geometrical characteristics of its plumbing system remain poorly understood. In fact, knowledge of the inner structure and the zones of magma storage is limited to the upper few hundred meters of the volcanic edifice [Chouet et al., 2003; Mattia et al., 2004], and P and S wave velocity models are available only in restricted areas [Petrosino et al., 2002].

DIGMAP: a computer program for accurate acquisition by digitizer of geographical coordinates from conformal projections

Abstract A computer program is described for the acquisition of geographical coordinates from maps in widely used cylindric and conic conformal projections: Direct Mercator, Lambert conformal, UTM, and Polar Stereographic. It uses the geometrical affine transformation of homogeneous coordinates and the direct and inverse analytical mapping functions of conformal projections. Selection between different reference ellipsoids and datum shift procedures is provided. Tests of accuracy were performed for direct and inverse mapping algorithms and for conversion of tablet coordinates to geographical coordinates. The results indicate that the uncertainty of capturing geographical coordinates is, in general, twice the graphic precision of 0.2 mm at any map scale and projection. Accuracy is increased by using good quality maps. The acquisition of calibration points is the most critical step. Mathematical expressions and source listing of the FORTRAN program are provided.

DAATUM: a FORTRAN 77 computer program for datum shift and conversion of geographical coordinates between different cartographics systems

Abstract A computer program is described for the datum change of geographical coordinates over large areas. The transformation to geocentric coordinates and ordinary least-squares techniques are applied to a set of coordinates of points (minimum three) in the two reference systems. The parameters of rotation, translation, and scaling between the two ellipsoids are output, and are suitable to be tabulated to provide for further shifting of data in the same area. The results indicate that accuracies of a few meters can be achieved over an entire country by using a single set of parameters. Higher accuracies may be obtained by local conversion procedures. The program also is used for accurate direct and inverse geographical-to-metric coordinate conversion for nine most used mapping projections. Mathematical expressions and the FORTRAN 77 source listing are provided.

Mediterranean seabed in digital shaded relief

Relief-shaded images made from a large digital depth model (DDM) provide a fresh view of regional tectonism and submarine geomorphology in the central Mediterranean. The 1-km spatial resolution is better than that afforded by the 5-arc-minute worldwide digital bathymetry (DBDB5). These computer pictures enable information on seafloor morphology to be visualized and communicated in much the same way that relief maps made from digital elevation models are used to interpret subaerial topography. The images shown here (partially joined along 19°E) were taken from a new shaded-relief map of the entire Mediterranean seafloor and parts of the Bay of Biscay and Black Sea between latitude 307deg;–46°N and longitude 6°W–37°E [Mark et al., 1990].

Transfer zones in an oblique back‐arc basin setting: Insights from the Latium‐Campania segmented margin (Tyrrhenian Sea)

New multichannel seismic reflection profiles were acquired to unravel the structure of a portion of the eastern margin of the Tyrrhenian basin. This extensional feature is part of an Oligocene to Present back-arc basin in the hangingwall of the west directed Apennines subduction system. The basin provides excellent conditions to investigate the early stage processes leading to the development of rifted passive margins and to the emplacement of oceanic crust in an oblique setting. The interpreted post-stack-migrated seismic profiles highlight the geometry and kinematics of the Pontine escarpment that connects the Latium-Campania continental margin to the Vavilov basin. The latter is the main feature of the area, related to the early Pliocene extension of the Tyrrhenian Sea. Several morphological variations are pointed out along strike, mirroring different structural settings of the margin itself: a steeper margin to the north corresponds to high-angle possibly transtensional faults, whereas a smooth slope in the southern portion corresponds to several more distributed listric faults. This work contributes to the understanding of the interplay between extensional and transtensional tectonics along the margins of an oblique back-arc basin.

The observation system of the Italian coastal zones

The study of the marine coastal areas is of great importance, for both the protection and management of the environment. Due to the always increasing anthropic pressure, the availability of real time and model data can be critical for decisions, expecially during emergencies In addition, the availability of time-series and of observation points is necessary for proper problem setting and tuning of models and knowledge-based systems. We present some experience of Real Time data transmission and handling from stations and platforms, aiming toward a better integration into an Observation System.