Understanding Mediterranean Tectonics to Recognise Earthquake-Prone Zones

Understanding Mediterranean Tectonics to Recognise Earthquake-prone Zones Professor Enzo Mantovani UNDERSTANDING MEDITERRANEAN TECTONICS TO RECOGNISE EARTHQUAKE-PRONE ZONES Precisely predicting when earthquakes will happen is still a distant goal. However, local authorities could reduce the damage caused by such disasters if scientists could identify zones that are most likely to be affected by earthquakes. Gaining this information requires an in-depth knowledge of the ongoing tectonic situation in a given area. In the Mediterranean region, this knowledge is surrounded by considerable uncertainty, as different researchers have different hypotheses to explain tectonic processes in this area. Professor Enzo Mantovani and his team at the University of Siena, Italy, propose a new geodynamic interpretation that offers a plausible explanation for all major tectonic features observed in this area. Using their hypothesis, along with the seismic history of the region, the team has recognised a connection between the short-term development of tectonic processes and the distribution of major earthquakes. Tectonic Evolution of the and Eurasian plates. However, Mediterranean since convergence mostly produces ‘compressional’ deformations, causing Over the last 30 million years, the the Earth’s crust to become thicker and tectonic and morphological situation mountains to form, some researchers in the Mediterranean region has propose that other driving forces undergone profound change. In this caused the formation of basins in the area, elongated regions of deformation Mediterranean. where the African and Eurasian tectonic plates converge – called ‘orogenic The most frequently cited hypothesis belts’ – migrated by distances of several assumes that basin formation is hundreds of kilometres and some also driven by the gravitational sinking of underwent strong distortions. In the subducted crust, called a ‘slab’. As the wake of the migrating belts, crustal slab sinks, the so-called ‘subduction stretching occurred, forming thinned zone’ at the surface retreats, inducing Mantovani and his colleagues at the zones called ‘Back Arc’ basins, which are crustal thinning in the overlying plate University of Siena. now part of the Mediterranean seafloor. and forming a basin. This explanation is called the ‘slab-pull’ model. However, In particular, it is very difficult to It is largely accepted that the this hypothesis cannot account for reconcile the complex evolution of the driving force of tectonic processes some of the main tectonic features Tyrrhenian basin, developed in three in the Mediterranean region is the that are observed in the region, as distinct phases in both space and time, convergence between the African extensively discussed by Professor Enzo with the main implications of the slab WWW.SCIENTIA.GLOBAL pull model. For instance, numerical as an effect of ‘extrusion’. This process In the Balearic basin, a stretching simulations of the tectonic mechanism occurs when an orogenic belt becomes of the crust began in the Oligocene predict the subsidence of the belt compressed between two converging epoch (34–23 million years ago), when instead of the observed uplift in the plates, being flanked by a region the Alpine belt lay along the eastern Apennines and Calabria. of oceanic crust, called an ‘oceanic side of the Iberian domain. Crustal domain’. The shortening of the belt, thinning developed in the wake of For over 40 years, Professor Mantovani which happens due to the compression, this belt, which, being stressed by the and his colleagues have gathered is accommodated by the lateral escape convergence of the African and Eurasian vast amounts of data from multiple of buoyant orogenic material (called an plates, was forced to migrate eastwards scientific disciplines, mainly geology, extruding wedge), which overthrusts the and undergo considerable south- geophysics, tectonics, seismology adjacent denser oceanic domain. The eastward bowing. and mathematical modelling, to load of this buoyant material perturbs reconstruct the past deformation the equilibrium in the collision zone, In another Mediterranean region, the pattern in the Mediterranean region. triggering the downward flexure of the southern Tyrrhenian basin, the crust They accurately tested the compatibility denser oceanic domain. Once initiated, began to stretch in the early Pleistocene, of this reconstruction with geodynamic slab sinking continues, driven by plate about 2.6 million years ago, in the wake models. In doing so, they developed a convergence. of the migrating Calabrian wedge. hypothesis that can provide plausible and coherent explanations for a far Experiments demonstrate that this ‘Explaining the generation of this greater number of the tectonic features extrusion process can generate system as an effect of a slab-pull driving observed in the Mediterranean, extensional deformations in the wake force is particularly difficult, since the compared to previous hypotheses. of a migrating orogenic belt, which can migrating Calabrian wedge underwent move much faster than the converging a fast uplift, south-eastward bowing and Explaining Crustal Thinning plates. ‘The available evidence indicates considerable fracturing,’ says Professor that this process has generated the Mantovani. ‘In particular, it is hard to Professor Mantovani and his Balearic basin and the Central-Southern believe that an uplift of more than 2000 team suggest that the occurrence Tyrrhenian basin, involving the metres in one to two million years was of extensional deformation in a subduction of large old and cold Tethys caused by the pull of the sinking slab.’ compressional context can be explained domains,’ explains Professor Mantovani. WWW.SCIENTIA.GLOBAL The basic role of extrusion processes in the Mediterranean evolution is clearly suggested by an important piece of evidence: the fact that subduction of oceanic domains only occurred beneath extruding wedges. When this condition did not occur, for instance during the collisional phase of the Africa- Adriatic plate with Eurasia, the Ionian oceanic domain could very efficiently transmit compressional stresses without undergoing any subduction. This behaviour, also observed in other zones of the world, is explained by the fact that during long (geological) time intervals, the horizontal strength of the oceanic domain is not lower than that of a continental plate. Finally, one can underline the fact that with the extrusion explanation, there is no need to assume the action of other driving forces in addition to the one provided by Plate Tectonic theory – that is, the convergence of the confining plates. tries to move roughly northward. team to recognise the Central-Northern Tectonics and Earthquake This motion is not continuous over Apennines as a priority zone, where Distribution time, being mainly allowed by the some years later in 2016, strong fault slidings that occur during major earthquakes took place. ‘Any attempt at predicting the earthquakes along the boundaries of next development of a physical the Adriatic plate. Informing Prevention Policies phenomenon requires the knowledge of its genetic mechanism,’ says Professor ‘Each strong shock triggers the motion ‘The most important problem Mantovani. Thus, exploiting their of the just decoupled Adriatic sector, concerning earthquake mitigation in knowledge of the real tectonic situation which then propagates through the Italy is the fact that a large proportion in the Mediterranean area, he and his plate with rates controlled by the plate’s of the buildings were built without any research group have tried to obtain properties,’ says Professor Mantovani. anti-seismic criteria,’ says Professor insights into the connection between ‘This process tends to involve more Mantovani. ‘Thus, the restoration of the short-term development of tectonic and more northern zones along the such a vast building heritage would processes and the distribution of major lateral borders of the Adriatic plate, up require a very long prevention project. earthquakes in both space and time. to reach the northern front of the plate This problem could be addressed by in the Eastern Alps. This hypothesis the recognition of the zones most prone This attempt has the highest chance of is compatible with the distribution of to next strong earthquakes, where success in the zones where the tectonic major earthquakes that occurred along prevention could be prioritised in the situation is relatively simple, such the Adriatic boundaries in the last four short-term.’ as the central Mediterranean region, centuries.’ where most seismic activity occurs in His team’s work aims to help local the orogenic belts that surround the The above interpretation may provide authorities to achieve this goal, by Adriatic plate, which forms the seafloor a tool to locate zones where ongoing recognising patterns of seismic activity of the Adriatic Sea between Italy and the stresses, induced by the driving forces driven by ongoing tectonic processes. Balkans (see the figure). and past seismicity in tectonically Such information could be essential connected zones, are closest to the in mitigating the effects of next major This activity is due to the fact that strength of the crust. This approach seismic disasters. the Adriatic plate, stressed by the has allowed the University of Siena convergence of the confining plates, WWW.SCIENTIA.GLOBAL Meet the researcher Professor Enzo Mantovani Department of Environment, Earth and Physical Sciences University of Siena Siena

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