Seismic tomography of Italy reveals new details on the deep structure and geodynamic evolution of the Alps and Apennines
Through the application of a seismological technique called Seismic Tomography, a team of researchers from the National Institute of Geophysics and Volcanology (INGV) has provided a homogeneous representation of the three-dimensional structure of the entire Italian region up to a depth of 80 km, so as to improve the understanding of the birth and evolution of the Alpine and Apennine mountain ranges. The results of the study "Lithosphere Structure, Processes, and Physical State of the Alpine-Apennine System" were recently published in the "Journal of Geophysical Research".
“The formation of mountain ranges is the result of the collision between the tectonic plates into which the lithosphere is divided, i.e. the most superficial, cold and rigid part of our planet”, explains Pasquale De Gori, a researcher at INGV. "The spatial, lateral and in-depth definition of the limits of these structures, whose reciprocal movement gave rise to the current structure of the Italian peninsula, is therefore fundamental for defining its geodynamic evolution".
The tectonic plates are in perennial and slow motion and the collision process to which they are subject determines the sinking of large portions of volumes of the lithosphere inside the earth's mantle (subduction).
“The phenomenon of subduction causes important variations in the chemical and physical state of rock volumes which respond differently to elastic stresses”, continues Pio Lucente, also a researcher at INGV. “One way to study these heterogeneities is to calculate how the speed of propagation of the seismic waves that pass through them varies following the genesis of an earthquake”.
Through the use of Seismic Tomography, a technique similar to Computerized Axial Tomography (TAC) which is used in the medical field, it was possible to determine how the seismic waves P (compressional waves) and S (transverse waves) propagate below the Italian peninsula, identifying the 'fast' zones, ie associated with dense and cold rocks, and the "slow" ones, composed of less dense and 'warmer' rocks. Through the analysis of the P and S waves generated by an earthquake, seismic tomography therefore makes it possible to obtain three-dimensional images of the interior of the Earth, just as the medical CAT scan allows the reconstruction of the inside of the human body through the use of X-rays .
“Starting from the recording of the earthquakes detected by the stations of the INGV National Seismic Network, in addition to those of a further very dense seismic network installed mainly in the Alpine arc as part of the European project AlpArray, we have developed tomographic models capable of describe the propagation of the waves inside the lithosphere and to provide a homogeneous representation of the three-dimensional structure of the entire Italian region”, adds Irene Menichelli, PhD student in Geological Sciences at the University of Roma Tre in collaboration with INGV and first author of the study, which continues "it was thus possible to highlight how the Alps derive from the subduction of the European plate below the African one, while the Adriatic plate plunges into the earth's mantle below the Apennine chain. The geometries and lateral variations of the lithospheric plates reconstructed through the tomographic images have made it possible to determine the characteristics of the subduction beneath the Alpine orogen. The detail of the three-dimensional images obtained has allowed us to distinguish the nature of the subducting lithosphere, i.e. whether it is continental or oceanic lithosphere, and to highlight the onset of a process of progressive sinking of the deepest and densest portion of the Adriatic continental lithosphere below below the Apennine chain, a geodynamic process known as 'delamination'. Furthermore, the comparative analysis of the speed models of the P and S waves makes it possible to identify the volumes, inside the lithosphere, where fluid components accumulate, which play a very important role in the evolution of seismogenic processes", concludes the researcher.
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picture 1 - On the left are the layers of the Vp and Vp/Vs velocity models at a depth of 40 km with the traces of the profiles (A-A', B-B', C-C'). The structure of the western (A-A'), central (B-B') and eastern (C-C') Alps clearly shows the subduction of the European plate without evidence of a polarity change of the slab. The black lines indicate the depth of the lithospheric mantle of Adria (AM) and Europa (EM). Dashed lines indicate the location of loose boundaries. BR= Brianza area, SL= Sesia-Lanzo, DM= Dora Maira massif.
Picture 2 - On the left, the layers of the Vp and Vp/Vs velocity models at a depth of 40 km with the traces of the profiles (D-D', E-E'). Above, the tomographic section along the Northern Apennines (D-D') and the corresponding geological interpretation. Below is the tomographic and interpretive section that crosses the Calabro arc (E-E'). The black lines indicate the position of the Ligurian lithospheric mantle, of Adria (AM) and of Europe (EM). Dashed lines indicate the location of loose boundaries.