INGV has made the video of the propagation animation on the earth's surface of the seismic waves generated by the Mw 6.0 earthquake of 03.36 on 24 August 2016 which involved the provinces of Rieti, L'Aquila, Perugia, Ascoli Piceno, Teramo.
Waves in blue indicate that the ground is moving rapidly downwards, those in red indicate that the ground is moving upwards. The color intensity is higher for faster vertical shifts.
Each second of the animation represents one second in real time.
The speed and amplitude of seismic waves depend on the characteristics of the seismic source, the type of soil they pass through and also the topography. Therefore, they do not propagate uniformly in space and places located at the same distance from the epicenter are affected by the earthquake in a completely different way.
The animation is generated through this procedure:
1) the seismic waves are recorded by the seismometers of the INGV National Seismic Network and are analyzed to determine the fundamental parameters of the earthquake such as epicenter, origin time, magnitude. For earthquakes with a magnitude greater than 3.5, the "seismic moment tensor" is also calculated, which is a mathematical description of the forces at play on the fault that generated the event.
2) a three-dimensional model of the interested region is built which includes geological complexities such as the Moho and the presence of "soft" soils (such as the alluvial sediments of the Po Valley and some Apennine basins).
Seismic tomography is a method used to study the interior of the earth in order to determine the speed of seismic waves through the analysis of seismograms. In the figure some sections of the 3D tomographic model used in this simulation, the colors represent the different speeds of the seismic waves P, from red (2000 m/s) in areas with lower speeds (alluvial basins) up to dark blue (8000 m/s ) in the fastest areas. (Di Stefano & Ciaccio 2014)
3) using the 3D model and the seismic moment tensor, the propagation of the seismic waves is simulated taking into account the local seismic response, such as the amplification of the waves in alluvial basins (soft soils) and the increase of wave velocity in rocky soils .
The equations are solved using the SPECFEM3D software (Peter et al. 2012, https://github.com/geodynamics/specfem3d), whose development collaborates with INGV researchers.
4) seismograms and the evolution of ground velocity values on the earth's surface are saved and displayed through Paraview (http://www.paraview.org)
This type of simulations has only recently been possible, since supercomputers have become available that allow for parallel calculations. For this (relatively small) simulation, 512 processors were used, for a total of 5000 minutes of calculation time and 256 GB of memory.
The analysis of the differences between the seismograms produced by this type of simulation and those actually measured offer crucial information not only for determining the seismic source and the characteristics of the subsoil but also for predicting the ground shaking produced by hypothetical seismic events.
Technical detail:
to reduce calculation times and given the limited current knowledge of subsoil details, the simulation in this animation is relatively "low-frequency", i.e. it displays wave frequencies down to 0.5 Hz. This means that the wave front " interacts” with objects the size of 1.5-2 km. The local seismic response is therefore limited to the effects of geological structures of this size. By increasing the frequency content, smaller details and, for example, amplification due to sediments would be highlighted.
edited by Emanuele Casarotti and Federica Magnoni (INGV).
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Bibliography:
Peter Daniel, Dimitri Komatitsch, Yang Luo, Roland Martin, Nicolas Le Goff, Emanuele Casarotti, Pieyre Le Loher, et al. 2011. “Forward and Adjoint Simulations of Seismic Wave Propagation on Fully Unstructured Hexahedral Meshes.” Geophys. J Int 186(2):721–39. doi:10.1111/j.1365-246X.2011.05044.x.
Global Shakemovie: http://global.shakemovie.princeton.edu
R. Di Stefano, MG Ciaccio, The lithosphere and asthenosphere system in Italy as inferred from the Vp and Vs 3D velocity model and Moho map, Journal of Geodynamics, Volume 82, December 2014, Pages 16-25, ISSN 0264-3707, http://dx.doi.org/10.1016/j.jog.2014.09.006.