The National Institute of Geophysics and Volcanology (INGV) has made the video of the propagation animation on the earth's surface of the seismic waves generated by the Mw 5.9 earthquake of 21.18 on 26 October 2016 which involved the provinces of Macerata, Rieti, L Aquila, Perugia and Ascoli Piceno.
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 first 85 seconds from the origin of the seismic event are represented.
This is not an artistic animation but the solution of the equations describing the propagation process.
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. These, therefore, do not propagate uniformly in space and, consequently, places located at the same distance from the epicenter are affected by the earthquake in a different way. In this case it can be observed that the waves propagated with more intensity and for longer towards the North-East, along the coasts of the Adriatic regions, towards Lazio and Southern Tuscany.
The animation is generated through the following 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: epicenter, time of origin and magnitude. For earthquakes with magnitude
higher 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 velocities of the seismic waves P, from red (2000 m/s) in areas with lower velocities (flood basins) to dark blue (8000 m/s) in faster areas. (Di Stefano & Ciaccio 2014)
3) using the 3D model and the seismic moment tensor, the propagation of seismic waves is simulated taking into account the local seismic response, such as wave amplification in alluvial basins (soft soils) and wave velocity increase 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 the details of the subsoil, the Italian-scale simulation of this animation is relatively "low-frequency", that is, it displays the frequencies of the waves down to 0.5 Hz. This means that the front of The wave "interacts" with objects of 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. To improve the resolution in the region near the epicenter, the first 30 seconds of animation were obtained through a higher resolution higher frequency (up to 2 Hz) local simulation. For visualization reasons the topography has been increased by 3 times.
(edited by Emanuele Casarotti and Federica Magnoni INGV).
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