The 2016 Mw 6 Amatrice earthquake : role of source and topography in the observed ground motion

Titre : The 2016 Mw 6 Amatrice earthquake : role of source and topography in the observed ground motion
Laboratoire de rattachement : ISTerre

Encadrants : Mathieu Causse, EMeline Maufroy, Emmanuel Chaljub

Téléphone : +33 (0)4 76 63 51 79

Mots clés : seismic hazard, rupture, site effects, source inversion, numerical modeling

Contexte et objectifs de la mission de stage :

In 2016, Italy (central Apenins moutain range) was struck by one of the most important seismic sequence ever recorded in this country. The sequence started with a Mw 6 event next to the village of Amatrice (on August 24th), causing about 300 fatalities due to the collapse of several buildings close to the epicenter. Like for many other earthquakes, the spatial distribution of the observed ground motion is highly variable. At the fault vicinity, where damage is the strongest, this variability reflects the heterogeneity of the geological structures and/or the complexity of the fault rupture.
Preliminary analyses of strong motion data have revealed some gross features of the fault rupture process, like directivity effect (that is, focusing of the seismic waves in the direction of the rupture propagation), which may explain the large ground motion (Peak Ground Acceleration of about 0.8 g) observed at the Amatrice strong motion station (Tinti et al. 2017). Furthermore, high resolution satellite images have shown that damage seems to be mainly located on topographic ridges. The amplification of seismic waves due to geometrical focusing on topographies, known as “topographic site effect”, may then be an additional effect that contributed to the observed large ground motion.
This internship proposes two different research approaches to better understand the origin of the near-fault ground motion and damage observed during the Amatrice earthquake. The candidate will have the opportunity to deepen one of the two approaches :
• Rupture process : we propose to build an image of the rupture by determining the space-time evolution of the fault slip (velocity of the rupture propagation, complexity of the slip distribution, ...). This image will be obtained from the ground motion observed at about 20 near-fault stations by using a non-linear inversion algorithm (Causse et al. 2017).

• Topographic effects : we propose to analyze synthetic waveforms obtained by 3D numerical simulations (spectral element method) over the high resolution topographic model of the Amatrice area. The goal is to quantify the ground motion amplification induced by the topography and to investigate how the curvature of the free surface impacts the ground motion in case of extended fault rupture compared to the point source case (Maufroy et al. 2012, 2015).

Skills : quantitative seismology and/or engineering seismology, signal processing, programming (Matlab, Fortran, ...)