Strong motion assessment in the near-field

Sujet de thèse : Strong motion assessment in the near-field

Doctorante : Rose Marie FAYJALOUN

Encadrants : Cécile CORNOU, Christophe VOISIN, Mathieu CAUSSE

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**Résumé :

Assessing ground motion due to a potential large earthquake is a key issue to reduce seismic hazard. This is especially true in moderate seismicity areas, but characterized by the presence of large faults (e.g. the 2010 Haiti earthquake) due to : (1) large uncertainties on seismic hazard ; (2) ruptures on large faults associated with potential devastating effects (directivity, supershear rupture, site effects) ; (3) poor a priori knowledge on the likely rupture mechanisms ; (4) partial knowledge on the geological structures.
One of the actual biggest challenge in seismology is then to develop reliable methods to assess ground motion in the near field that account for the lack of a priori knowledge on the rupture process and for the lack of instrumental data (earthquake recordings). This PhD topic thus proposes to couple the complexity of the rupture process with an innovative approach based on seismic noise to extract information about the wave propagation between the fault and the site (Denolle et al., 2013, 2014). The first stage pertains to the rupture modeling on the fault plane. We propose to account for the natural variability of the rupture process (Causse et al., 2010, 2014) by defining joint probability distributions of source parameters like rupture velocity, stress drop, slip heterogeneity, etc. These distributions will be derived from the database of finite-fault rupture models SRCMOD (Mai & Thingbaijam 2014) and from the database of source functions SCARDEC (Vallée et al. 2011) They will be compared to worldwide observations of the ground motion variability.
Once the source model has been defined, the second stage is to propagate the rupture information towards the target site. This information is represented by the Green’s functions, defined for each fault point. Green’s functions are classically computed in simple structures (e.g. AXITRA, Cotton & Coutant 1994). The originally of this PhD is to use seismic noise recorded at different stations along the fault and at the site. We will first define the velocity model. Next we will account for the wave attenuation between the source and the site, and tempt to interpolate Green’s functions laterally and in depth.
Finally, we propose to apply this new methodology to the city of Beirut, where large strike slip ruptures are expected on the Yammouneh fault. We will use the continuous seismological data recorded by a dense array (3-component broadband sensors) installed in Lebanon during 18 months (LIBRIS project 2011-2014, Figure 1). Assuming various rupture scenarios, we will finally compute the probability density function (pdf) of ground motion. This pdf will be compared to an independent assessment using populations of speleothemes (Lacave et al. 2014).