Can seismic ambient noise serve as a proxy to ground motion spatial variability ?

Titre : Can seismic ambient noise serve as a proxy to ground motion spatial variability ?

Laboratoire de rattachement : ISTerre

Encadrant : Cécile Cornou
Emmanuel Chaljub
Eleni Koufoudi

Téléphone : 0476635251

Mots clés : spatial variability, coherency, seismic noise

Contexte et objectifs de la mission de stage :
Many past earthquakes have outlined spatially heterogeneous damage distribution over short distances (tens to hundreds of meters ; e.g. Loma Prieta earthquake in 1989, Northridge in 1994) even for similar engineering structures (e.g. Boumerdes, 2003 ; Christchurh, 2010). Beyond peculiar structural failures, such short distance damages might be caused by variable ground motion in both amplitude and phase, leading to significant differential displacements, especially important for long-bearing structures (bridges, dams, pipelines, …). The origin of such spatial variation of ground motion is ranging from near-fault rupturing effects, regional heterogeneity and local site effects such as the presence of topography, sedimentary basin, spatial variability of soil properties.
In earthquake engineering, spatial variability of ground motion is usually described through the spatial coherency of ground motion, which is derived from ground motion data measured at dense arrays scale (Argostoli in Greece, Pinyon Flat & Parkfield in the US, Chi-Chi & Hualien in Taiwan, St Guérin in France…). However, the actual lack of dense array ground motion recordings prevents existing empirical coherency models to be representative of spatial variability properties at other sites.
Within this context, the objective of this research topic is to explore how the seismic noise can help to overcome the lack of instrumental data to provide robust and quantitative indicators on both the amplitude and the phase spatial variability of seismic motion and, hereafter, the coherency functions. This master thesis will use seismic ambient noise acquired continuously for six months by a dense array of 62 broadband stations in Argostoli Valley (Greece) within the European project NERA and 20 broadband stations in Saint Guerin dam (French Alps). Coherency functions from earthquake recordings have been already derived at those sites. In this work, time variation of noise properties and composition (Rayleigh/Love waves) will first be investigated by means of array or polarization analysis. Next, coherency functions will be derived from the noise recordings (including use of specific seismic phases or not, study of the time stability of coherency functions) and compared to the actual coherency.

Prérequis : engineering seismology, signal processing