My thesis project is about investigating changes of seismic anisotropy in seismogenic zones and it includes three different aspects of research : theoretical analysis, numerical investigation and real data processing.
In fact seismic anisotropy is related to the stress field in the crust, knowing that it could be induced by the alignment of cracks subject to a stress field. Thus monitoring seismic anisotropy can be used to detect changes in the stress field in seismogenic zones specially before and after earthquakes.
The first part of the project consists of theoretical and numerical investigations in order to understand the effect of anisotropy on the propagation of surface waves, in particular on their polarization.
Numerical investigations were performed using the spectral element method (RegSEM, Cupillard et al. 2012). We computed cross-correlations (CCT) of synthetic seismograms in an anisotropic medium and investigated the effect of anisotropy on these CCTs. The aim is to have an observable that is the signature of anisotropy on the CCT. Results showed that this observable is very significant compared to travel time measurements. Hence using CCT of ambient noise which is dominated by surface waves permits the monitoring of seismic anisotropy.
The second part of the project consists of developing a method in order to monitor continuously seismic anisotropy using cross-correlation of ambient seismic noise and applying the method to different tectonic contexts starting with the 2008 Iwate-Miyagi earthquake.