Calcuation of the seismic movement associated with a reference earthquake for a given site, taking the site effect into account. Linear empirical method and modeling of the nonlinear site effect
Doctorate thesis of the University of Joseph Fourier - Grenoble 1 (Speciality: Geophysics - Geochemistry - Geomechanics) defended on the 29th October 2001.
Two methods are used to estimate the seismic motion for a given site, considering site effect. In the first part of the thesis, the empirical method is used. The data of the Kyoshin network are used in order to develop several attenuation laws with new classification systems for taking into account site effect. The site classification systems are based on two parameters : Vs30 and Fo Vs30 is the the shear velocity averaged over the upper 30 meters and For is the fundamental resonance frequency deduced from horizontal to vertical spectral ratio (HVSR) computed on seismic events. The new classification systems are applied to acceleration response spectra and to other ground motion parameters (duration, Arias intensity, etc). In the second part of the thesis, the seismic motion is estimated with the « site specific » approach. In this case, the considered site is described as a layered media with homogeneous laterally infinite layers. Three codes are used to model the non-linear behaviour of soils : SHAKE (equivalent linear), NOAH and CYBERQUAKE (non-linear codes). Several tests are performed to compare the three codes, the third one uses in situ recorded data at Port Island and Kushiro Port (Japan). This tests show the importance of time history modelling especially for frequencies higher than 5 Hz. Finally the site of Synchrotron (Grenoble, France) is studied in order to evaluate the infuence of shallow layers on seismic motion. This study show that, in that particular case, the seismic motion is due to global site effect (3D geometry of the valley) rather than local site effect (non?linear behaviour of shallow layers).