IN SITU SHEAR MODULUS REDUCTION COMPUTATION USING SEISMIC INTERFEROMETRY BY DECONVOLUTION FROM BOREHOLE AND SURFACE DATA: THEORY AND EXAMPLES
We use seismic interferometry by deconvolution to estimate the travel time between two receivers located at the surface and at the bottom of a soil column having strong motion recordings. Since the distance between sensors is known, the computed travel time can be used to estimate the velocity of propagating waves in the medium. In previous studies, several records having different PGA's have been used to estimate the material degradation by computing the lag time either by cross correlation or deconvolution techniques. In this study we show that it is possible to obtain reliable results using a single couple of records if nonlinear effects are triggered. First, some numerical tests are conducted to validate the method with a monolayer soil column, with well known mechanical properties. As a borehole input, we use the record of the Northridge earthquake of 1994 (Sylmar station rock site) while the ground motion at the surface is computed using a 1D SH nonlinear wave propagation code. Furthermore, the code allows to set constant/variable shear velocity profile for each layer and controlling the hysteretic damping following a prescribed damping ratio curve. The results show the capability of the method to compute the G/G0 curve that is compatible with the theoretical one. In addition, we assess the effect of hysteretic damping control during wave propagation. We found that this introduces artificial degradation of the backbone curve, which in turn produces permanent fictitious material degradation. Finally, we use borehole and surface accelerograms from the Mw9 Tohoku earthquake in 2011 recorded at the KiK net IBRH16 station. We estimate the plastic degradation of the first 5 m of loosely packed soil layers and monitor the temporal changes of shear modulus at this site.