Nonlinear seismic response for the 2011 Tohoku earthquake: borehole records versus one-directional three-component propagation models

The seismic response of surficial multilayered soils to strong earthquakes is analysed through a non-linear one-directional three-component (1D-3C) wave propagation model. The three components (3C-polarization) of the incident wave are simultaneously propagated into a horizontal multilayered soil. A 3-D non-linear constitutive relation for dry soils under cyclic loading is implemented in a quadratic line finite element model. The soil rheology is modelled by mean of a multisurface cyclic plasticity model of the Masing-Prandtl-Ishlinskii-Iwan type. Its major advantage is that the rheology is characterized by few non-linear parameters commonly available. Previous studies showed that, when comparing one to 3C unidirectional wave propagation simulations, the soil shear modulus decreases and the dissipation increases, for a given maximum strain amplitude. The 3-D loading path due to the 3C-polarization leads to multiaxial stress interaction that reduces soil strength and increases non-linear effects. Non-linearity and coupling effects between components are more obvious with decreasing seismic velocity ratio in the soil and increasing vertical to horizontal component ratio for the incident wave. This research aims at comparing computed ground motions at the surface of soil profiles in the Tohoku area (Japan) with 3C seismic signals recorded during the 2011 Tohoku earthquake. The 3C recorded downhole motion is imposed as boundary condition at the base of soil layer stack. Notable amplification phenomena are shown, comparing seismograms at the bottom and at the surface. The 1D-3C approach evidences the influence of the 3-D loading path and input wavefield polarization. 3C motion and 3-D stress and strain evolution are evaluated all over the soil profile. The triaxial mechanical coupling is pointed out by observing the variation of the propagating wave polarization all along the duration of seismograms. The variation of the maximum horizontal component of motion with time, as well as the influence of the vertical component, confirm the interest of taking into account the 3C non-linear coupling in 1D wave propagation models for such a large event.