Finite difference simulations of seismic wave propagation are performed in the Niigata area, Japan, for the Mw 6.6 Niigata-ken Chuetsu-Oki earthquake at low frequencies. We test three 3D structural models built independently in various studies. First aftershock simulations are carried out. The model based on 3D tomography yields correct body waves in the near field, but later phases are imperfectly reproduced due to the lack of shallow sediment layers; other models based on various 1D/2D profiles and geological interpretation provide good site responses but generate seismic phases that may be shifted from those actually observed. Next, for the mainshock simulations, we adopt two different finite source models that differ in the near-field ground motion, especially above the fault plane (but under the sea) and then along the coastline. Each model is found to be calibrated differently for the given stations. For engineering purposes, the variations observed in simulated ground motion are significant, but for seismological purposes, additional parameter calibrations would be possible for such a complex 3D case.

International audienceThe earthquake motion generated by the multiple scattering due to the complexity of the underneath soil structure can be referred to as a diffuse wave field. Under the assumption of the well-diffused wave field it is accepted that the average autocorrelation of a single receiver is proportional to the imaginary part of the Green's function when both source and receiver are located at the same point. In this study we focus on sites where the site effect can be described using a 1D model. Previous studies show that the imaginary part of the Green's function at the free surface is proportional to the square of the absolute value of the corresponding transfer function for a plane, vertically incident wave with unit amplitude. It is then possible to carry out an inversion of the 1D velocity structure using the relationship between the horizontal-to-vertical (H/V) spectral ratio and the ratio of horizontal and vertical transfer functions. We verify that the average H/V spectral ratio computed with a sufficiently large number of earthquake data depends only on the underneath geological structure and not on the set of data used to compute it. We then carry out inversions of the velocity structures for 10 sites of the K-NET and KiK-net networks in the Tohoku area, Japan, following the proposed theory for earthquake H/V spectral ratios.We verify that there is a good match between the observed H/V spectral ratios and the theoretical ones corresponding to the proposed velocity structures for the 10 target sites studied in the present work

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A hybrid method combining finite element and 4 th -order finite difference techniques is developed to model SH and P-SV seismic wave propagation in a 2D elastic medium with irregular surface topography. Both the classic staggered grid finite difference scheme and the partially staggered grid scheme are tested. The accuracy of the hybrid method is studied by comparison with a semi-analytical and another numerical method. Subsequently, to study the amplification, numerical simulations of seismic wave propagation in a series of hills are carried out and compared with the single-hill case. Depending on the position of the source in relation to the topography, the ratio between the heights and lengths of the hills or the ratio between the lengths of the hills and the wavelength, the presence of several hills as opposed to a single one can increase the amplification effect due to topography. This study highlights the fact that, when evaluating topographic site effects, surrounding topography must be taken into account in addition to local topography.