Inversion of Soil Structure and Analysis of the Seismic Wavefield From a Vertical Array

Abstract eng:
Currently, considerations of site effects in codes account for impedance contrast effects but do not consider the effects of laterally irregular structures. We need a better understanding of the relation between irregular subsoil structure, diffracted wavefield, and intensity measures. In this paper we analyze the wavefield that propagates in Euroseistest, a small basin in Northern Greece where a natural scale laboratory for seismology and earthquake engineering has been in operation for the last 15 years. The geometry and properties of the subsoil formations are well determined and a permanent accelerograph network, including a vertical array of six triaxial accelerometers is in operation. We present the preliminary results of the analysis of eight earthquakes recorded by this array using two approaches. We first use simulated annealing to minimize the difference between spectral ratios between neighbouring accelerometers and those computed using a 1D model. This allows us to improve previous determinations of seismic velocities and Q factors in the soil layers. The second approach is seismic interferometry. The analysis of traces deconvolved either with the surface or bottom records gives us further constraints on the subsoil structure and on the nature of the recorded waves. Our results are useful to improve the soil profile. The analysis of the records shows that local structure conditions the recorded wavefield independently of the source location. Thus, the consideration of the lateral heterogeneity is essential to predict ground motion at this site.

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