About the Efficiency of Numerical 1-D and 2-D Modelling of Site Effects in Basin Structures

Abstract

In the present study we compare results obtained from experimental estimates of local site amplification effects with those from numerical modelling using four different techniques. We benefit from an extremely precise knowledge of the near-surface structure and experimental estimates of the local amplification factors which are determined from seismic weak-motion data recorded by a dense array across a sedimentary basin at a European test-site in Northern Greece. The possibilities and limitations of the different modelling techniques (a 1-D technique, and three 2-D techniques) to model the effects of local amplification effects are tested. Amplifications calculated by the numerical techniques are only qualitatively compared with observed data from experimental studies in the time domain and directly in the frequency domain.As a result we conclude that, in the case of a complex subsurface geometry, 1-D modelling underestimates the amplification patterns in terms of absolute amplification level, and cannot correctly account for resonant frequencies, at least for modes higher than the fundamental mode. If a more realistic incident wave field than just a plane wave is taken into account, 2-D modelling reveals the fundamental frequency and shows adequate amplifications not only at the fundamental frequency but also at higher frequencies. The general trend of the amplifications at the lowest frequencies is well determined by the 2-D numerical modelling, and can therefore supply information for seismic risk analysis.

The use of direct boundary element method for gaining insight into complex seismic site response

“…In it they found that the amplification levels for frequencies higher than 1 Hz is better modeled. Following the conclusions in [1] and this study, it could be inferred that these high amplifications at the edges can be assessed more reliably by a combination of a more realistic: geometry of the near-surface soil-as the one shown in model II, and of the incident radiation on the rock basement (to be performed in future works).…”

“…Notice the amplification at the edges of the basin (STC, STE) for frequencies higher than 1.5 Hz. This last specific characteristic of the seismic response has been underestimated in other numerical studies using a standard finite-difference method [1], where the authors pointed out that it could be due to several reasons concerning the simplification of models: the nonexisting surface weathered layer in their model, some neglected thin layers, the smoothing of the inhomogeneous fault zone, more complex 3D effects,... In the same work the authors performed a more complex computation of the response of the basin applying a hybrid method to account for a more realistic incident wave field.…”

“…Transfer functions computed along the surface of the Graben's models have been analyzed within the framework of other results coming from data recorded at the site and other numerical investigations [1,34,35].…”

“…Taking into consideration the above remarks of the survey carried out in [1] this misfit could be explained in terms of the above mentioned neglected surface layers (soft and weathered layers). This issue is to be investigated in forthcoming works.…”

“…Refs. [1][2][3][4][5][6][7] are just examples of the vast bibliography devoted to the understanding of seismic site effects. Seismic codes are echoing the importance of site effects by approaching a more detailed quantification of soil effects [8].…”