1

Title of the paper

Author Journal name and publication year

This paper considers the Boundary Element Method (BEM) to analyze the seismic site effects of the Volvi basin located about 30 km from north-east of the city of Thessaloniki in Greece. Authors have adopted two types of model viz. simplified model with two-layer on elastic bedrock and detail model with six different layers on elastic bedrock having distinct physical properties.

Results

Simplified model

The simplified model has shown interesting results. At the lower frequency (1Hz) largest amplification occurred in the deepest part of the basin and seems to correspond to the fundamental mode of vibration of the basin but the amplification factor was not very high (Fig. 1). For the frequency 0.8 Hz, two areas of the large amplification appeared at the free surface of the central part of the basin with maximum amplification of 9.5. But with increasing frequency, the area of higher amplification has been shifted towards the right of the model. For 1.8 Hz frequency, amplification reached up to 8.3 and the area corresponds to the extreme left of the medium depth of the basin (Fig. 1). For 2.4 Hz frequency, the shallow right of the basin predicted larger amplification.

Figure 1. Amplification values in the basin estimated numerically at various frequencies.

Comparison with the complete model

In the complete model all six layers on elastic bedrock were considered. The main aim of this comparison is to understand how the soil layering and basin geometry affect the site amplification.

2

The frequency domain comparison has shown that at the lower frequency (0.85 Hz) there was large difference between simplified and complete model almost 40% on maximum values. The complete model leads to amplification value up to 15 at the central part of the basin (Fig. 2). The amplification patterns were also different because the complete model has given four distinct amplification areas as compared to two areas in simplified model. With increasing frequency (1.75 Hz), amplification values for both models are slightly different. For frequency 4.75 Hz, the differences are significant on maximum values but not so large on amplification location (Fig. 2).

These results have indicated that the amplification level estimated for the complete model is larger than the simplified model because the basin effects strengthen the seismic wave amplification. Therefore, this research has clearly highlighted the effect of basin geometry and soil layering on site effects.

In time domain comparison, upward propagating SH-wave described by a Ricker signal was considered. The time domain solution derived from the simplified model has clearly shown the effect of lateral heterogeneities and the amplification of the first arrival also obvious. Seismic wave effect on Volvi basin influenced by soil layering and basin edge were also observed. In contrast, in complete model the amplification of the first arrival was larger than the simplified model. Since the soil layer was considered more precisely, amplification due to velocity contrast was more pronounced in the complete model. The signal duration was significantly increased showing the combined influence of basin effect and soil layering.

This study has clearly shown the effect of basin geometry and soil stratigraphy considering simplified model with two layers of greater thickness and complete model with complex soil stratigraphy. Based on this study, it can be concluded that the detail soil layering gives stronger heterogeneities and consequently larger amplification than the simplified geometry gives. Thus seismic wave amplification in alluvial basin is mainly affected by basin geometry and soil layering.

Figure 2. Amplification factor vs distance for both models.