A Proxy for Assessing Topographic Site Effects and Its Application to K-NET and KiK-net

Abstract

Topographic site effects were examined based on the spectral ratios of seismic records on a crest of a simple-shaped ridge and a lower station with flatness topography (reference point). The topographic effects more clearly appeared in the orthogonal direction to the ridge axis than in the ridge axis direction, and the spectral characteristics showed from around 2 Hz to a nearly flat peak up to 5 Hz to the reference point. The topographic effect was greater at the edge of the ridge elongation than at the center of the ridge, even on the same ridge crest. Based on the numerical simulations using DEM, the three-dimensional topographic model better reproduced the difference in the topographic effect between the center and the edge of the ridge compared to the two-dimensional model, indicating the importance of considering three-dimensional topography. The correlation between the obtained topographic site effects and topographic indices was examined to identify a proxy for assessing topographic site effects. The results showed that the ridge-valley degree, defined from positive and negative openness, has a higher correlation than the general topographic index, the shape ratio (width-to-height ratio of the ridge) and is effective in evaluating the topographic site effects. Furthermore, the ridge-valley degrees at K-NET and KiK-net stations were investigated, and the effectiveness of the ridge-valley degree in evaluating topographic site effects was confirmed by comparing AKTH04, which has a significant ridge-valley degree, with the nearby station with a less ridge-valley degree through the site condition proxy, the ridge-valley degree, and peak accelerations during the 2008 Iwate-Miyagi Nairiku earthquake.