Interpretation of Seismic Vertical Amplification Observed at an Array Site

Abstract

Strong ground motions were recorded at Port Island, Kobe, by a borehole array during the 1995 Hyogo-ken Nanbu (Kobe) earthquake. These records indicate that, while the horizontal peak accelerations were reduced as seismic waves traveling from the bottom to the surface, motions in the vertical direction were significantly amplified at the surface, with peak value of 1.5 to 2 times larger than the horizontal components. Some studies have discussed local site effects on the horizontal motions, whereas the available study on the vertical amplification is limited. In this article, we present a possible mechanism to explain the observed vertical amplification in detail. We consider that the observed large vertical amplification is mainly associated with incomplete saturation of near-surface soils, which causes substantial amplification of P waves but does not affect the propagation of S waves. Based on the concept of homogeneous pore fluid and Biot's theory of two-phase media, we discuss the characteristics of P-wave velocity, Poisson's ratio and degree of saturation in shallow soil layers, and show evidence of incomplete saturation of near-surface soils at the array site. We analyze a simple model to theoretically investigate the effects of saturation on vertical-motion amplification. The results show that the degree of saturation may produce substantial influence on the amplification, both amplitude and frequency content. Using a solid-fluid coupled finite element procedure, we perform a simulation of the observed vertical motions at the array site by including the effects of saturation. The results demonstrate the mechanism of large amplification caused by incomplete saturation of near-surface layers. The present study indicates that vertical-component motions may significantly be affected by pore-water saturation of soils, suggesting that we may need to carefully examine the condition of saturation in the study of vertical site amplification.

Numerical and analytical investigation of compressional wave propagation in saturated soils

“…This is based on the assumption that the effect of the vertical component of the ground motion, induced predominantly by vertically propagating compressional waves, is less significant, due to its smaller magnitude and higher frequency content compared to the horizontal ground motion component (Yang and Yan, 2009). However, since 1990s, strong vertical ground motions have been repeatedly observed, leading to significant damage of engineering structures in the form of vertical compression (Papazoglou and Elnashai, 1996;Yang and Sato, 2000;Bradley, 2011;Lee et al, 2013;Riches, 2015). In particular in the recent 2011 Christchurch earthquake, strong vertical ground motions were widely observed (Lee et al, 2013), which induced significant damage on crib retaining walls due to the contact loss of the crib units in the vertical direction (Riches, 2015).…”

Site characterization in the Vega Baja, SE Spain, using ambient-noise H/V analysis

“…Recent tests (e.g. Yang and Sato 2000;Mucciarelli et al 2003;Chatelain et al 2008;Gosar et al 2010) have shown that the effect of the water table variation on the results is only a slight shift of the frequency peak towards lower values, unless measurements are performed on water saturated ground which could produce artificial high frequency peaks. In order to avoid possible biases due to this kind of effects, reproducibility of measurements was checked by repeating measurements under different conditions (day time and meteorological) at a number of sites (31) on different dates (2005,2006,2007,2008) and through different acquisition systems.…”

Numerical investigation of multi-directional site response based on KiK-net downhole array monitoring data

“…However, evidence of strong vertical ground motions and compressional damage of engineering structures have been increasingly observed in recent earthquakes (Papazoglou and Elnashai, 1996 [28]; Yang and Sato, 2000 [38]; Bradley, 2011 [7]). There is therefore a need for a systematic investigation of the site response subjected to both the horizontal and vertical components of the ground motion.…”