Influence of Local Site Condition on Vertical-to-Horizontal Spectrum Ratio – Insight from Site Response Analysis

ABSTRACT

A vertical-to-horizontal (V/H) response spectral ratio of 2/3 is commonly adopted in earthquake engineering for assessing the magnitude of vertical motion. However, higher V/H ratios are found in some earthquake records. This study evaluates the factors that can influence the V/H ratio on the basis of vertical and horizontal site response analyses. Two sets of rock motions representing near-source and far-field conditions are propagated through four selected sites that vary in soil column thickness, groundwater table, soil type, and wave velocity profile. The V/H ratio on the ground surface is calculated and compared with that at the bedrock. Results show that the V/H ratio is amplified by the local site effect, and increases at low wave velocity and high intensity of input motion owing to the different nonlinearities presented in the vertical and horizontal directions. The groundwater table considerably varies the V/H ratio and should be considered in assessing the vertical motion or V/H ratio.

KEYWORDS:

• Vertical-to-Horizontal Spectrum Ratio
• Site Effect
• Site Response Analysis
• Vertical Ground Motion
• Ground Water Table

Acknowledgments

This work was supported by the Ministry of Science and Technology, Taiwan under Award No. (MOST 105-2628-E-005-002-MY3). The authors gratefully acknowledge such support.