Abstract
A number of vertical array records during eight destructive earthquakes in Japan are utilized, after discussing criteria for desirable requirements of vertical arrays, to formulate seismic amplification between ground surface and outcrop base for seismic zonation. A correlation between peak spectrum amplification and Vs (S-wave velocity) ratio (base Vs/surface Vs) was found to clearly improve by using Vs in an equivalent surface layer wherein predominant frequency or first peak is exerted, though the currently used average Vs in top 30 m is also meaningful, correlating positively with the amplification. We also found that soil nonlinearity during strong earthquakes has only a marginal effect even in soft soil sites on the amplification between surface and outcrop base except for ultimate soil liquefaction failure, while strong nonlinearity clearly appears in the vertical array amplification between surface and downhole base. Its theoretical basis has been explained by a simple study on a two-layered system in terms of radiation damping and strain-dependent equivalent nonlinearity.
Highlights
In determining design motions for superstructures, site amplifications have to be evaluated in soil layers above an engineering bedrock at which the design motion is prescribed
The appreciation of the significant role of the SH-wave in site amplification stems from the pioneering work by Kanai et al in 1956 [1] and 1966 [2], wherein simultaneous earthquake observations were conducted at the ground surface and the subsurface level below in a tunnel, demonstrating that seismic horizontal ground motions can be idealized essentially by the SH-wave vertically propagating in one-dimensional horizontal layers
In order to investigate the effect of soil nonlinearity on the site amplification using actual earthquake data, aftershock records were compared with corresponding mainshock records in the same sites
Summary
In determining design motions for superstructures, site amplifications have to be evaluated in soil layers above an engineering bedrock (base layer) at which the design motion is prescribed. The appreciation of the significant role of the SH-wave in site amplification stems from the pioneering work by Kanai et al in 1956 [1] and 1966 [2], wherein simultaneous earthquake observations were conducted at the ground surface and the subsurface level below in a tunnel, demonstrating that seismic horizontal ground motions can be idealized essentially by the SH-wave vertically propagating in one-dimensional horizontal layers. The site amplification in the 1D soil profile seems to depend on S-wave velocities, soil densities, internal damping of the individual layers. Vertical array earthquake observation systems have been increasingly deployed, in Japan, wherein site-specific amplifications can be investigated at the same sites along with the depth, having various geological profiles between the ground surface and downhole base. It is generally accepted that Geosciences 2021, 11, x FOR PEER REVthIEeWengineering bedrock can serve as a common scale to define relative site amplifica3toifo1n9 among sites sharing the same bedrock, though deeper geological structures may have some effect on absolute amplification (e.g., Trifunac 1990 [9])
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