Abstract
A prediction model for offshore vertical-to-horizontal (V/H) spectral ratios of peak ground acceleration (PGA) and 5%-damped elastic response spectra for periods ranging from 0.01 to 10 s for an offshore area off Sagami Bay was developed. To compare differences between offshore and onshore ground motions in the V/H spectral ratios, an onshore V/H prediction model was derived for onshore stations adjacent to the studied offshore sites. The offshore dataset includes 738 three-component records, and the onshore dataset includes 2219 records; both datasets are derived from the same set of 233 earthquakes selected for this study. The moment magnitude, hypocentral distance, focal depth, tectonic source type, and individual site correction term are used as independent variables in the V/H models. A comparison drawn between the offshore and onshore models shows that the V/H spectral ratios of offshore ground motions are obviously smaller than those of the onshore motions over short periods (< 1.0 s) but are comparable for periods of longer than 1.0 s. Water layer reduces vertical ground motions, especially over short periods, and as the periods increase, the effect of water layer decreases. The effect of the moment magnitude on offshore V/H ratios is stronger than that of onshore ratios, especially for long periods; the effect of hypocentral distance and focal depth is considerable for long periods for offshore V/H ratios, and the V/H ratios between offshore sites are very different due to complex local site geologies beneath the offshore stations.
Highlights
Earthquake ground motions on the seafloor differ from onshore ground motions in vertical components and vertical-to-horizontal (V/H) response spectral ratios (Chen et al 2015)
The ground motion prediction equation (GMPE) of offshore sites is different from that of onshore stations (Hu et al 2020), and we have presented a direct comparison of V/H spectral ratios of the offshore and onshore ground motion, from which we can understand how V/H spectral ratios vary by period, moment magnitude, hypocentral distance, etc
The validity of the proposed models is examined by a detailed residual analysis, and the analysis shows that the offshore V/H model is not biased with regard to the moment magnitude, hypocentral distance, or focal depth
Summary
Earthquake ground motions on the seafloor differ from onshore ground motions in vertical components and vertical-to-horizontal (V/H) response spectral ratios (Chen et al 2015). The ground motion prediction equation (GMPE) of offshore sites is different from that of onshore stations (Hu et al 2020), and we have presented a direct comparison of V/H spectral ratios of the offshore and onshore ground motion, from which we can understand how V/H spectral ratios vary by period, moment magnitude, hypocentral distance, etc. Simplified period-dependent models for V/H or even a constant V/H ratio is used in some codes to obtain a vertical response spectrum from corresponding horizontal spectrum (Zolfaghari and Darzi 2019b), and Chen et al (2017) analyzed vertical-to-horizontal response spectral ratios for offshore ground motions by presenting the mean V/H ratios of nine earthquakes and proposed a simplified design equation for the V/H ratio of offshore ground motions, in which only the moment magnitude and the perioddependent variation are considered. The results of direct comparisons show that the V/H ratios of offshore motions are significantly smaller than those of onshore motions for short moderate spectral periods (T < 1.0 s) and are comparable for periods longer than 1.0 s
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