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.

对于相模湾外的海上区域，在0.01到10 s的时间范围内，建立了峰值地面加速度（PGA）和5％阻尼弹性响应谱的海上垂直/水平（V / H）光谱比的预测模型。为了比较V / H频谱比中近海和陆上地面运动之间的差异，陆上V / H为与研究的海上站点相邻的陆上站点推导了预测模型。离岸数据集包含738个三部分记录，而陆上数据集包含2219个记录。这两个数据集均来自为此次研究选择的同一组233次地震。在V / H模型中，矩震级，震中距离，震源深度，构造源类型和单个部位校正术语被用作自变量。海上和陆上模型之间的比较表明，V / H在短时间内（<1.0 s），近海地面运动的频谱比明显小于陆上运动的频谱比，但在超过1.0 s的时期内，可比。水层会减少垂直地面运动，尤其是在短时间内，并且随着时间的增加，水层的影响会降低。矩量级对海上V / H比的影响要比陆上V / H比的影响要大，特别是对于长期而言；对于海上的V / H比，在较长的时间内，下偏距距离和震源深度的影响是相当大的，并且由于海上平台下方的复杂的现场地质条件，海上平台之间的V / H比差异很大。