An empirical vertical-to-horizontal (V/H) spectral ratio model for shallow-crustal and upper-mantle earthquakes: insights from the S-net seafloor stations in the Japan Trench region
Abstract
Abstract This study develops an empirical vertical-to-horizontal (V/H) spectral ratio model for offshore ground motions of shallow-crustal and upper-mantle earthquakes from the S-net in the Japan Trench region, integrated with seafloor station based on dominant frequency identification and site classification (SC1 - SC4). Using the Mw 7.5 offshore earthquake off northeastern Japan on 8 December 2025, as a representative event, we quantitatively compare offshore and onshore V/H spectral characteristics and systematically assess the influence of moment magnitude, focal depth, rupture distance, fault mechanism, station burial condition, and site condition on offshore V/H spectral ratios. Model performance is evaluated through between-event and within-event residual analysis and benchmarked against established onshore V/H ratio models from global onshore regions. The results indicate that offshore V/H spectral ratios are generally significantly lower than those observed on onshore ones, which is consistent with the pronounced attenuation of high-frequency vertical ground-motion components by the water layer. The effect of moment magnitude exhibits clear period dependence, while focal depth shows an attenuating effect primarily in the intermediate- and long-period ranges. Overall, offshore V/H tends to increase with increasing rupture distance, and burial conditions exert a significant influence on the offshore V/H ratios. In the long-period range (T > 1.0 s), offshore V/H…
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