Investigation of the Effect of Seawater on Seismic Motions Observed at Seafloor

Abstract

With the recent development of ocean bottom seismograph (OBS) networks, the use of OBS data in earthquake early warning (EEW) systems for railways has been considered. To apply existing P-wave EEW methods—originally validated using inland seismic data—to OBS data, it is essential to understand the amplification characteristics at the seafloor and the effects of seawater on the seismic motions recorded by OBSs. In this study, we investigated the influence of seawater on seismic motions at the seafloor by analysing spectral ratios between land and seafloor seismic motions. Our result show that troughs in the spectral ratios can be attributed to the transmission of P-waves into the seawater. We also found that the frequencies of these troughs vary according to the quarter-wavelength approximation, depending on the installation depth of each OBS. Furthermore, analysis of time history waveforms from OBS data revealed that the amplitude of the initial P-wave observed at the seafloor is less than twice that of the upgoing wave from the seafloor. Additionally, the phase of the P-wave at the seafloor is affected by reflections from the sea surface, and this effect varies with OBS installation depth. These factors reduce the accuracy of existing Pwave EEW methods when applied to OBS data. However, by taking into account the findings of this study regarding P-wave propagation through seawater, it will be possible to develop EEW systems that utilise P-waves of OBS data.