Tsunami early warning based on the data assimilation (DA) approach is a recent technique (Maeda et al., 2015). It directly uses offshore observations to reconstruct the tsunami wavefield and does not require tsunami source information for forecasting. Therefore, it is applicable to tsunamis generated by various sources, such as earthquakes, landslides, and volcanic eruptions (Gusman et al., 2016;. Offshore tsunami observations are primarily provided by offshore bottom pressure gauges (OBPGs). They measure the water pressure and convert it to sea surface elevation in real time. The tsunami height is derived from these records and used as input for DA (Kaneda, 2010;. DA using OBPG records has been successfully applied to past events, such as the 2009 Dusky Sound (New Zealand) tsunami (