Offshore tsunami records recorded by deep ocean bottom pressure gauges are very helpful in developing a real-time tsunami forecast system and substantially improving theoretical understanding. In these studies and systems, a simplified proportional relationship in which the ocean bottom pressure changes are linearly proportional to tsunami wave height is usually used under the long-wave approximation. However, the ocean bottom pressure changes should be attenuated by the product of the wavenumber of the tsunami and the water depth at the location of observation. In order to evaluate this effect of this attenuation, we investigated a short-wavelength tsunami in real-time tsunami forecasting.
Widely distributed offshore tsunami observation networks, such as the Deep-ocean Assessment and Reporting of Tsunamis (DART; Bernard and Meinig, 2011; Table 1; Fig. 1), have tremendously improved the theoretical understanding of tsunami propagation, such as confirming the existence of dispersive waves (e.g., Saito et al., 2010; Saito et al., 2011; Miyoshi et al., 2015; Baba et al., 2017). DART stations, which has 51 stations in the world ocean as of March 22, 2019 (Fig. 1), are located at sites in regions generating historical destructive tsunamis to perform real-time tsunami forecast. Each station is located at the range of water depth from approximately 2,000 m to 6,000 m (Table 1). In addition, recent dense offshore observation networks will possibly help us to develop a real-time tsunami forecast system for reducing damage (Tsushima et al., 2009; Baba et al., --- Yamamoto et al., 2016a; Takahashi et al., 2017). These observation networks consist of a number of ocean bottom pressure gauges connected by a satellite network or optic fiber cables to transfer the data in real time.
For example, the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET; Kaneda et al., 2015; Kawaguchi et al., 2015; Fig. 2) has been constructed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and started observations in August, 2011. DONET is consisted of 51 stations in Kumano-nada and off Kii Channel, to monitor earthquakes and tsunamis in the Nankai Trouph region southwest Japan. These stations are located at the depth of approximately from 1,000 m to 4,500 m. The data is transmitted to research institutes and universities in real-time, and improves precision and warning times of earthquake early warning and tsunami warnings and/or advisories by Japan Meteorological Agency (JMA). DONET is currently operated by the National Research Institute for Earth Science and Disaster Resilience (NIED).