Establishing offshore tsunami monitoring network system is an urgent task for countries facing ocean. Up to present, the tsunami-alarming information has been based solely on earthquake acceleration records due to difficulties in direct tsunami wave measurement. Nevertheless, as earthquake motion intensity does not
correspond well with tsunami height, reliability of the tsunami alarming information has not yet reached sufficient level. Therefore, direct tsunami wave observation was recommended using existing seabed installed coastal wave gauges. This paper introduces the observed high frequency seabed pressure data obtained through the Japanese nation-wide coastal wave observation network (NOWPHAS) at the attack of the 2005 Miyagi-Prefecture Off-Earthquake, the 2007 Noto-Penisula-Earthquake, the 2007 Chuetsu-Off Earthquake and the2008 Iwate-Miyagi Inland Earthquake events.
　Following conclusions were obtained;
1)Improvement of the NOWPHAS (Nationwide Ocean Wave information system for Ports and HarborS) enabled us to observe high frequency seabed pressure data due to earthquake seabed motion before tsunami attack, which are supposed direct earthquake action to sea-water useful for tsunami prediction.
2)Duration time of earthquake action and the high frequency seabed pressure fluctuation was compared.
3)Amplitudes of the observed and estimated high frequency seabed pressure fluctuation were compared. Sea-water acoustic characteristics may take an important rule to determine the amplitudes.
4)Frequency energy distribution was compared between the high frequency seabed pressure fluctuation and the earthquake acceleration.