This paper explained results of the GPS Buoy observed offshore tsunami records caused by the 2001 Peru-Earthquake, the 2003 Tokachi-off Earthquake and the 2004 Tokaido-off Earthquake. Observed GPS buoy tsunami data were compared with seabed installed acoustic and pressure wave gauges nationwide network data and coastal tide stations data. It was concluded that GPS Buoy successfully observed tsunami profiles with good precision at the three events. GPS buoy of 100m deep observed the first tsunami wave 10 minutes earlier than the coastal tide station, which indicated that GPS buoy will be useful for rapid coastal evacuation and tsunami disaster reduction. Tsunami amplification characteristics from offshore GPS buoy observation points to coastal tide gauge points were also clarified at the two ports area, the Ofunato Port and the Murotsu Port, by conducting tsunami profile wave-to-wave analysis and frequency spectrum analysis.
Offshore observation of tsunami and storm surge before arriving to the coast is very important for coastal disaster prevention. But up to ten years ago, coastal tide stations had been supposed to be the only means to observe tsunami and storm surge profile, for difficulty of offshore observation (Goda et al. 2002). Recently seabed installed coastal wave gauges were repeatedly reported to successfully observe various tsunami profiles by conducting continuous data acquisition (Goda et al. 2001; Nagai, 2002a; Nagai et al. 1996, 2000, 2002b).
Nevertheless seabed installed types of wave gauges should be installed in the limited area with water depth less than 50m for their maintenance necessity. On the other hand, buoy type wave gauges with acceleration sensors are not able to detect long period tsunami, for acceleration is very small in such long period fluctuations. Therefore, a new offshore observation system had been desired.
