The US Pacific Tsunami Warning Center also said there was no threat to coastlines beyond Japan. What is the difference between a tsunami warning, advisory and forecast in Japan? A red tsunami warning is meanwhile used when larger waves, generally of up to three metres, are possible. A purple major tsunami warning is the most serious category and indicates the possibility of waves exceeding three metres. The alert issued after the Kumamoto earthquake forecast waves of up to one metre for the Ariake and Yatsushiro seas.