Japanese who are also buddhists followers usually believe in two Great Bodhisattvas. The first one is "Kannon/Kanzeon" or "Avalokiteshvara" (觀世音菩薩) and another one is "Jizo" or "Ksitigarbha" (地藏王菩薩). Both Bodhisattvas have great compassion and great power in saving all sentient beings. They both are said to answer all prayers of all beings, especially when being in danger.
Understanding that both of Them are always ready to save all beings, we should think that they are also willing to save during tsunami. In fact, prayers in Buddhism are not only necessary in danger but we should pray and follow Their teachings every day. Reciting the Sutras and repentance are two important things in learning Buddhism. Since countless eons, we have created immeasurable evil deeds or you can call as evil karma through our mind, speech and action.
Whether natural disasters / calamity or just personal misfortune are not things that unrelated to our karma. They are actually the bitter fruit derived from the karma. We should seek repentance every day for the mistakes or evil deeds that we do every day. Not for those mistakes/evil deeds of this life only. We have past lives, and you don't know what you have done in those past lives.
Praying, chanting Sutra, bowing homage, reciting their Sacred Names are actions done for repentance. These are actions to avoid calamities. If they are done by mass of people altogether, power to avoid calamities can be stronger. That's also why after a calamity, there will always have more Buddhist ceremonies held in Hong Kong, Taiwan, China, and other buddhist countries.
At the end of the ceremonies, Buddhist do merit transference which means transferring the merit of the chanting to all beings especially those who are in natural disasters. Hoping that the destruction caused by the disaster will be stopped or reduced and also hoping that the future disasters can be avoided.
I will pick up a part of "Universal Gateway of Avalokiteshvara" (觀世音菩薩普門品) which tells you that the Great Compassionate Avalokiteshvara will save those beings by just reciting His Name. When a being is in the danger of flood (or we say seriously tsunami), by just reciting His Name, this being will be saved. Luckily, I found a animated video on this description. Also in "Immeasurable Life Sutra" (無量壽經), the Buddha have also advised us to seek refuge in Avalokiteshvara when in difficulties and dangers.
Another Sutra that also tells us about avoiding calamities is Ksitigarbha Bodhisattva. The Lord Buddha also advised us to seek refuge, recite Ksitigarbha Bodhisattva's Name, bowing homage and reciting Ksitigarbha Sutra. Buddha also tells those devas, kings of ghosts, deities and those Bodhisattvas who are the audience of the Ksitigarbha Sutra to protect those who are sincerely seeking refuge and follow this Teachings of Ksitigarbha Bodhisattva.
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I realized that my knowledge about this is very limited, but I hope that this short post of information will help those who are seeking for a solution about how to prevent tsunami and other natural disasters. Saving the world is not the duty of one personal, but the duty of everybody.
Whether you are Buddhist or not, the Truth is that there won't be no reason behind a calamity. We should accept the Truth of Cause and Effect or the Law of Karma.
I also find that those social workers who are heading towards Japan to offer their helps are very brave and courageous. No matter which method you use to help those pitiful Japanese people or others harmed by natural disasters, the most important thing is that you have done something for them, whether by direct-hand-help, donating or praying for them. May all beings be free from harm and danger. Amitabha!!!