Once the flood, fire, earthquake or tsunami has happened, we can't undo it. But it's always useful to send Love energy to such situations, and to the people in them and the places where they occur. If you're a Reiki practitioner, Reiki is one of the easiest ways to do that. Beam it at the TV screen when you're watching the news footage — either to the general scene or to individuals who are shown, or both. When you see those scenes in your head later, as you're bound to, again visualise and intend to 'zap' them with that energy or surround them in it.
This is a huge act of faith, as you will never know if it did any good. It is a time to recall the many instances when you have been able to get feedback and to know that Reiki helped a person or situation. Knowing that, trust that it will help in the large-scale disasters also, and remember that some Reiki is better than none.
Sometimes weather magick can prevent threatened disasters. Witches would probably just light a candle. In his book Urban Shaman, Serge Kahili King recounts instances of shamans banding together to successfully turn away destructive weather from population areas. Group energy is very powerful; for that reason it is also effective to take part in such events as world-wide meditations at specific times.
For people who aren't shamans or witches and don't know how to use Reiki, heartfelt prayer is also powerful. So are 'Love projections' where you get into a semi-meditative state and focus Love towards the person / place / situation. You can visualise it as white light if that makes it easier — or rainbow light, or pink light, or green, or gold — so long as you hold the understanding that this light is an expression of Love energy.
Humanity has been told by many teachers over the centuries that Love is the essential for making our lives work, having ourselves and our planet be healthy. It's a very difficult thing to do, to stay always in that state. Most of us have to catch ourselves —eventually! — when we fall out of it, pick ourselves up and start again. It is worth persevering. There is nothing that is so much worth doing.
I am now in my 'senior' years, with a great deal of life experience behind me. It becomes more and more apparent to me that Love is indeed the only thing that really matters. It's vital.