Volunteering through an organization provides a better chance of insurance and liability protection. There are many tasks to do after a disaster - cleaning up and rebuilding are two of the biggest. Both voluntary agencies and the local government may be aware of opportunities for volunteer labor in the long and difficult recovery phase. Watch the local media carefully to see what volunteer coordination efforts are being organized.
Also, before the next disaster strikes, consider getting some disaster training. You will be in a better position to find meaningful volunteer work at the time of a disaster.