IRS. Publication 3833, Disaster Relief: Providing Assistance Through Charitable Organizations is the place to start for those interested in disaster relief efforts.
One issue that often arises is whether the "charitable class" to be benefited by the charitable organization engaging in disaster relief efforts is large and/or indefinite enough that providing aid to members of the class benefits the community as a whole. See Disaster Relief and Emergency Hardship Programs in the 1999 EO CPE Text.
A limited group of beneficiaries may also affect the deductibility of a charitable contribution for the benefit of such group. In this context, the issue is whether the contribution has been earmarked to assist a particular individual or small group of individuals.
For more information:
Disaster Relief and Recovery: The Role of Nonprofits Beyond Ground Zero and the Legal Implications of Their Work – Lawyers Alliance for New York
California Disaster Relief Tax Provisions, Southern California Wildfires: Southern California, October 2003, FTB Pub. 1034A-4