Immediately after the 2008 Sichuan Earthquake, the netizens reacted and participated in the disaster response in many ways. This work examined a popular Chinese discussion forum during the first week after the earthquake, adopting both quantitative and qualitative approaches. A new message classification scheme was developed to categorize 2,266 discussion threads posted to the forum. Four important roles played by the forum were identified and further examined. Design implications for disaster response systems were drawn.