In this article we explore the behavior of Twitter users under an emergency situation. In particular, we analyze the activity related to the 2010 earthquake in Chile and characterize Twitter in the hours and days following this disaster. Furthermore, we perform a pre-liminary study of certain social phenomenons, such as the dissem-ination of false rumors and confirmed news. We analyze how this information propagated through the Twitter network, with the pur-pose of assessing the reliability of Twitter as an information source under extreme circumstances. Our analysis shows that the propa-gation of tweets that correspond to rumors differs from tweets that spread news because rumors tend to be questioned more than news by the Twitter community. This result shows that it is posible to detect rumors by using aggregate analysis on tweets.