This study explores how micro-bloggers react to disasters in social media by examining the discursive-semiotic activities activated in Twitter and Weibo in responding to the 2013 West explosion in the US and the 2015 Tianjin explosions in China. By analyzing 1322 Weibo posts and 1387 Twitter posts, the article shows how users of Twitter and Weibo mobilized alternative repertoires of representatives, expressives, directives, commissives and eliciting to make sense of disasters, in ways which in turn evoked a contrasting sense of communities of collective purpose. While the discourse of the 2013 West explosion reflected a strong sense of the creation, sharing and distribution of knowledge, as well as showing support and sympathy to the suffering, the discourse of the 2015 Tianjin explosions, displayed strong critical and oppositional properties that disrupted the official discourse of the accident. This study contributes not only to a better understanding of social media in disaster communication, but also to the methodologies for studying social media data in relation to disasters.