On 11th March 2011, a 9.0-magnitude megathrust earthquake occurred in the ocean near Japan. This was the first large-scale natural disaster in Japan since the broad adoption of social media tools (such as Facebook and Twitter). In particular, Twitter is suitable for broadcasting information, naturally making it the most frequently used social medias when disasters strike. This paper presents a topical analysis using text mining tools and shows the tools' effectiveness for the analysis of social media data analysis after a disaster. Though an ad hoc system without prepared resources was useful, an improved system with some syntactic pattern dictionaries showed better results. Copyright is held by the International World Wide Web Conference Committee (IW3C2).