Twitter has just published a couple of videos that visualize the flow of tweets before and after the March 2011 earthquake.
The first video shows @reply tweets in the hour preceding and following the earthquake, to (pink) and from (yellow) Japanese Twitter accounts. You can clearly see the sudden jump in the volume of tweets following the earthquake.
The second video shows tweets originating in Japan (red) and then being retweeted (green) around the globe in the hour following the earthquake.
Twitter reports that the volume of tweets spiked to 5000 tweets per second five times in the hours following the earthquake and tsunami. A vivid example of Twitter's reach as a global communications tool.