Twitter added more choices of cities on its trending keywords in Japan on February 2. There were only all-Japan and Tokyo rankings before.
6 cities, Osaka, Nagoya, Sapporo, Kyoto, Fukuoka and Sendai are the new ones.
View Twitter Trends On New Japanese Cities in a full screen map
There are many bigger cities than Sendai, but most of them are satellite cities of the largest ones (Yokohama, Kawasaki, Saitama, Chiba for Tokyo, Kobe for Osaka) so they were probably selected to make geographic balance. Hiroshima, which has slightly more population than Sendai, could have been chosen though.
I am unsure if different cities will make much difference on trends. We will report if there are anything interesting observed.