Sansa Odori is Morioka's big summer festival in early August where thousands of people dance in the streets while playing drums and flutes. The festival's origins stem from the defeat of a pesky devil that plagued the area. A local god made the devil place his hand upon a stone to signify his pledge never to bother the people again. The stone handprint remained and that is where the prefecture Iwate got its name - "stone hand."
Whatever the origins, it's a fun festival with a lot of energy.