What Buddhist materials describe how to increase the number of people with good Karma in a geographic area?
Though not directly affecting karma, there is a variation of Metta Meditation mentioned in the Visuddhi Magga that increases universal love and brotherhood. Though the object is universal, its obvious that nearer vicinities would be felt more profound an effect.

If there is one thing in this world that can influence peoples' Karma, it is meditation. Karma is nothing but the result of aeons of conditioning of the five aggregates, and only with insight (pradnya) can they be positively affected.

As a practical experiment, just arrange a series of group meditation sittings among the people of the geographic area you want to improvise. After each sitting, just feel the metta vibrations in the air!