A powerful 7.5-magnitude earthquake has struck off northern Japan, injuring over 20 people and triggering a tsunami. The quake hit late Monday, about 80 kilometers off the coast of Aomori. Officials have reported a tsunami of up to 70 centimeters in some areas. The Fire and Disaster Management Agency says 23 people are injured, mostly by falling objects. Nuclear power plants are conducting safety checks, and some train services are suspended. The Japan Meteorological Agency is warning of possible aftershocks and increased risk of a megaquake.