Complex sentence: Continuing movement on the subduction zone associated with the Japan Trench is one of the main causes of tsunamis and earthquakes in northern Japan, including the megathrust Sendai earthquake and resulting tsunami that occurred on 11 March 2011.
Split into simpler sentences: Continuing movement on the subduction zone associated with the Japan Trench is one of the main causes of tsunamis and earthquakes in northern Japan. The Japan Trench is also the site of the megathrust Sendai earthquake and resulting tsunami that occurred on 11 March 2011.