Earthquakes destroy many buildings and take lives. One cause of earthquakes is an active fault. The impact of a sudden displacement of the ground shakes the Earth and causes an earthquake. Japan has compiled a database of active faults found to date and is preparing for earthquakes by examining each one in detail to predict its magnitude. Surveys are also being conducted to find new active faults. In this episode, we'll see how one researcher is searching for active faults hidden in the seafloor by using ultrasonic waves sent out from a ship.