What are earthquakes and tsunamis? If you've seen the newspaper or news on television or the Internet over the past few days, you have heard about the terrible earthquake and tsunami that hit Japan, killing and injuring thousands of people. We thought it might be helpful to define some of the words you've been hearing. Earthquake: An earthquake is more than the earth shaking; it's when energy is suddenly released in the Earth's crust. The magnitude of an earthquake is a measure of its size. An earthquake with a magnitude of 3 is almost impossible to feel. Magnitude 7 and above cause significant damage. The Japanese earthquake, at 8.9, was the fifth most powerful earthquake since 1900. Tsunami (pronounced soo-nah-mee): A tsunami is a series of waves caused by huge movements in the water. Earthquakes, volcanic eruptions and landslides can cause tsunamis. Japan is one of the places in the world where tsunamis are most common. The word is Japanese and means "harbor wave."