The most common cause of tsunami is seismic activity. Although over the past two millennia, earthquakes have produced 82.3% of all tsunami in the Pacific Ocean, tsunamigenic earthquakes are rare. Between 1861 and 1948, over 15,000 earthquakes produced only 124 tsunami. This low frequency of occurrence may simply reflect the fact the most tsunamis are small in amplitude and go unnoticed. Two thirds of damaging tsunamis in the Pacific Ocean region have been associated with earthquakes with a surface magnitude of 7.5 or more. The majority of these earthquakes have been teleseismic events affecting distant coastlines as well as local ones.

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How earthquakes generate tsunamis?

Most tsunamigenic earthquakes take place at the great ocean trenches, where the tectonic plates that make up the earth’s surface collide and are forced under each other. When the plates move gradually or in small thrusts, only small earthquakes are produced; however, periodically in certain areas the plates catch. The areas where the plates are hung up are known as “seismic gaps” for their lack of earthquakes. The forces in these gaps continue to build until finally they overcome the strength of the rocs holding back the plate motion. The built-up tension (or compression) is released in one large earthquake, instead of many smaller quakes, and these often generate large, deadly tsunamis. In fact the tsunamis often prove more deadly than the earthquakes themselves.
Most tsunamis occur in the Pacific Ocean, because the Pacific basin is surrounded by a zone of very active features in the earth’s crust: deep ocean trenches, explosive volcanic islands, and dynamic mountain ranges. Frequent earthquakes and volcanic eruptions make the rim of the Pacific basin the most geologically active region on earth, this place is called the ring of fire. But this is not the only place tsunamis occur. One of the most devastating tsunamis in history occurred in the Atlantic Ocean, produced by the famous Lisbon earthquake of 1755.

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