Earthquakes and tsunamis often go hand-in-hand. Tsunamis are generated when chunks of the planets crust separate under the ocean floor causing an earthquake. It can travel over the surface of the ocean for many hundreds of miles. One slab of lifting crust acts as a giant paddle, and transfers its energy to the water.
Tsunamis can also be caused by volcanic eruptions, under water detonations and landslides. Predicting a tsunami is difficult. The waves resulting from any of the reasons mentioned above are hard to predict as is its affect on the ocean floor until hours, days or even months after the event. Tsunamis are also hard to notice on the open ocean, as it only rises to its full force near the shore. There are many tsunami-sensing buoys which cover the ocean following the effects of the 2004 Indian Ocean tsunami. Still the waves could be missed. Not every seafloor earthquake generates a tsunami. If the friction between the plates occurs very deep below the floor of the ocean and causes just a minimal paddle effect, then a tsunami is not likely to generate.
While a number of devastating tsunamis have occurred in history, one of them levelled Lisbon, Portugal in 1755 while another was generated by an explosion in Indonesia which drowned an estimated 36,000 people. While the 2004 quake off the coast of Sumatra, Indonesia was recorded at a magnitude 9.3 – an 8.7 magnitude earthquake in 2005 in the very same location did not generate a tsunami according to scientists. The reasons are yet unknown.