THERE are natural disasters that humans can predict. There are others whose timing is difficult to know. What happened in Indonesia last month fell into the latter group. On October 25, a tsunami   caused by a 7.7 magnitude   earthquake in the Mentawai islands of West Sumatra, Indonesia hit shores and damaged whole villages. By last Friday, it had left about 450 dead and forced around 15,000 people to leave their homes. Tsunami is Japanese for "harbor wave". It is usually caused by a sudden rise or fall of part of the Earth's crust   under the ocean. It is most common in the Pacific and Indian oceans. Normally a tsunami is a series of waves. The waves can be very long - as much as 100 kilometers - and be as far as one hour apart. They are able to cross oceans without losing much energy. When the ocean is deep, tsunamis can travel unnoticed at speeds of up to 800 kilometers an hour. They can cross a whole ocean in a day or less. The wave may only be a few meters high but when they are near the shore and reach shallow water, they build up height very quickly. Some witnesses in the Mentawai islands reported seeing up to 6-meter high waves crash   onto the shore. Failed alert system Many countries set up early warning systems in their waters in the hope of giving people enough time to escape to higher ground in the event of a tsunami. But Indonesia's warning system, completed with German aid, stopped working about a month ago because of disrepair  , according to the country's Meteorology and Geophysics Agency. As a result, not a single siren   sounded after the earthquake. However, it was unclear if the sirens could have made a difference because the islands affected were so close to the epicenter   that the tsunami would have reached them within minutes. Which of the following statements about tsunamis is TRUE according to the article?
High-speed tsunamis can be easily detected. Tsunamis can travel across a whole ocean in a few hours. Tsunamis are usually caused by quakes under the ocean. The power of tsunami decreases quickly as it moves toward land.