it produced small but measurable tsunamis along the western coasts of North and South America, typically around . At Manzanillo, Mexico, a crest-to-trough tsunami was measured. As well, the tsunami was large enough to be detected in Vancouver, which puzzled many scientists, as the tsunamis measured in some parts of South America were larger than those measured in some parts of the Indian Ocean. It has been theorized that the tsunamis were focused and directed at long ranges by the mid-ocean ridges which run along the margins of the continental plates. Early signs and warnings Despite a delay of up to several hours between the earthquake and the impact of the tsunami, nearly all of the victims were taken by surprise. There were no tsunami warning systems in the Indian Ocean to detect tsunamis or to warn the general population living around the ocean. Tsunami detection is not easy because while a tsunami is in deep water, it has little height and a network of sensors is needed to detect it. Tsunamis are more frequent in the Pacific Ocean than in other oceans because of earthquakes in the "Ring of Fire". Although the extreme western edge of the Ring of Fire extends into the Indian Ocean (the point where the earthquake struck), no warning system exists in that ocean. Tsunamis there are relatively rare despite earthquakes being relatively frequent in Indonesia. The last major tsunami was caused by the 1883 eruption of Krakatoa. Not every earthquake produces large tsunamis: on 28 March 2005, a magnitude 8.7 earthquake hit roughly the same area of the Indian Ocean but did not result in a major tsunami. The first warning sign of a possible tsunami is the earthquake itself. However, tsunamis can strike thousands of kilometres away where the earthquake is felt only weakly or not at all. Also, in the minutes preceding a tsunami strike, the sea sometimes recedes temporarily from the coast, which was observed on the eastern earthquake rupture zone such as the coastlines of Aceh, Phuket island, and Khao Lak area in Thailand, Penang island of Malaysia, and the Andaman and Nicobar islands. This rare sight reportedly induced people, especially children, to visit the coast to investigate and collect stranded fish on as much as of exposed beach, with fatal results. However, not all tsunamis cause this "disappearing sea" effect. In some cases, there are no warning signs at