单项选择题

Quite different from storm waves are the giant sea waves called tsunamis, which derive their name from the Japanese expression for "high water in a harbor". These waves are also referred to by the general public as tidal waves, although they have relatively little to do with tides. Scientists often refer to them as seismic (地震的) sea waves, far more appropriate in that they do result from undersea seismic activity.
Tsunamis often occur in the Pacific because the Pacific is an area of heavy seismic activity. Two areas of the Pacific well accustomed to the threat of tsunamis are Japan and Hawaii. Because the seismic activity that causes tsunamis in Japan often occurs on the ocean bottom quite close to the islands, the tsunamis that hit Japan often come with little warning and can therefore prove disastrous. Most of the tsunamis that hit the Hawaiian Islands, however, originate (起源) thousands of miles away near the coast of Alaska, so these tsunamis have a much greater distance to travel and the inhabitants of Hawaii generally have time for warning of their arrival.
Tsunamis are certainly not limited to Japan and Hawaii. In 1755, Europe experienced a disastrous tsunami, when movement near the Azores caused a massive tsunami to sweep onto the Portuguese coast and flood the heavily populated area around Lisbon. The greatest tsunami on record occurred on the other side of the world in 1883 when the Krakatoa volcano (火山) underwent (经历) a massive explosion, sending waves more than 30 meters high onto nearby Indonesian islands, the tsunami from this volcano actually traveled around the world and was witnessed as far away as the English Channel. It is implied in the passage that tsunamis ______.