Part of a lecture about tsunamis — what causes these great waves, how

Section 4. You will hear part of a lecture about tsunamis — what causes these great waves, how they behave, and how people are warned. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. WOMAN: Good afternoon. Today we're looking at tsunamis, a word that comes from Japanese and means, quite simply, harbour wave. I want to clear up a misunderstanding at the outset. A tsunami has nothing to do with the wind, ordinary waves are made by wind blowing over the surface, but a tsunami is caused by a sudden movement of the sea floor itself. By far the commonest trigger is an underwater earthquake, though a landslide or a volcanic eruption can produce one too. WOMAN: Here is what makes them so dangerous. When the sea floor jerks upward, it lifts the entire column of water above it, from the seabed to the surface, and that is why a tsunami carries so much more energy than a wind wave, which only disturbs the top few metres. Out in the deep ocean, though, the wave is almost invisible, it may be less than a metre high, and a ship might pass over it without noticing. But it travels at astonishing speed, as fast as a jet aircraft, crossing an entire ocean in hours. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. WOMAN: Everything changes as it nears land. As the wave reaches shallow water, it slows down, and because the water behind it is still racing forward, the wave grows dramatically in height, piling up into the towering wall of water we picture. There is often a strange warning sign, just before it strikes, the sea may suddenly draw back, exposing the seabed, as the trough arrives ahead of the crest, anyone who sees the water retreat like that should run for high ground at once. WOMAN: And a tsunami is not a single wave. It arrives as a series of waves, minutes apart, and, alarmingly, the first is often not the largest, which is why people who return to the shore too soon are caught out. The destruction comes not only from the water rushing in, but from the powerful backwash dragging debris, and people, back out to sea. WOMAN: Finally, warning systems. Because we cannot prevent tsunamis, everything depends on warning people in time. A network of sensors on the ocean floor detects the pressure change as a wave passes overhead and sends an alert to shore. But for coasts very close to the earthquake, there may be only minutes, so the most important warning of all remains a natural one, the ground shaking beneath your feet. In those regions, the earthquake itself is the signal to head inland. That's where we'll stop today. That is the end of Section 4. You now have some time to check your answers.