Sam Melville, of Broad Bay school, asks :-

How does an earthquake cause a large tidal wave and why are they called 'tidal' waves or tsunamis if they are actually caused by tremors?

Ross Vennell, an oceanographer at the University of Otago, responded.

The tremor you feel from a distant earthquake does not cause tsunamis and not all earthquakes cause tsunamis. Tsunamis are usually caused by catastrophic vertical movements of the ocean floor, either by rapid uplift along a fault line or by underwater 'landslides' (avalanches) of mud and sediment. These 'landslides' are often triggered by earthquakes. A large meteor striking the ocean would also cause a tsunami.

Rapid vertical movements due to earthquake uplift of the seafloor or landslides can raise or lower a several kilometer thick layer of ocean above the sea floor. The oceans raised or lowered surface spreads outwards forming the crest or trough of a tsunami - similar to the rings of waves radiating outwards from a rock tossed into a pond. Tsunamis travel fast, up to 800kmph in the deep ocean, where they are usually less than 1m high. Near the coast, tsunamis slow down and can grow to be many meters high. A tsunami may have several crests and troughs, with crests arriving around 10-20 minutes apart. Sometimes a trough arrives first, causing the sea to recede over several minutes, leaving fish and boats stranded. So if you see this, get to high ground or to the top of a large tall concrete building fast!

When they hit the coast, tsunamis dont appear as the large towering walls of water that you might see in movies, but as rapid surges in water level which occur over several minutes. I suspect it is this surging which caused then to originally be called tidal waves, because they were like a rapid tidal change in sea level. Tsunamis are not caused by the tidal forces. Their old name, tidal waves is gradually fading out as we adopt their Japanese name tsunami, which literally means 'harbour wave'.