Underwater landslides can cause tsunami as can onshore landslides which slump into the ocean.
The 1998 tsunami in northern Papua New Guinea was caused by an earthquake that is believed to have triggered an undersea landslide.
Landslides can happen on the seafloor, just like on land. Areas of the seafloor that are steep and loaded with sediment are more prone to undersea landslides, such as the edge of the continental slope.
When an undersea landslide occurs (perhaps after a nearby earthquake) a large mass of sand, mud and gravel can move down the slope. This movement will draw the water down and may cause a tsunami that will travel across the ocean.
As the tsunami moves across the open ocean it is almost undetectable on the ocean surface.
In this example, the tsunami waves are only about half a metre high but have a wavelength of 200 kilometres. Travelling at speeds of up to eight or nine hundred kilometres an hour (the speed of a commercial passenger jet), it will take each wave about 15 minutes to pass a slow moving ship.
As the tsunami leaves the deep water of the open ocean and approaches the shallower waters near the coast, it slows down and may grow in height depending on the shape of the seafloor.
A tsunami that is unnoticeable by ships at sea may grow to be several metres or more in height near the coast.
Our example tsunami is now 1.5 metres high with a wavelength of 100 kilometres and is moving at about 400 kilometres an hour.
Depending on whether the first part of a tsunami to reach the shore is a crest or a trough, it may appear as a rapidly rising or falling tide, and in some cases the tsunami may appear as a series of breaking waves. People near the beach may also hear a roaring sound, like an approaching train.
In this example, the first crest of our tsunami arrives without warning and inundates the beach and low lying land causing extensive damage.
After the first wave, the water will draw back and then the second and third waves will repeat the process at 15 to 20 minute intervals. The first wave may not be the biggest. Reefs and offshore islands may help to protect the coast from the devastating effect of a tsunami.
Landslide generated tsunami animation