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PARIS - THE magnitude 7.0 earthquake that struck Haiti in January, killing a quarter of a million people, also unleashed a string of tsunamis on the country's western coast, scientists reported on Sunday. Several waves measuring up to 75cm were caused by a combination of earth movement and coastal landslides. They included banks of sediment on the sea bed, accumulated at river deltas, which were displaced by the shock and unleashed the waves as they moved, the study says. Waves were reported west, north and south of the epicentre, which was within a few kilometres of the surface on the Enriquillo-Plaintain Garden fault, on the boundaries of two microplates in the Caribbean. Such 'slide-generated' tsunami are rare, but their risk is underestimated, according to the paper, published online in the journal Nature Geoscience. 'Even modest, predominantly strike-slip earthquakes can cause potentially catastrophic slide-generated tsunami,' it says. A strike-slip earthquake occurs when one side of the fault moves along horizontally and in parallel to the other side of the fault, rather than down or up. Vertical displacement, especially of the seabed, is best known for creating tsunamis. The study was led by a team led by Matthew Hornbach of the University of Texas at Austin. The January 12 quake was in tectonic terms a surprisingly complex affair, according to new research. Two-thirds of the movement was strike-slip, and a third was a thrust, or upward, movement. The quake inflicted huge damage to the capital, Port-au-Prince, injuring 300,000 people and leaving 1.5 million homeless. -- AFP

The causes of landslides are usually related to instabilities in slopes. It is usually possible to identify one or more landslide causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time. Landslide causes are listed in the following table, and include geological factors, morphological factors, physical factors and factors associated with human activity. Causes may be considered to be factors that made the slope vulnerable to failure, that predispose the slope to becoming unstable. The trigger is the single event that finally initiated the landslide. Thus, causes combine to make a slope vulnerable to failure, and the trigger finally initiates the movement. Landslides can have many causes but can only have one trigger as shown in the next figure. Usually, it is relatively easy to determine the trigger after the landslide has occurred (although it is generally very difficult to determine the exact nature of landslide triggers ahead of a movement event). Occasionally, even after detailed investigations, no trigger can be determined - this was the case in the large Mount Cook landslide in New Zealand 1991. It is unclear as to whether the lack of a trigger in such cases is the result of some unknown process acting within the landslide, or whether there was in fact a trigger, but it cannot be determined. Perhaps this is because the trigger was in fact a slow but steady decrease in material strength associated with the weathering of the rock - at some point the material becomes so weak that failure must occur. Hence the trigger is the weathering process, but this is not detectable externally. In most cases we think of a trigger as an external stimulus that induces an immediate or near-immediate response in the slope, in this case in the form of the movement of the landslide. Generally this movement is induced either because the stresses in the slope are altered, perhaps by increasing shear stress or decreasing the effective normal stress, or by reducing the resistance to the movement perhaps by decreasing the shear strength of the materials within the landslide.

A tsunami is a series of waves generated in an ocean or other body of water by a disturbance such as an earthquake, landslide, volcanic eruption, or meteorite impact. The picture at the left shows how an earthquake can generate a tsunami in the overlying water.

he huge waves moved quickly across the ocean before hitting land. The waves caused a huge amount of destruction to towns and resorts along the coast. Over 150,000 people were killed as the tsunami waves hit the coasts of Indonesia, Malaysia, Myanmar, Thailand, Bangladesh, Sri Lanka, India, and the east coast of Africa. There was very little time to sound alarms and get people away from the coast. The more you know about how the Earth works, the more you can keep yourself and others as safe as possible from natural disasters. We at Windows to the Universe would like to congratulate Tilly Smith, a 10-year-old British girl who used her knowledge about how tsunamis works to save 100 people during the event. Tilly learned about tsunamis at school two weeks before the tsunami hit where her family was vacationing in Phuket, Thailand. She saw the water drawing out quickly from the shore and remembered that this can happen before a tsunami wave hits the coast. Thanks to Tilly, her mother, and the hotel staff, everyone was cleared off the beach minutes before the wave arrived. Last modified May 21, 2008 by Lisa Gardiner.