• Subsea landslides. Along both compressive and passive margins sediments accumulate, sometimes forming cushions that are several kilometers thick. The city of New Orleans, for example, is built on a very thick sediment wedge, that originated from deposits carried by the Mississippi River. Very often, these sediments are poorly lithified, and can slide towards the ocean floor, to form (turbulent flow)= turbidite currents. Deltas do collapse at times. The Congo Delta, for example, spontaneously collapsed at the Miocene/Pliocene time boundary, some 6 million years ago and for unknown reasons. The Agadir Canyon in Morocco is an equally young incision feature, possibly caused by a super-strong earth quake. Its associated turbidite fan reaches far out into the open ocean. Some sub-sea landslides can be triggered by earth quakes, others or spontaneous release of methane from the sediment. The sliding avalanche forms a turbulent, and often high-density flow. Ahead of a turbidite flow, water is compressed and displaced on a massive scale. This can trigger a different kind of tsunami, although it is not very well known what amount of damage this may cause.

• Volcanic eruptions. During volcanic eruptions, the volcano’s rock vibrates and can easily form fractures. Water can penetrate these fractures, and reach areas of hot rock creating overheated, and highly pressurized steam. Several volcanoes at the Indian plate margin have exploded that way – Krakatoa and Tabora are examples that occurred fairly recently, in geologic terms. Some volcanoes are formed by walls of unconsolidated sediments. These become unstable at times and can cause massive landslides, at times.

Which areas of the world might become particularly tsunami-prone in the near future?

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  The Pacific Ocean is the classic terrain of tsunamis. Surrounded by compressive margins with active subduction and strike-slip (lateral movement fault) zones, the Pacific is quivering with seismic life. An area of particular concern is the Pacific Plate Boundary, which reaches from Northern California to Alaska. Recent studies seem to indicate, that stress has been building up along this margin for several hundreds of years, and that a strong sea quake is overdue. Tsunamis could potentially have serious consequences even in remote areas - I wonder what would happen if a strong tsunami would hit the Antartic shelf ice. Massice slabs of ice might become unstable, detach itself from the bedrock, and float into the ocean, leading to a sudden seal level rise.
  


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  The eastern Indian Ocean. The spectacular earth quake witnessed on Dec 26 may have shifted the ocean floor by several tenths of meters. This means, as the examples in California and Turkey have shown, that compressive energy is shifted farther up along the plate margin. Strong earth quakes, with associated tsunamis, will occur along the Andaman and Nicobar margin, but it might take several years, or hundreds of years before the next huge earth quake is due. A tsunami in the Bay of Bengal could endanger millions of people living in the densely populated Ganges Delta area.

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  The Atlantic margins. Two volcanoes of the Canary Island group appear to have unconsolidated and highly unstable flanks, that could yield massive landslides. Cities such as Casablanca, New York, Miami or even Rio de Janeiro could be threatened by tsunamis, if these volcanoes were to collapse into the Atlantic Ocean.

• The Southern Mediterranean. Coastal areas, in particular Algeria, have seen many violent earth quakes in recent history. Areas of the huge Nile Delta might prove unstable- several recent fault escarpments are seen on the sea floor. A tsunami originating along the North African margin could endanger dense populated areas both along the North African and the Italian/Greek coast lines.
  


• The Texan and Louisiana Gulf coast. Massive submarine landslides have occurred along this margin throughout the Tertiary, given the accumulation of massive, unstable sediments deposited at the mouth of the Mississippi River. A big, submarine landslide might cause a tsunami with the potential of hurting coastal cities such as New Orleans, or Galveston.
  

What can be done to prevent tsunamis? Nothing! Tsunamis do happen. Improved tsunami warning systems, however, could help evacuate some areas before the tsunami strikes. It makes sense when a targeted coastline is located several hours away from the tsunami's origin. Alerting the population early enough can help the evacuation of people from the vulnerable coastlines to the safety of higher grounds. Areas located close to an epicenter such as the 26 December Aceh, however, are doomed and will be doomed in future events – there won't simply be enough time to alert the coastal population.

The author holds a PHD in sedimentology and has worked 20 years in the oil and gas exploration business. He has been teaching deepwater geology courses in Australia, Norway and Egypt.