On a dark winter's night in January 1700 a tsunami struck Japan. It flooded fields, swept away villages for miles inland and cost many lives. Even as far back as 1700 the Japanese had made the connection between earthquakes and Tsunami, but this time there was no earthquake, no warning to allow the people time to evacuate to higher ground. The tsunami was called the 'orphan tsunami' because it had no 'parent' earthquake. For more than 300 years the origin of the orphan tsunami remained a mystery.

In the 1980s Hiroo Kanamori and Tom Heaton published a paper that said the 1700 tsunami was caused by a massive rupture of the Cascadia fault line that runs off the west coast of the United States from California to Vancouver. In 1987 Brian Atwater studied soil samples far inland across the length of the fault and discovered that the United States had also suffered a tsunami at the same time as the Japanese. He concluded that Kanamori and Heaton were correct, a massive earthquake had sent a tsunami out from the source of the quake inundating the coasts on both sides of the Pacific.

Recent studies by researchers at the University of Pennsylvania has concurred on the findings of previous studies.

The Cascadia Fault is a subduction zone, an area where one of the tectonic plates is forced underneath the plate it collides into. In the case of Cascadia the Juan de Fuca Plate is moving under the North American Plate. These plates have been locked together since 1700, 313 years. A particular feature of the Cascadia Fault is that it doesn't produce small quakes, it remains still and silent until the pressure gets too much and one of the plates rise to a mega-thrust earthquake of massive force. These quakes can be compared to the Indonesian quake of 2004 and the Japanese quake of 2011.

The Cascadia fault is long, very long, just over 800 miles (1300km) in length. Based on the findings of the scientists the tsunami caused by the 1700 event moved inland for more than 60 miles, wiping out everything in its path. Of course in 1700 Seattle wasn't there, neither was Vancouver, San Francisco, Portland or any of the other cities and metropolitan areas currently occupied by millions of people.

The Cascadia, courtesy of its length will give rise to a long quake if the whole fault 'unzips' at the same time, as it's believed to have done in 1700. The first P waves will travel the length of the fracture in a minute or two; the S waves that follow, the ones that cause the real damage are slower and will cause shaking and movement for about five minutes though their speed can vary depending on the rock they are traveling through.

Any buildings not built to withstand earthquakes will collapse. Depending on the duration and magnitude of the quake it cannot be assumed that even 'hardened' buildings will remain standing. Many areas have sedimentary basins under urban sprawl and this amplifies the waves and the damage they cause.

Within approximately 20-30 minutes the tsunami will roll in. Unlike normal waves which have a few yards between them the tsunami waves can have hundreds of miles between them. The entire wall of water displaced when the plates slip hurtles outwards in all directions from the epicentre of the quake. As the waves approach the continental shelf the water at the back of the wave starts to catch up with the water at the front of the wave, which has slowed down as it moves up the incline that marks the start of the continental land mass. Its this that allows the water to build up and give the tsunami its characteristic 'wall of water' appearance.

The water will just keep on coming, flowing forward taking almost everything in its path with it. It may or may not be followed by more waves, there is no way to know if it will be a solo wave or a series until it happens. There is also no way to know in advance how deep the water will be. The tsunami caused by an earthquake in Lituya Bay Alaska in 1958 reached 1720 feet.

Lituya Bay is an extreme example, an un-survivable example unless you happen to be Howard G Ulrich or his son. The Indonesia earthquake on boxing day 2004 has been estimated to have had a wave height of between 30-50 feet.

So what do you do with your 20-30 minute warning? Obviously you want to get as high as possible. This is often easier said than done. In a city littered with debris, the dead and dying and glass falling from skyscrapers without warning getting out is going to pose many problems. Highways will be buckled and impassable and there is a great danger from gas main explosions and fires.

Some countries are investing in vertical evacuation centres. High platforms/shelters on stilts that allow the water to pass through the base of the structure without putting extra stresses on masonry that maybe already damaged from the earthquake. FEMA have issued a community guide detailing where such structures will be built / are being built along the West Coast of the United States.

If you don't have access to a vertical evacuation shelter, or you prefer not to be at the mercy of FEMA, multi-story parking lots with open sides can offer some possibility of escape. The water is able to flow through the building and out the other side, reducing the chances of collapse due to the weight of the water pressing against it. Be prepared to stay in such a place for a considerable time, help and/or escape is not going to be coming fast. Carry as much food, water and emergency equipment as you can without the weight slowing you down. Think Katrina multiplied by 1000.

It can take anything from a few hours to many days for the floodwaters to subside and even when it does the emergency is far from over. Disease and death with be rife and you should head out of the area as soon as is humanly possible to give yourself the best chance of survival.

Cascadia is locked, loaded and ready to fire; it's not a case of if, but a case of when. Sadly with so many variables, an earthquake tsunami combination is almost impossible to prepare for adequately. The only sure-fire way to survive is not to be in the area in the first place.