Experts warn a mega quake will trigger a devastating 100ft-high tsunami, cause carnage for seven million people and destroy 140,000 square miles. Here they share the key actions needed to improve your odds of survival

Not far off the coast of the Pacific Northwest, a crack exists in the Earth’s crust. This fissure, along a fault line that stretches for almost 700 miles, will herald what experts say will be the worst natural disaster to hit North America.

For this is the ‘Big One’ – the long-anticipated earthquake that could one day bring devastation to the continent. Worst-case scenarios suggest it could devastate 140,000 square miles of the US and affect at least seven million people, many of them swept away by the tsunami that would come in the wake of the quake.

It would make the earthquake that struck off the coast of northern California on Thursday – a magnitude 7 on the Richter scale – seem a mere tremor.

At a magnitude 9, it’s been predicted the eruption of the Cascadia Subduction Zone – a so-called megaquake – could kill 14,000 people and severely damage or destroy a million buildings.

Losses to the local economy would exceed £100 billion – while the impact to Wall Street and economies around the world is impossible to calculate.

Experts estimate there is a one-in-ten chance that this devastating earthquake will come within the next 50 years. That may sound remote, until one realises that one-in-ten events happen all the time.

Seismologists and disaster planners have painted a cataclysmic picture of how the Cascadia quake would wreak havoc in a huge area that includes the states of Washington and Oregon, northern California and the Canadian province of British Columbia.

Earthquakes happen where tectonic plates collide. In the case of the Cascadia Subduction Zone, the heavier oceanic plate is being forced downwards (subducting), beneath the continental plate, and a sudden slip between the two will release a vast amount of energy in the form of seismic waves.

Dogs will sense it first, barking madly as their senses detect so-called ‘primary’ seismic waves travelling deep beneath the earth. Humans will feel a sudden but gentle jolt. Thirty to 90 seconds later, the slower-travelling ‘secondary’ waves will hit – and these will be far more violent. Objects will fall off shelves and pictures clatter from walls.

Then, in a series of strong pulses, the violent shaking will begin. For at least three minutes, people will have difficulty even standing while their ears are filled by a loud, rumbling sound as buildings sway and creak. Anything not bolted down will move several inches at a time.

Basement water boilers could topple over and start fires. The floors of older, multi-storey buildings will start to collapse in on one another, crushing anyone inside.

The prolonged shaking will set off endless landslides – one estimate says there will be 30,000 in Seattle alone. In wetter areas, the shaking will create ‘liquefaction’, where once solid ground turns to fluid. In Seattle, which has 755,000 people – some 15 per cent of the city is built on ‘liquefiable’ land.

The worst place to be, however, will be close to the coast. There the movement of the ground will be even more violent, even tossing people into the air.

Within just 15 minutes of the first tremors, a tsunami which has been estimated at 100ft high will hit.

Some 70,000 residents – many of them retirees – would have no option but to try to get to higher land as churning seawater floods the towns. Many of the slower ones won’t stand a chance. Anyone who ends up in the water – a maelstrom of fishing boats, cars, telephone poles and ripped-up trees – will be unlikely to get out alive.

When, some 12 to 24 hours later, the water starts to recede, the land will have permanently dropped by more than 3ft (1m).

Far inland, powerful aftershocks will continue for at least a day, preventing people from going back into buildings and complicating the nightmarish recovery.

Many emergency service members will be among the casualties while damage to some 7,000 road bridges could cut off millions of people from help. A sea rescue by the US Navy won’t be able to reach the shattered coast for a week.

It’s no wonder disaster planners are urging people to keep emergency supplies that will last them at least two weeks.

FEMA, the US government’s disaster management agency, calculates 85 per cent of ports and drinking water plants would be seriously damaged or destroyed. That’s along with 77 per cent of fire stations and two-thirds of airports, hospitals, railways and schools. Gas and electricity networks would be crippled.

Life wouldn’t take months to return to normal – but years.

And seismologists fear this event could even set off other lethal quakes, including along California’s notorious San Andreas fault line, which snakes down the Golden State and covers San Francisco, Los Angeles and San Diego.

In 1906, San Francisco was flattened by an earthquake that killed more than 3,000 people and destroyed more than 80 per cent of the city. However, the Cascadia megaquake – named after the Cascade mountain range – would bring carnage on a truly biblical scale. In terms of energy, it is thought it would be 33 times more powerful than the 1906 disaster.

The last time the ‘Big One’ struck was in 1700, when the Pacific Northwest was still inhabited by Native American tribes who kept no written records.

Seismologists only discovered in the 1980s that it had happened because evidence of the tsunami it caused was detected in Japan, almost 5,000 miles away.

Meanwhile, a spate of recent smaller earthquakes up and down the Pacific coast – including the quake on Thursday – has focused fresh attention on the possibility of the Big One happening soon.

At an enormously powerful magnitude 9, the Cascadia quake would shake the ground for three to five minutes.

Japan suffered a magnitude 9 – the Great Tohoku Earthquake – in 2011. Lasting six minutes, it created a 130ft-high tsunami that flooded more than 1,200 miles of coastline. The quake and tidal wave together killed 18,500 people.

Japan, however, is far better-prepared for earthquakes than the US or Canada. Estimates of the impact of the Cascadia megaquake have varied – but that, seismologist Chris Goldfinger tells me, is because it would make a huge difference precisely where and when the eruption takes place.

Professor Goldfinger, the leading scientist researching this quake, says in terms of casualties, the crucial variable would be whether the Cascadia megaquake comes on a working day or a weekend.

If it occurs at the weekend, people are generally going to be at home. In the timber-rich Pacific Northwest, that usually means houses with wood frames which are more likely to stay standing.

On a weekday, however, far more people are likely to be in concrete office buildings. Local construction standards didn’t take into account a magnitude 9 earthquake until 1994, meaning any buildings older than this are likely to collapse. These include hospitals and some 3,000 schools, which are made of unreinforced masonry such as bricks and concrete.

These materials, which are not strengthened to resist large earthquakes, will collapse quickly.

And yet progress to try to prevent catastrophe remains sluggish. Retro-fitting buildings to better withstand quakes is expensive and even now continues to be resisted by developers and homeowners.

Robert Ezelle, director of emergency management at Washington state’s military department, last month said preparing the region for the Big One was ‘like trying to drain an Olympic-sized swimming pool with a teaspoon’. He predicts it will be ‘the worst natural disaster our country has ever seen’.

So, when it comes to preparations, the situation looks dire.

The Pacific Northwest has an earthquake early-warning system, but it is basic compared to Japan’s, which automatically shuts down railways and power stations, opens the doors of lifts and fire stations and alerts hospitals to halt surgeries.

Instead, Americans and Canadians who have subscribed to the early-warning service on their phones will receive an alert and hear an automated voice saying ‘Earthquake!’ This will be followed by instructions to ‘drop, cover and hold on’.

This is the official advice on what to do as a quake hits: drop down wherever you are, cover your head – preferably under a sturdy table or desk – and hold on there until the shaking stops.

It’s not the perfect advice for every situation, admits Professor Goldfinger. Anyone on the ground floor of a vulnerable building would be better advised to dash outside and crouch where they won’t be hit by falling trees or poles. It’s almost always better to be outdoors during an earthquake, he stresses.

How much time people have to prepare depends heavily on how close they are to the quake. And understanding what to do in those precious moments before the shaking really starts can be the difference between survival and death.

The big cities, where so many people in the Cascadia danger zone live, are all much nearer to the northern end of the fault line. If the rupture starts there, before spreading along the full length of the line, they may have no more than 30 seconds. But if it starts on the southern end, they could get up to a minute and a half while the seismic waves travel northwards.

‘That will make a difference for a lot of people,’ says Professor Goldfinger, who experienced the 2011 magnitude 9 quake when he was in Japan.

And the destruction could spread. Professor Goldfinger says the evidence is ‘quite strong’ that the 1700 Cascadia eruption triggered a quake on the San Andreas fault as far south as what is now San Francisco.

He only wishes there were photos of the last time the ‘Big One’ struck, as there are of the 1906 San Francisco disaster.

‘It’s an uphill battle to get people’s attention,’ he says – even though another megaquake of this scale is ‘a certainty’. Chillingly, he adds: ‘It’s the most airtight case in science that I know of.’