How did anyone survive the Christchurch gas explosion?

A devastating gas blast in a Christchurch suburban street. Six people were injured, one critically, in the Northwood blast at 10.14am on Friday, no-one was killed.

*Mercury Rx (see:'other functions') conjunct Vulcanus (intense power, might forces). Mercury enters Cancer (retrograde). Neptune (liquids, gases).quincunx Zeus (fireplaces, fuels)  bi-quintile Mars (heat, destructiveness). Venus (appreciative)  trine Deucalion (magic (miracle)) sextile Mors-Somnus (conscious, aware of immediacy of life).

*The blast caused roof tiles and insulation to rain down across the neighbourhood - live updates

“I’m just really thankful no-one lost their lives, it’s a miracle when you look at that house.

Nearby resident, Brad Culver, was astounded the five occupants of the obliterated Marble Court house survived.

"It's a miracle when you look at that house," he said.  

While investigators will soon sift through the rubble, the current working theory is that an LPG leak linked to the occupants' gas fire led to the explosion.

Although LPG has an odour, it could have built up in the house without the occupants noticing. The gas could then have been ignited by something as simple as a hot lamp.

How could the occupants have survived when 20 houses nearby were damaged and houses 100 metres away had doors blown out.

There will be many theories until further facts are available.

 Mining blast expert Professor David Cliff, of Queensland University, who gave evidence at the Royal Commission into the Pike River mine disaster, said he could only speculate.

The occupants survival could have been due to being a distance away from the epicentre of the blast or protection from walls or even furniture, he said.

The blast, which would follow a path of least resistance, might also have travelled away from the victims, he said.

Blast waves could gather energy as they surged from the centre of the explosion so could continue to do damage a distance away.

Fast-moving objects blown around by an explosion, not to mention heat and fire, can cause a great deal of terrible injury and damage but the most dangerous force in an explosion is the blast wave, experts say.

In a blast, a large amount of air is pushed outward in a very short time.

Ordinarily the air around us puts about 96 kPa pressure on the body. During an explosion, an even sphere is created around the explosion and no new air can get in. What's left is a partial vacuum. The body is forced to contend with a hit of increased pressure and then a near vacuum in a split second.

A sudden change of pressure – anywhere between 137kPA and 275kPa – can be fatal.

Most explosion injuries, experts say, are caused by flying objects but the blast wind can also throw victims against walls and floors.

Lower-level explosions could be survivable, but they can cause serious internal injuries.

The ears are the first organs to suffer and then the lungs, filled with tiny air sacs, can be ruptured and bleed. The bowels, filled with liquid and gas, are susceptible to tearing.

Gas explosions are rare in New Zealand but more common in areas of the world where gas is piped into homes from a central producer.

As recently as May, a gas explosion in a suburban house in southern Indiana killed one person and injured two others.

But people do survive gas blasts in other "miracle escapes".

In February a home in Pontiac, near Detroit, was demolished after a gas leak that lead to an explosion. Scenes of the damage look similar to the Northwood site.

The 60-year-old occupier was blown into the street and found thanking God he was alive.

 Looking at the wreckage, his neighbour Greg Dixon said:

"How does anybody survive that?"

Another example is Jimmy Brown, an English pensioner who in 2007 survived a gas explosion in his cottage, despite it being reduced to rubble. Debris from the blast also damaged 15 vehicles.

Explosives experts believed Brown escaped with only cuts and bruises because he was almost directly at the centre of the blast.

If Brown had been further away from the centre of the explosion, the full brunt of the shockwave would have killed him, one expert said.

Guardian science writer Ian Sample said the effect had been noticed by inspectors investigating the 1974 explosion at a chemical plant in Flixborough near Scunthorpe that killed 28 workers and injured 36.

"Inspectors noticed that away from the centre of the blast, lampposts had been twisted and flattened. But near the centre of the explosion, they were still standing upright."

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Source: i.stuff.co.nz