The cascade of catastrophe that has befallen Japan highlights the vulnerability of megacities to disaster, including fallout from a nuclear accident, say experts on urban risk.

Greater Tokyo, home to 35 million people, mostly escaped the devastation wrought by the March 11 9.0-magnitude earthquake and tsunami that swept the coast of northeastern Honshu.

Tokyo is also, according to the International Atomic Energy Agency (IAEA), beyond the reach of the radioactive plume emanating from the crippled Fukushima nuclear plant – at least for now.

But what if the quake had struck nearer the city, like the Great Kanto earthquake of 1923? Or if the tsunami had occurred opposite Tokyo Bay? Or if the nuclear plant that had been crippled was an ageing facility at Hamaoka that lies 200 kilometres south, and thus upwind, of the capital?

“This incident puts in clear evidence the fragility of megacities in every aspect: physical, social, economic and ecological,” said Fouad Bendimerad, head of the Earthquakes and Megacities Initiative, an international scientific organisation that analyses disaster risk.

“Many previous assumptions about the resiliency of megacities will be put into question.”

Tokyo is hardly the only supercity facing multiple threats from natural and manmade disaster.

For Chris Ipsen, head of the Emergency Management Department for the city of Los Angeles, the drama unfolding in Japan is a reminder of how things might go horribly wrong in a crowded metropolis.

“It is something we definitely relate to. Obviously we have a lot of earthquake threat, it’s our number one hazard. We face tsunami hazard as well,” he said by phone.

The scenario that most haunts emergency planning experts is how to evacuate millions of people from a city when key infrastructure is down.

“The major problems in the evacuation of a megacity where you have a disaster are about transport, access roads, clogging and energy supplies that stop working,” said Helena Molin Valdes, deputy head of the UN International Strategy for Disaster Reduction.

All the experts interviewed agreed that cultural factors weigh heavily and Japan – especially Tokyo – is as well prepared for disaster as any society can be.