For modern Olympic host cities, the twin threats of terrorism and financial ruin constantly loom.

But spare a thought for Tokyo 2020 organisers, who face the added risk of a devastating earthquake or tsunami.

Japan is already stepping up efforts to reassure top athletes and hundreds of thousands of visitors when they flock to Tokyo for the Games, which begin on July 24, 2020, that safety will be paramount.

Tokyo and its surrounding areas sit precariously at the junction of shifting tectonic plates and have suffered violent quakes in the past, notably the 1923 Great Kanto Earthquake that killed more than 100,000 people.

In 2011, a giant quake and tsunami which killed more than 18,500 people in northeast Japan -- though the majority died as a result of the latter rather than the earthquake itself.

The disaster left millions in the Tokyo area to cope with rolling power blackouts and a wide-scale breakdown of transportation.

It also triggered a nuclear meltdown at Fukushima, 240 kilometres (150 miles) northeast of Tokyo.

Naoshi Hirata, head of the Earthquake Prediction Research Center at the University of Tokyo, believes a powerful quake before the 2020 Olympics could even cripple Tokyo's ability to host the event. 

"If an earthquake directly hitting the capital caused catastrophe before the 2020 Tokyo Olympics, it could be serious enough to prevent the Japanese economy from functioning properly," Hirata told AFP.

"We would have little option but to seek an alternative Olympic host."

Tokyo and neighbouring Yokohama topped a list of global quake or tsunami-risk metropolitan areas, according to a 2013 survey by insurance firm Swiss Re.

Tokyo 2020 organisers insist Olympic facilities have all cleared the nation's strict quake-proof building standards.

"I cannot think of any city that is completely safe," admitted Tokyo 2020 chief executive officer Toshiro Muto.

"But Tokyo is one of the best-prepared for natural disasters." 

Many venues are reinforced by shock-absorbing technology, they say, such as the $320 million Ariake volleyball arena, which features giant rubber cushions.

"Japan is vulnerable to earthquakes, but building standards are the strictest in the world," said Satoru Sunada, a Tokyo government official in charge of Olympic facilities.

"We will do everything possible to prepare venues so that we can safeguard people's lives in Tokyo in the event of a major earthquake."

- Evacuation manual -

Sunada added that waterfront facilities, including the Olympic village, are built on embankments or protected by sea walls high enough to withstand a tsunami of almost two metres (6.5 feet) -- the maximum height predicted inside Tokyo Bay.

But Kojiro Suzuki, a tsunami researcher at Port and Airport Research Institute -- where staff study tsunamis in a huge wave pool -- warned there is a limit to what preventative measures can do.

"The damage could well be beyond people's anticipation," he said. "We should not fully rely on breakwaters. Evacuation is important."

The Tokyo metropolitan government is busy drawing up a multilingual evacuation manual with instructions for overseas Olympic visitors due to be ready early next year, while disaster awareness exercises have also begun.

These include earthquake simulation systems, which replicate the shaking sensation and people can walk into, and also drills instructing people where and how to take shelter.

Toshitsugu Fujii, a retired professor at the University of Tokyo and leading expert on quakes and volcano studies, warned: "People who are not used to (earthquakes) may get into a panic even at minor jolts." 

A sizable quake occurring directly beneath Tokyo is a lingering fear but regional tremors can also affect the Olympics.

A potential Nankai Trough mega-quake, which could affect large areas of western Japan and threaten an estimated 300,000 lives, could also deal a crippling blow to the nation's economy, experts predict.

But it is not only earthquakes that worry Olympic organisers -- a volcano could also pose a potential threat to the Games.

In 2014, following the eruption of Mount Ontake which killed 63 people, scientists warned Japan's highest volcano Mt. Fuji was also at increased risk of doing so.

Situated some 100 kilometres west of Tokyo, it last erupted in 1707 covering the capital with a thick layer of ash. 

Experts warn that if it happens again, the financial hit would be in excess of 2.5 trillion yen ($22.5 billion).

"It is difficult to predict precisely but it would be no surprise if Mt. Fuji erupts in the near future," Fujii said. "We have to keep that risk in mind."

Source: AFP