The organisers of next year’s Olympic Games in Tokyo have decided to cut the number of spectators for the sailing events by a third so they can be quickly evacuated to higher ground in the event of a tsunami.

The 2020 Tokyo Olympics organising committee had initially planned to allow up to 5,000 people to watch the sailing events off Enoshima island, just south of the Japanese capital, according to the public broadcaster NHK.

They lowered the number to 3,300, however, after discussing tsunami evacuation measures with local authorities, NHK added.

Japan is one of the most seismically active countries in the world. Its Pacific coast, where the sailing events will take place, is susceptible to powerful tsunamis triggered by earthquakes.

In less than two months’ time the country will mark the eighth anniversary of the March 2011 triple disaster, in which more than 18,000 people were killed by a huge tsunami along the north-east coast.

While there is disagreement on the likelihood of Tokyo being struck by a destructive earthquake in the foreseeable future, government experts believe there is a high probability of a major earthquake occurring in the Tokyo metropolitan area within the next 30 years.

A powerful offshore quake could trigger a large tsunami that would reach the shore within minutes, NHK said, citing government projections.

The decision to cut spectator numbers at Enoshima is not the only adjustment organisers of the Olympics, which will run from 24 July to 9 August 2020, have had to make in view of Japan’s vulnerability to powerful natural forces and extreme weather.

The Yomiuri Shimbun reported last week that organisers had decided to shift the start time for the men’s and women’s marathons to 6am – an hour-and-a-half earlier than originally planned – to avoid the health risk to runners and spectators from the intense heat later in the day.

The men’s 50km walk will start two hours earlier, at 5.30am, the newspaper said, citing sources close to the Games.

In July last year a record 133 people died from heatstroke or heat exhaustion in Japan, with thousands of others admitted to hospital. The country experienced its highest temperature on record on 23 July, when the mercury reached 41.1C in Kumagaya, a city north-west of Tokyo. Several other cities recorded temperatures of about 40C.