Such an earthquake is projected to leave up to about 8 million people stranded in the metropolitan area.

Because railways and roads are expected to be halted in such an emergency, the ministry is considering using the Arakawa, Tamagawa and other rivers as urgent transportation routes, the sources said. It intends to start full-fledged transportation drills based on the plan as early as fiscal 2018.

According to an estimate by the government’s Central Disaster Prevention Council, it would take JR and private railway companies about one month, and subway companies about one week, to restore services to normal following a major quake beneath Tokyo.

Roads in Tokyo’s 23 wards are expected to take more than one month to reopen because liquefaction would disrupt many sections.

Up to 8 million people are projected to have difficulties returning home in the metropolitan area after a major quake beneath Tokyo, exceeding the 5.15 million people who faced such difficulties after the 2011 Great East Japan Earthquake.

The ministry plans to have people board ships at docks along rivers and transport them toward their homes. With an eye on the 2020 Tokyo Olympics and Paralympics, the ministry is also considering water routes from river mouths to Haneda Airport to allow foreign tourists to return to their home countries with ease.

The ministry intends to seek cooperation from private companies operating cruise services in Tokyo Bay and elsewhere, or secure ships for emergencies from among the vessels it owns.

Government-run docks exist at nine locations along an about 27-kilometer stretch of the Arakawa river — which flows through Saitama Prefecture and Tokyo — starting from its mouth.

The locations include Toda, Saitama Prefecture and Adachi Ward, Tokyo. The docks are used mainly for operating tourist ships and yakatabune pleasure boats.

Three docks exist along an about eight-kilometer stretch of the Tamagawa river, which flows through Kanagawa Prefecture, Tokyo and other areas, starting from its mouth.

The ministry has already formulated a plan to carry necessary equipment and food for restoration activities in ships via the Arakawa river. It is therefore considering expanding the scope of the plan to include transporting people.

The ministry wants to expand the plan so that people in need could be transported along the Tamagawa, Edogawa and Tsurumi rivers, in addition to the Arakawa river.

The ministry says the possibility of a tsunami surging upriver from the sea, as was observed during the 2011 earthquake, would be low in the case of a quake with a focus directly under Tokyo. Thus, rivers could become effective transportation routes.

A ministry official in charge said, “Having the option of telling people, ‘Head for docks of rivers, not central Tokyo’ when a quake with a focus directly below Tokyo occurs would surely reduce the number of people facing difficulties returning home.”