Osaka said Thursday a massive temblor could trigger tsunami that could flood an area of 9,394 hectares in the prefecture, three times more than earlier estimated by the central government.

Tsunami 30 cm high could even reach busy downtown streets around Osaka Station in Kita Ward, Osaka, the prefecture said.

A massive earthquake is expected to occur in the Nankai Trough off the Pacific coast of central and western Japan, triggering waves of up to 5 meters.

The Cabinet Office last August estimated multiple tsunami could flood a total area of 3,050 hectares in Osaka Prefecture, with seawalls working to resist the waves. But the prefecture took into account the possibility of seawalls sinking due to shaking and ground liquefaction and estimated the tsunami could flood a far wider area.

"We have to be prepared for the worst case, learning lessons from the March 2011 Great East Japan Earthquake," a prefectural official said.

The prefecture will revise its regional disaster reduction plan to enhance measures for wooden houses, underground malls, high-rise buildings and petrochemical complexes, prefectural officials said.

The prefectural estimate said tsunami could flood a total area of 871 hectares in Nishiyodo Ward, Osaka, at the mouth of the Yodo River, 43 times more than forecast by the Cabinet Office.

In Kita Ward, which includes busy downtown streets and high-rises, tsunami could affect 195 hectares, against the less than 10 hectares indicated by the state.