TOKYO — A powerful earthquake with a preliminary magnitude of 6.4 knocked over houses in southern Japan on Thursday evening, and police said people may be trapped underneath.

There were no immediate reports of casualties from the 9:26 p.m. quake, and no risk of a tsunami.

“There was a ka-boom and the whole house shook violently sideways,” Takahiko Morita, a resident of Mashiki, the town at the epicenter, said in a telephone interview with Japanese broadcaster NHK. “Furniture and bookshelves fell down, books were all over the floor.”

Morita said some houses and walls collapsed in his neighborhood, and water supply was cut off.

Police in Kumamoto prefecture said they have received reports of a number of collapsed houses and people possibly trapped inside.

Mashiki is east of Kumamoto city, about 1,300 kilometers (800 miles) southwest of Tokyo.

Japan’s Chief Cabinet Secretary Yoshihide Suga told a news conference that damage was being assessed, but there were no abnormalities at nearby nuclear facilities.

The epicenter was 120 kilometers (74 mile) northeast of the Sendai nuclear plant, the only one operating in the country.

NHK showed Mashiki town hall in the dark, apparently having lost power. Footage also showed rubble on the road, shards of glasses and broken windows, and fire breaking out in some places, with firefighters battling an orange blaze.