A powerful earthquake with a preliminary magnitude of 7.1 struck Japan's northern island of Hokkaido (search) early Monday, injuring at least 13 people, swaying buildings and triggering a small tsunami wave that reached the shore.

There were no immediate reports of damage following the 3:32 a.m. quake.

Japan's Meteorological Agency quickly issued a tsunami (search) warning for the eastern shores of Hokkaido's Pacific coastal area. Tsunami are potentially dangerous waves triggered by seismic activity.

But agency official Masahiro Yamamoto later told a televised news conference that the agency had lifted its warning, saying it had detected a 4-inch tsunami and expected only small changes in the ocean's surface.

The quake was centered off Hokkaido's east coast, about 550 miles northwest of Tokyo (search), 30 miles below the sea surface. The rocking was felt throughout northern Japan, including Hokkaido's largest city, Sapporo, and Kushiro, on the island's eastern shore.

A 4.6-magnitude aftershock followed the initial tremor about a half hour later.

"We expect the aftershocks to continue," Yamamoto said.

National broadcaster NHK showed footage of desks rumbling in their offices and convenience store goods thrown from shelves by the force of the quake.

Most of the injuries were in the cities of Kushiro, Nemuro and Bekkaicho. An 81-year-old woman living in Nemuro was thrown to the floor and treated for a broken arm, and several others, mostly elderly, had suffered minor injuries such cuts from falling objects or broken glass, Hokkaido prefectural police spokesman Tsuneo Sasaki said.

More than 400 homes in Hokkaido briefly lost power and natural gas services, Sasaki said.

Cracks were discovered in Nemuro city's docks and at least two schools had broken windows and cracked water pipes, city spokesman Atsushi Maeda said.

Railway services on several lines in Hokkaido were suspended for a short time, according to NHK.

A magnitude 7 earthquake can cause widespread, heavy damage.

Last month, a magnitude 6.8 earthquake and several large aftershocks in the prefecture of northern Niigata killed 40 people and damaged more than 16,000 homes. That quake was the deadliest in Japan since 1995, when a magnitude 7.2 quake killed 6,000 people in the western city of Kobe.