TOKYO — Thousands of people living along northern Japan's Pacific coast fled to higher ground yesterday after a powerful undersea earthquake prompted tsunami warnings as far away as Alaska.

The 8.1-magnitude quake struck an area claimed by both Russia and Japan, but the waves did not swell higher than 23 inches. There were no reports of casualties or damage.

Japan's meteorological agency withdrew its tsunami warning after about three hours. Tsunami warnings posted for Russia and coastal areas of Alaska also were canceled, as were tsunami watches for the Philippines, Taiwan, Indonesia and several Pacific islands.

But for several tense hours, communities along Hokkaido's northeastern shore braced for larger waves, with thousands fleeing the coastline for higher ground. Fishermen scrambled to secure their boats, police cars made rounds of deserted piers, and ad hoc evacuation centers were set up in schools and town halls.