A strong earthquake rocked a large swath of Indonesia's Sumatra Island (search) on Tuesday, shaking buildings and causing panic, witnesses and a meteorological official said. There were no immediate reports of injuries or damage.

The U.S. Geological Survey (search) issued a preliminary report saying the quake measured 6.8.

It struck off the west coast of the island at 8:52 a.m. local time and was "strongly felt," across west Sumatra and outlying islands, said Budi Waluyu, from the government's geophysical and meteorological agency.

Callers to el-Shinta radio station from Medan, a large city on Sumatra, said tall buildings shook and some residents ran from their homes.

Earthquakes have struck the region regularly since a monster 9.1 magnitude earthquake on Dec. 26 that triggered a tsunami (search), killing more than 176,000 people in Indonesia and 10 other countries across the Indian Ocean.