An undersea earthquake measuring six on the Richter scale, struck off Sumatra island today, but no tsunami threat or casualty were reported. The Meteorology and Geophysics Agency said, the quake jolted southwest Mukomuko of Bengkulu province of Sumatra and with a depth at 28 km under sea bed. Indonesia is prone to earthquakes as it lies on a vulnerable quake-hit zone, so called the Pacific Ring of Fire.