WASHINGTON -- A strong 6.6-magnitude earthquake struck Thursday near the Aleutian Islands in southwestern Alaska, the US Geological Survey reported.

The quake's epicenter was located at a depth of 10 kilometers (6.2 miles), 65 kilometers (40 miles) west of Adak, Alaska, the westernmost municipality in the United States with a population of some 300 people.

The earthquake occurred at 0133 GMT in the Andreanof Islands, one section of the earthquake-prone volcanic island chain, the survey said.

The reading was based on the open-ended moment magnitude scale, now used by US seismologists, which measures the area of the fault that ruptured and the total energy released.

A measurement of five indicates a moderate earthquake, six or higher a strong quake, seven a major quake and eight a great quake.