Unalaska, AK – Adak Island was rocked by a strong earthquake at about 4:30 p.m. local time Thursday.

According to the Adak clinic, there were no reported injuries from the 6.6 magnitude quake 40 miles west of the island, which was followed by hundreds of smaller aftershocks. The tremor was the third in the area over the past two weeks with a magnitude greater than 6.

Earthquakes, even strong ones, are common in the Aleutians, which sit along the boundary where the North American tectonic plate meets the Pacific Ocean plate. But seismologist Natasha Ruppert with the Alaska Earthquake Information Center at the University of Alaska Fairbanks said this tremor was unique in that it didn't occur along that boundary, and originated at a relatively shallow depth of just over 6 miles.

"This is kind of a rare sequence of events," Ruppert said. "Most of the earthquakes are associated with the contact between two plates, and shallow earthquakes don't occur too often."

Ruppert said the recent quakes were probably the result of friction between smaller pieces of the earth's crust far beneath the sea. She said the aftershocks from Thursday's quake could continue for several more days.