The powerful 7.9-magnitude earthquake that shook the Aleutian Islands on Monday was the largest quake to hit the U.S. in more than a decade, according to the U.S. Geological Survey.

The quake, centered about 13 miles southeast of Little Sitkin Island, triggered more than 21 aftershocks and prompted tsunami warnings and evacuations for the islands near the epicenter. The warning was later downgraded to an advisory as experts ruled out the potential of a destructive tsunami for other Pacific coastlines. The last U.S. quake as big as the one that occurred Monday was a 7.9 tremor near Alaska's Denali National Park on Nov. 3, 2002, and the last major earthquake of magnitude 7.0 or greater occurred in the Aleutian Islands on Aug. 13, 2013.

Monday's earthquake struck at 12:53 p.m. Alaska Daylight Time (4:53 p.m. Eastern time) near Little Sitkin Island in the far western part of the Aleutian Island chain, just west of the International Date Line.

According to the U.S. Geological Survey, the main jolt was followed by a series of aftershocks. Two significant aftershocks, measuring magnitude 6.0 and 5.8, occurred 18 and 37 minutes after the main shock, respectively. About four hours after the 7.9 temblor, a magnitude-6.9 aftershock rocked the same area.

The National Tsunami Warning Center in Palmer, Alaska, issued a tsunami warning for a portion of the Aleutians stretching from Nikolski to Attu. "Widespread dangerous coastal flooding accompanied by powerful currents are possible and may continue for hours after tsunami arrival," the NTWC bulletin said.

Residents of Adak, Alaska, evacuated the town site and gathered on a nearby hill.

"We're seeing water leave our bay, so we do have everybody up on the Bering Hill area, where our primary evacuation center is at," City Manager Layton Lockett told The Associated Press by telephone as he gathered some last paperwork before heading out himself to join about 300 residents at the center.

The agency downgraded the bulletin to a less-serious tsunami advisory at 2:44 p.m. local time, just under two hours after the quake. At 4:35 p.m. local time, all tsunami advisories were canceled.

The first tsunami measurement came from Amchitka, Alaska, about 25 miles from the quake's epicenter. A tsunami of 0.6 foot (about 7 inches) was recorded at 1:36 p.m. local time. The NTWC said that the depth of the earthquake, some 71 miles below the earth's surface, would mitigate the extent of the tsunami. Later, at 3:22 p.m. local time, Amchitka recorded an 0.7-foot tsunami wave.

Other nearby communities reported tsunami heights of 0.2 to 0.6 foot.

Jeremy Zidek, a spokesman for the Alaska Department of Homeland Security and Emergency Management, told the Associated Press there were no immediate reports of damage from either the earthquake or the small tsunami.

Natasha Ruppert, a seismologist with the Alaska Earthquake Center, said that because the communities that would have suffered damage are under a tsunami warning, people may not have been able to get out and check for damage yet.

The Aleutian Island chain stretches some 1,200 miles across the northern Pacific Ocean between the Alaska mainland and eastern Russia. The epicenter of Monday's earthquake was closer to Russia than it was to Anchorage, Alaska's largest city. At a distance of just over 2,000 miles from Tokyo, it was also closer to the Japanese capital than to Seattle.

Tsunami Concerns Ease for West Coast, Hawaii and Japan

The Pacific Tsunami Warning Center said the Alaska quake was too deep to pose a tsunami danger to Hawaii, and ruled that state out of the tsunami risk. California, Oregon and Washington were later excluded from the tsunami risk as well.

Large earthquakes in Alaska have caused significant tsunamis in the past. An Aleutian Islands quake in 1946 caused a tsunami that killed 96 people in Hilo, Hawaii, and prompted the establishment of the Pacific Tsunami Warning Center.

The much stronger magnitude-9.2 Good Friday earthquake of 1964, which took place farther east near Anchorage, caused over 100 tsunami deaths in Alaska, five in Oregon, and 13 in California.

After evaluating the data, the Japan Meteorological Agency ruled out any tsunami risk to Japan from the Aleutians earthquake. Japan has suffered damaging tsunamis from overseas quakes in the past, including the 1964 Good Friday quake. Major earthquakes in Chile also caused damaging tsunamis in Japan in 1960 and 2010.

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