'Swarm' of earthquakes hit Alaska's western Aleutian Islands without damage

A series of moderate to strong earthquakes that struck Alaska's western Aleutian Islands and offshore areas Sunday comprised what scientists call a swarm," but the state's earthquake monitors said the seismic events should not cause residents to worry.

There were no immediate reports of damage or injuries in the area around Adak, Amchitka and Kiska islands some 2,200 km west of Anchorage. Of the three remote islands, only Adak Island, with a population of about 300 people, is inhabited.

Nine quakes measuring at least magnitude 5.0 struck on or near the islands and in a cluster offshore to the south Sunday morning and afternoon, according to the Alaska Earthquake Centre.

Three of the quakes measured 6.0 or stronger, with the biggest a 6.3 temblor offshore.

In a summary posted on its website Sunday night, the earthquake center said the 6.3 magnitude quake at a depth of about 18 km did not raise concern on its own because of its offshore location and the lack of damage reports, while the events that followed within the hour above magnitude 5.0 were expected aftershocks.

But two additional aftershocks above magnitude 6.0 that began around 3 pm and spaced by 20 minutes were not normal. The quake series constituted a swarm, or a cluster of earthquakes with similar magnitudes, the centre said.

Michael West, the Alaska Earthquake Centre director and state seismologist, said the swarm included quakes that were larger than normal but should not alarm Alaska residents.

Swarms of moderately large earthquakes are common in the Aleutians and do not necessarily portend anything more substantial, West said in the summary. Because this area has been in, or on the edge, of numerous large historical earthquakes, we are keeping close watch on it.

There are small earthquake swarms every couple of years on average in the subduction zone along the Aleutian chain. A swarm occurred in May about 550 km east of Sunday's activity, the centre said.

The quakes on Sunday did not appear related to recent seismic activity at a volcano near Anchorage that last erupted in 1992.

Much of Alaska including the Aleutian Islands is on the Ring of Fire, a seismically and volcanically active zone that surrounds most of the Pacific Ocean.