An earthquake with a preliminary magnitude of 7 (M7.0) struck in Alaska’s Aleutian Islands on 30 August in the early hours of the morning local time. The tremor, which occurred just south of the Andreanof Islands at a depth of 34km and was followed by a number of aftershocks, was the largest in the state since the M7.3 in June 2011.
Although the tremor occurred offshore and was of a significant magnitude, no major tsunami was expected and not tsunami warning issued by the Pacific Tsunami Warning Center. At the time of writing there are no reports of any damage or injuries but the relative remoteness of the area suggests that the likelihood of and human impact is slight.
Tectonic Setting of the M7.0 of 30 August
Alaska is among the most seismically active areas of the world and is part of the so-called Pacific Ring of Fire. Here, the north-westward movement of the Pacific plate (at a rate of between 54mm and 78mm per year, increasing from east to west) brings it into collision with the North American plate. The old, dense ocean crust of the Pacific plate is subducted beneath the North American plate forming a deep trench with at the boundary between the two.
Earthquakes occur regularly along the length of this trench as friction between the two plates builds up and is released. Although it’s impossible to say what kind of motion generated the earthquake of 30 August, a typical subduction earthquake involves vertical movement along the interface, although movements may also occur within the over-riding or the subducting plate.
Earthquake History of Alaska
As a measure of the levels of seismic activity in Alaska, it’s worth noting that 43 tremors were recorded in the first ten hours of August 30. Many of these were minor: but it emphasizes that Alaska is subject to thousands of earthquakes each year. Earthquakes of increasingly larger magnitudes are increasingly rarer and the M7.0 can be regarded as significant for any area of the planet. (The first eight months of 2013 have seen just ten tremors of this magnitude, four of them aftershocks to an M8.0.)
That said, Alaska’s location means that major earthquakes such as this are by no mean uncommon. In 2011, 2007 and 2003 earthquakes of greater than M7.0 occurred, although previous years have seen maximum magnitudes of around M6.5, while the largest of recent years (also in 2003) was an M7.8.
The tectonic forces at play also mean that the Alaskan islands are vulnerable to so-called ‘megathrust’ earthquakes, where rupturing may occur along hundreds of kilometres of the subduction zone. The state has the dubious distinction of being the location of one of the largest earthquakes on record, and M9.2 in Prince William Sound in 1964 which generated a tsunami which reached as far as Hawaii and which killed 128 people.
Alaska Earthquake Information Center. Recent earthquakes. Accessed 30 August 2013.
USGS. Earthquake information by year. Accessed 30 August 2013.
USGS. Historic Earthquakes: Prince William Sound, Alaska. Accessed 30 August 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.