The United States Geological Survey has announced that an earthquake struck Alaska’s Aleutian Islands on the 17 July 2010. The earthquake took place at a depth of 10 km (just over six miles) in the Fox Islands part of the Aleutian chain. The magnitude of the earthquake was reported by USGS as 6.7M, although it may be subject to later adjustment (USGS, “Magnitude 6.7 - Fox Islands, Aleutian Islands, Alaska, earthquake.usgs.gov, 18 July 2010).
According to the AP news agency there have to date been no reports of damage despite the fact that the earthquake is significant in size and thus capable of causing a notable amount of damage. No tsunami was reported as a result of the ‘quake, despite its high magnitude (Associated Press, “Strong Earthquake Shakes Alaska Island Region”, 18 July 2010). A number of minor aftershocks continued to be reported at the time of writing.
Why Earthquakes Occur in Alaska
The Aleutian island chain is part of the so-called Pacific Ring of Fire, a circular belt of earthquake and volcanic activity which surrounds the Pacific Ocean and is associated with the movement of the earth’s tectonic plates. The Aleutian Island chain is an island arc, formed by the northwards movement of the Pacific Plate against the North American Plate (Open University, Evolution of Continental Crust, 1997).
The Pacific and North American plates are both composed of oceanic material at the point at which they collide. Because it is older, the Pacific Plate is the colder and therefore the denser of the two and is forced down (subducted) below the more buoyant North American Plate (Plate Tectonics, “Subduction Zones”, platetectonics.com, accessed 18 July 2010).
This downwards movement generates magma which rises to form a volcanically active chain of islands (the Aleutians). It also creates enormous amounts of friction between the two plates and it is this which gives rise to earthquakes. As a result of this, subduction zones such as the Aleutians are among some of the most active earthquake and volcanic belts on the planet. (Open University).
The Earthquake History of Alaska
As part of the Pacific Ring of Fire (described by USGS as ‘the earth’s most active seismic feature’) Alaska and the Aleutian Islands have a long and complex history of earthquake activity. Although at magnitude 6.7 the latest earthquake is generally considered by seismologists to be significant in size, it is by no means the largest to have struck the region in recorded history.
Alaska experiences over 4,000 earthquakes every year – more than in all the other states of the USA combined (USGS Earthquake Hazards program, “Alaska Earthquake History”, accessed 18 July 2010). Historically, the state – and in particular the Aleutians, which are Alaska’s most seismically active zone – are the location of some of the most significant earthquake events ever recorded.
One of the largest earthquakes on record, with a magnitude of 8.5 (later recalculated to 9.2) occurred in Alaska in 1964, causing considerable damage (though because of the area’s remoteness, very few deaths were recorded in the immediate area). Two earthquakes of greater than magnitude 8 occurred in 1899, and in 1957 a ‘quake of magnitude 8.8 took place. A further eight ‘great earthquakes (magnitude 7.0-7.9) took place between 1899 and 1969 (USGS).
Because the area is so remote and so sparsely settled, the greatest danger to humans from seismic activity comes not from the earthquakes themselves but from the giant waves, or tsunamis, which they generate. In 1964, the tsunami generated by the tremor killed over 100 people along the coast of the USA, Canada and as far away as Hawaii (USGS).
The significance of earthquakes in the state, therefore, is less to do with their immediate damage than the potential for generating tsunamis which will strike at a distance. The latest tremor of July 2010 did not do so and thus, once again, a significant seismic event has passed with minimal earthquake damage.
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