The magnitude 6.2 (M6.2) earthquake which struck 130 km from Anchorage, Alaska, on 25 September was the largest to occur in the inland zone of the state since an M6.3 in 2003.
The earthquake, which occurred at a depth of 103 km, resulted in light to moderate shaking of the immediate area – and residents of the city of Anchorage felt the quake.
September 25 Alaska Earthquake: Tectonic Setting
Much of Alaska is influenced by the subduction of the Pacific plate beneath the North American plate, a convergence which produces the highly seismically-active Aleutian Island chain.
At first glance, the location and depth of the earthquake appear to indicate that it was a subduction zone earthquake.
Given the complexity of the situation, however, this may be misleading.
The earthquake occurred some way inland from the eastern end of the subduction zone. In Alaska the angular nature of the boundary between the the North American and Pacific plates introduces a rotational element and the situation is further obscured by the existence of smaller slivers of crust.
The USGS summarises the tectonics in the area thus: “Crustal earthquakes occur as a result transmitted deformation and stress associated with the northwestward convergence of the Pacific plate that collides a block of oceanic and continental material into the North America plate”
Seismologist Robert Yeats further divides the area into smaller microplates and, looking at this subdivision, the evidence suggest that the most recent earthquake occurred at or near the boundary between two of these, the Bering and Southern Alaska microplates.
This interpretation, however, is weakened by the depth of the earthquake.
In the absence of any further information it therefore seems likely that the tremor is the result of crustal deformation because of many interacting stresses and faults.
Tectonic History of Central Alaska
Earthquakes in Alaska are many and varied. The subduction zone facilities larger ‘megathrust’ earthquakes. Further south the relative motion between the North American and Pacific plates is lateral, resulting in so-called ‘strike-slip’ earthquakes. And the compressional forces and change of direction of the two plates generates huge stresses away from the plate boundary with many, broadly parallel, thrust faults also providing the sources for earthquakes – as the map of Alaskan earthquakes for just the past 30 days clearly demonstrates.
The state’s largest tremors occur as a result of subduction and the area lays claim to the doubtful distinction of having been the source of the second largest earthquake on record – the so-called ‘great’ earthquake of 1964, which had a magnitude of M9.2.
Earthquakes of at least M6.0 resulting from crustal deformation, rather than slippage at the subduction zone, are unusual but not unheard of.
In 2007 and earthquake of similar magnitude and similar depth occurred in the same region; while in 2002 an earthquake of M7.8 struck further to the north west and at a shallower depth.
Understanding the Causes of the Earthquake
With so many factors at play it isn’t easy to determine the true source of the earthquake on the basis of the available information. Taking the preliminary data and location of faults into account, however, it seems most likely that today’s Alaska earthquake is the result of crustal deformation resulting from convergence of the two tectonic plates.
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