In the USA (excluding Alaska) the largest tremor was an M5.1 recorded offshore in southern Oregon, where the remnant Juan de Fuca plate (divided into a number of smaller microplates) is being subducted beneath the North American plate; part of the process that gives rise to the Rocky Mountain chain. (It is this mountain-building process that is likely to be responsible for the earthquakes this week in Wyoming, Nevada and California.)
The earthquake was not, however, caused by the subduction process but rather by the extensive faulting associated with fracture zones at the seaward edge of the plate. Here it is separating from the Pacific plate as new ocean crust is created: As a result, the spreading zones drive the edges of the two plates apart. The causes of this earthquake, therefore, are constructive, whereas an earthquake at the leading edge of the plate would be caused by subduction.
Constructive and Destructive: This Week’s Earthquakes
The seismic pattern of this week clearly demonstrates that earthquakes, though mainly associated with destructive margins (subduction zones) are also a key part of constructive zones, whether those zones are those of collision and uplift, or of the creation of new oceanic crust.
USGS. Real time earthquake map. (2013). Accessed 5 March 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.
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