Earthquake History of Oregon
At M6.0 the February 2012 earthquake is not globally significant but is nevertheless large for an earthquake at an ocean ridge system. In such settings frequent and shallow earthquakes are usually small in magnitude. Although large earthquakes do occur in such zones (the largest recorded being M8.2), major tremors are much more typical of subduction zones.
Interestingly, the recent earthquake also rates as one of the largest recorded in or off Oregon – despite the presence of a subduction zone (the Cascadia subduction zone) immediately off the coast, at the eastern edge of the Juan de Fuca microplate where it bends beneath the North American continent.
Although the whole of the continental west coast was affected by a major earthquake in 1700, Oregon has not in recent years been prone to major earthquakes: the USGS list of major tremors shows only three of M6.0 or more since 1900. Of these the second largest, at M6.3, occurred on the same fracture zone as the event of 2012.
The reason for this is the relatively slow convergence (and in places no convergence at all) of the Juan de Fuca system and North American Plate. Thus, although not significant by global standards, the Oregon 2012 quake will count as a major earthquake in its local context and within its tectonic framework.
USGS Earthquake Hazards Program. Magnitude 6.0 Off the Coast of Oregon. Magnitude 8 and Greater Earthquakes Since 1900. Accessed February 15, 2012.
University of Colorado. The Juan de Fuca Microplate System. Accessed February 15, 2012.
Decoding Science. One article at a time.