The Earth moved for many over this Valentine’s Day, but not due to romance – a magnitude 6.0 earthquake struck around 250 km off the west coast of Oregon on February 14, 2012. The Valentine’s Day quake is notable not only for its size (it is one of the largest ever to have occurred in the state or off its coast) but also in terms of the complex tectonic setting in which it occurred.
The Oregon Earthquake of February 2012
The Oregon earthquake occurred on an ocean ridge, at a divergent boundary – where new crust is created by upward movement of hot and buoyant rock from the earth’s interior – between two of the large slabs of crust (tectonic plates) which make up the surface of the earth. Preliminary information from the United States Geological Survey (USGS) shows that the quake occurred at a depth of 10km on the fracture zone associated with the Juan de Fuca Ridge, which marks the western boundary of the Juan de Fuca microplate.
The Tectonic Setting: Divergent Boundaries
Although plate tectonics is generally described in simplistic terms (large crustal plates and single faults) the local situation is inevitably more complex. At its simplest we might describe a setting in which a divergent boundary in the east drives the Juan de Fuca plate eastwards against the North American plate, forcing it beneath the North American continent.
In the case of the Juan de Fuca microplate, at least two other small plates, the Explorer Plate to the north and the Gorda plate to the south, are involved. The relative direction, type of movement (with convergent, divergent and conservative boundaries present within a relatively small area) and speed of these plates generates tensions and, ultimately, earthquakes.
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