The week of 14-20 November 2012 was relatively quiet, following the series of large earthquakes which occurred in the previous seven days. Just four tremors of greater than magnitude 6 (>M6.0) were recorded; there were 28 of ≥M5.0 and 98 of ≥M4.0. The distribution followed an expected pattern concentrated around the major subduction zones of the Pacific, though there was also a scattering of smaller tremors in south-central Asia and southern Europe.
The Week’s Largest Earthquake: M6.4 off Kamchatka
The largest earthquake to occur in the week was an M6.4 which occurred at a depth of 29km around 200km off the southern tip of Russia’s Kamchatka peninsula. Part of the Pacific Ring of Fire, this region is among the most seismically active on the planet.
Tectonically speaking, the region is complex: a wedge of the North American Plate tapers southwards between the Pacific and Eurasian plates, where it eventually forms the northern part of the Japanese archipelago. The Pacific Plate moves westwards against the static North American Plate at a rate of around 80mm and the old and dense oceanic crust is subducted. From Kamchatka the boundary southwards is marked by the island chain of the Kuril arc and characterised not just by frequent seismic activity but also by extensive volcanic activity.
Continental Convergence – the Western Himalyas
Most of the world’s earthquakes are associated with convergent boundaries at subduction zones, where cold, dense oceanic crust is forced downwards beneath warmer, more buoyant continental material. Where two continents collide there is uplift rather than subduction, with the boundaries typically less well defined than the narrow subduction zones.
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