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.
A number of small earthquakes that occurred to the west of the Himalayan arc neatly demonstrate the diffuse nature of such boundaries. Scattered across the countries of Pakistan, Afghanistan and Tajikistan, the largest of them was just M5.4 and there were four others between M4.1 and M4.9.
The Himalayan arc is the earth’s major zone of continental collision. The relatively rapid northward movement of the Indian subcontinent (between 40-50mm per year) has generated uplift across a swathe of southern Asia from the Pamir range in the west to the Burmese mountains in the east, an area which includes not only the Himalayan range but also the high Tibetan Plateau which lies to the north.
Such regions experience frequent shallow earthquakes over a wide area, and though they are not as notorious as subduction zones for major earthquakes of M8.0 upwards, major tremors of M7.0 are not infrequent across the region – and frequently cause considerable loss of life.
Two Types of Tectonic Convergence
The week’s earthquakes illustrate two different aspects of convergent plate tectonics, with earthquakes at both the narrow subduction zones resulting from collision of oceanic and continental crust, and along the more diffuse boundary between continents.
United States Geological Survey. Real Time Earthquake Map. (2012). Accessed 20 November 2012
United States Geological Survey. M6.4 – 163km SSW of Severo-Kuril’sk, Russia. Accessed 20 November 2012.
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