So the world didn’t come to an end on 21 December after all: there were no cosmic forces shearing the planet apart.
In fact, apart from one significant earth tremor, the world was a relatively quiet place, seismically speaking at least, during the week of 18-24 December.
The United States Geological Survey’s real-time earthquake map shows 95 tremors of M4 or more (≥M4.0) and of these just 20 registered ≥M5.0.
In terms of distribution, the majority of the tremors, as expected, were associated with the Pacific rim and the subduction zones of the eastern Indian Ocean, although there was also a scattering of tremors across the Himalayan collision zone and further west into the Caucasus.
It’s noteworthy, too, that the Atlantic Ocean, Europe, and the Mediterranean recorded nothing of ≥M4.0.
Largest Earthquake: M6.8 in Vanuatu
The M6.8 tremor in Vanuatu on 22 December (local time) was by more than an order of magnitude the largest to strike during the week. A product of the complicated collisional tectonics of the Pacific and Indian plates, this event caused no reported damage and was not even the largest tremor to strike in this notoriously earthquake-prone region during the year.
Compressional Earth Movements in the Caucasus
To the east of the Himalayan massif, the Caucasus mountain range is caused by collisional tectonics. The week saw two earthquakes in the region, an M5.5 in the Black Sea off Georgia and an M5.2 several hundred km to the southwest, in Iran.
The tectonic setting of this region is complicated. Squeezed between the African and Eurasian plates, the Arabian continent is moving northwards driven by crustal spreading (forming a potential future ocean) along the axis of the Red Sea, and is being forced against the Eurasian plate along the northern margins of the Persian Gulf. In consequence, the region is no stranger to major and damaging earthquakes: the week’s tremors are minor compared with recent events: Iran, for example, has seen eight tremors of ≥M6.0 in the past two decades, with considerable loss of life.
Alaska and California Earthquakes: They’re Moving (But Not Much)
After a flurry of excitement following an M6.3 off California in the middle of the month, the mainland US continued to be quiet, with little to report beyond minor activity along the San Andreas Fault Zone. A flurry of low-level earth movement continued in Alaska, however, as the collision between the Pacific and North American plates continued to generate small and medium-sized earthquakes along the state’s island arcs and inland fault zones.
United States Geological Survey Real time earthquake map. (2012). Accessed 24 December 2012.
United States Geological Survey Seismotectonics of the Middle East and Vicinity. (2012). Accessed 24 December 2012.
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