Plenty of Activity But No Large Earthquakes: 25-31 December 2012


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Earthquakes of M5.0 or greater, 25-31 December 2012. Image credit: USGS

Looking at the USGS real time earthquake map for the 25-31 December, you’d be forgiven for thinking that the planet, like so many of the rest of us, is in that sluggish state that comes from over-indulging at Christmas.

Of course it’s foolish, not to say scientifically inaccurate, to attempt to anthropomorphise physical earth processes: but it’s nevertheless worthy of comment that the largest tremor recorded is no more than magnitude 5.8 (M5.8).

That’s not to say that there was no activity – in fact there were 33 reported tremors of at least M5.0, and over a hundred registering at least M4.0.

These were scattered across the globe and covered most tectonic settings – not just the major focuses of collision zones but also a scattering along convergent boundaries at mid-ocean ridges and, for good measure, and M4.8 occurring in the center of Africa in the Congo.

Largest Earthquake: M5.8 in Afghanistan

Continental collision was the mechanism for the M5.8 tremor which struck in northeast Afghanistan. The northwards-moving Indian plate is in collision with the Eurasian plate and the result, with both plates made up of relatively buoyant continental crust, is mechanical uplift. The boundary between these two plates is traced, on the Eurasian side, by the mountain ranges of Pakistan and Burma to west and east and the strongly-defined line of the Himalayas to the north.

Such convergence causes thrust faulting, where one block of land is forced upwards, and mountain ranges in areas of uplift are characterised by broad belts where such movements are typical. The configuration and relative movements of the two pltes, however, means that other types of faulting can occur and its possible that this week’s tremor may have involved lateral, or strike-slip, faulting, although detailed information is not available.

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