After a fortnight of relative quiet, the week ending October 2, 2012 saw the earth begin moving again. The United States Geological Survey records show a total of almost 1300 earthquakes of all magnitudes within the US and of greater than or equal to M4.0 elsewhere. Of these, 25 registered magnitudes of M5.0 or more, and one fifth of these were greater than, or equal to, M6.0.
Looking at the spatial distribution of these tremors, the pattern of the week’s seismic activity shows that all but one of the larger tremors occurred along the rim of the Pacific Ocean – the so-called Pacific Ring of Fire. The smaller earthquakes also raise an interesting pattern, showing a scattering along the diffuse zone across central Eurasia from the Mediterranean to Tibet where continental collision is taking place between Eurasia in the north and Africa/India in the south.
Colombia’s M7.3 Earthquake
The largest earthquake of the week was some 8 times greater in magnitude than, and released over 20 times as much energy as, the next largest. This large quake was the M7.3 event in Colombia. Here, the Nazca Plate which makes up the southern part of the Pacific Ocean is subducted beneath the South American plate – a mechanism which causes continental uplift, volcanism and frequent earthquakes, and which finds its topographical expression in the form of the Andes mountain chain.
Movements within subduction zones are often complex. Information from the USGS indicates that the tremor was caused by normal faulting (essentially, where extension causes one side of a fault to drop down) within the descending Nazca Plate. Interestingly, the depth of the earthquake (around 100 miles) presents a situation in which the actual location of the earthquake (the focus) is in one tectonic plate but its epicentre (the point on the earth’s surface immediately above the focus) is in another.
The Andean margin of South America was more active than in recent weeks, with 15 other earthquakes of greater than, or equal to M4.0 showing on the USGS map – almost all of these between 100-200 miles depth. Without detailed geophysical reports it isn’t possible to comment on exact mechanisms, but it seems likely that these, like the Colombian tremor, have their origins in crustal deformation within the descending Nazca plate.
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