The week of 31 October-6 November 2012 saw a total of 1,217 earthquakes recorded on the United States Geological Survey’s real time earthquake map.
These included all of those quakes observed on US territories and those greater than magnitude 4.0 (≥M4.0) elsewhere: the latter numbered 217.
Large earthquakes were few, with just 26 ≥M5.0 and one, an M6.1 in the Philippines, larger than M6.0.
So what were the points of interest in a week with no major tremors?
A glance at the map shows two particular events worthy of a closer look – the M5.2 which occurred to the north of New Zealand’s North Island, and the M4.6 which occurred in Tanzania.
M5.2 in North Island, New Zealand
New Zealand is complex and seismically active. Located between the Pacific and Australian plates, its very existence largely a product of their collision, New Zealand is prone to earthquakes which can be damaging and traumatic as the Christchurch earthquake of 2011, which killed 181 people, demonstrates.
The plate boundary itself varies in nature: in the south of the country it is a transform boundary, exhibiting lateral movement, becoming a destructive boundary further north and extending offshore into the Tonga Trench. Further north still, the boundary curves sharply to the west and becomes diffused among the many microplates on the western Pacific. This week’s M5.2 tremor occurred at a depth of around 42 km, some 200 km west of the plate boundary at a point at which subduction is the dominant movement.
M4.6 Earthquake in Tanzania
An unusual feature of the earthquake map was the M4.6 in Tanzania. Although by no means large, this tremor is certainly worth a second look. The map shows that it is remote from plate boundaries and in what looks to be a tectonically stable area. So, what caused an earthquake?
The location of the tremor, in the East African Plateau, suggests that it is likely to be associated with the African Rift Valley which bounds the high ground to the west. Earthquakes in this part of the world are not generally large but nor are they uncommon. According to Open University, “in profile and scale the East African Rift is similar to rifts on the summit of ocean ridges.” Yet as the system is very much distant from the oceans, what is going on?
As in ocean ridges, the extensional tectonics of the African rift and its associated earthquakes are driven by crustal stretching: molten rock rising within the earth puts a strain on the crust and its surface expression is deep, fault-bounded valleys. These are earth-changing processes: it may be that this rift system (which extends over thousands of kilometres from the Red Sea) is the early stage of continental separation and could eventually form a new ocean with the two sides eventually drifting apart.
Complex Crustal Processes
Large earthquakes, widely felt, may be the ones which make the headlines: but it’s interesting to note that even relatively small tremors which depart from the expected pattern of activity at plate boundaries, such as that in Tanzania, show how widespread and complex the processes of crustal evolution can be.
Open University Course Team From Rifting to Drifting : Mantle Plumes and Continental Break-Up (2001). Open University.
Oregon State University Department of Geosciences. New Zealand. Accessed 6 November 2012
United States Geological Survey. Real time earthquake map. (2012). Accessed 6 November 2012