The USGS real-time earthquake map recorded little in the way of major seismic activity in the week of 28 November-4 December – unlike the previous two weeks, each of which recorded a tremor of at least magnitude 7 (≥M7.0).
This week the largest tremor, at M6.4, was one of just two to reach M6.0: there were 27 ≥M5.0 and these were impressively widespread, with events of at least this magnitude occurring in every tectonic setting, in three oceans (Atlantic, Indian and Pacific), and in North and South America, Asia and, unusually, in Africa.
The Week’s Largest Earthquake: M6.4, Southern Indonesia
In the Banda Sea, south Indonesia, a number of current and former faults break the Earth’s crust into small tectonic plates (microplates). This area lies close to the junction between the Sunda microplate and the Philippine and Australian plates. Robert Yeats, in his comprehensive review of faulting in the area, identifies a northward-dipping former subduction zone to the south of the island of Timor and a southwards-dipping active one to the north, concluding that over time the direction of subduction has reversed.
This amply illustrates the complexities of tectonics in the region. The week’s largest earthquake, at M6.4, is by no means unusual for such a highly seismically-active region and occurred close to the northern (active) subduction zone, where the Philippine Sea plate subducts beneath the Australian plate. The quake’s location and depth suggest that it was the result of deformation and faulting in the over-riding plate rather than being directly associated with movement along the plate interface.
Earthquakes in the East African Rift
Africa, an ancient continent, is notable for its lack of seismic activity (a map of earthquakes ≥M5 in the last 30 days shows just two – both of them which occurred within the space of 24 hours this week). So what’s happening? Africa, for all its age, is subject to the force of the deep Earth and the African Rift Valley, which stretches along much of the east of the continent from the Red Sea into Mozambique, is likely to be the beginning of a new tectonic margin. Rifting, driven by deep mantle processes, is splitting the continent into microplates (the Nubian and Somalian) and in time the rift may become a new ocean.
Steep-sided rift valleys with active volcanoes in the centre characterise the splitting of continents. This week’s earthquakes occurred in the Central African Republic, to the west of the western arm of the rift system, where the two parts of Africa are moving apart at around 2-5mm per year. Typically, the dominant fault mechanism is normal faulting and it’s likely that this was the course of this week’s two tremors.
Earthquakes in the US: Touchdown
Football fans take their sport so seriously that some really believe the results are earth shattering. This week there’s evidence to suggest that they may be right: News media reported that crowd celebrations following a touchdown at the game between Seattle Seahawks and the New Orleans giants generated a detectable seismic event – an earthquake – of between M1-M2. Such an event is less uncommon that you might think: there are many recorded events of human activity causing detectable earth movements. Recent examples of man-made events detected by seismologists include the Oklahoma fertiliser factory explosion of April 2013 and nuclear tests undertaken by North Korea.
Quakes: A Diverse Week
Although the majority of earthquakes are associated with obvious plate boundaries, most notably subduction zones, the week’s earthquake map shows a few oddities. Earthquakes, albeit generally small, can occur away from these margins as part of the continuing process of cycling and recycling of continents – and even as a result of non-terrestrial action. Something to remember next time your team makes a touchdown!
BBC news online. Seattle Seahawks football fans ’caused minor earthquake’. (2013). Accessed 4 December 2013.
USGS. Real time earthquake map. (2013). Accessed 4 December 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.