There were no real seismic fireworks in the week of 13-19 March. A quick overview of the statistics shown on the USGS’s real time earthquake map shows just 10 tremors of at least magnitude 5 (≥M5.0) and just one of greater than M6.0. A map of earthquakes ≥M4.0 shows the distribution to be as might be expected: most earthquakes occurred around the Pacific Ring of Fire with the odd outlier – most notably, the week’s largest tremor (at just M6.1) which occurred in the South Atlantic, and a shallow tremor of M4.1 in Romania.
How Many Earthquakes Might We Expect in a Week?
So, it was a very quiet week seismically – but how quiet? Well, the statistics are inevitably incomplete – the USGS map shows only those reported worldwide of at least M4.0. A comparison of statistics drawn from the map and covering the past ten weeks shows that in fact this level of seismic activity isn’t that unusual – the data series shows that it has the second lowest figure for earthquakes of ≥M4.0, although in terms of earthquakes of ≥M5.0 this week is the quietest.
It’s interesting to observe that major earthquakes tend to skew the statistics on a short-term basis, as they are often followed by significant aftershocks. Although there is relatively little variation in the total number of tremors of ≥M4.0, the M8.0 earthquake off the Solomon Islands in February 2013 produced a major cluster of aftershocks of M5.0 in the West Pacific which produced a figure almost four times higher than any other week in the series.
This Week’s Largest Earthquake: M6.1, South Atlantic
The week’s largest earthquake was an M6.1 which occurred in the southern Atlantic Ocean to the east of the South Sandwich Islands. Although no detailed information is available on this tremor, a glance at its tectonic setting suggests that it is the result of subduction. In the south Atlantic, the sliver of crust known as the Sandwich microplate is squeezed between the South American and Antarctic plates: a spreading ridge at its western margin drives it against the South American plate, which is subducted beneath it.
The South Sandwich Islands themselves, which lie on the overriding plate, are the product of this collision: subducted crust is melted and rises to the surface as volcanoes, breaking the surface to form the South Sandwich island chain.
The Largest US Earthquake
In such a quiet week, at M4.8, the earthquake which struck in Alaska ranks in the top 20 most significant seismic events to have struck in the world, although it is by no means unusual for its tectonic setting. Here, the northwards-moving Pacific plate is subducted beneath the North American plate to create a highly seismically active zone. The epicentre of the earthquake lies behind the main subduction zone and is probably associated with thrust faulting in continental crust.
Even in a week without major earthquakes, the map and associated statistics clearly show that movement between the earth’s tectonic plates continues to generate a broadly constant level of seismic activity against which occasional major events are noteworthy.
Leat, P. T. Smellie, J. L. Millar I. L. and Larter R. D. Magmatism in the South Sandwich Arc. (2003). Geological Society, London, Special Publications. Accessed 19 March, 2013.
USGS. Real time earthquake map. (2013). Accessed 19 March, 2013
Yeats, R Active Faults of the World. (2012). Cambridge University Press.
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