The week of 2-8 August 2013 saw little significant seismic activity.
It’s unusual, though not exceptionally so, for no tremor of magnitude 6.0 or larger to occur on the planet in any given week, as happened over the past seven days.
The pattern of tremors, however, was broadly as expected with a focus on the Earth’s subduction zones, and the overall number of earthquakes of all magnitudes recorded on the United States Geological Survey’s real time earthquake map fell well within the range of a typical week.
The Week’s Largest Earthquake – M5.9, Vanuatu
The largest earthquake of the week was an M5.9 which struck off the Pacific island of Vanuatu on 7 August at a depth of 13.9km, followed very shortly after by an M5.7 in almost exactly the same location and at the same depth.
Vanuatu lies on the complicated tectonic margin between the Australian and Pacific plates, where the Australian plate is being subducted beneath the Pacific plate along the New Hebrides Trench, which runs roughly north to south.
The depth of the two earthquakes, and their location in the overriding Pacific plate close to the subduction zone, implies that they are a direct result of subduction. It should be noted, however, that there is also extensional activity resulting from sea-floor spreading in the North Fiji Basin to the east, which creates additional, complicating stresses.
The Himalayas and Tibet
Compressional tectonic forces are at play in many areas of the globe and mostly involve oceanic-oceanic or oceanic-continental crust combinations, resulting in subduction and accounting for most of the planet’s large earthquakes.
In some areas, however, continental crust collides with continental crust, resulting in uplift and mountain building.
At present there is no clearer or more dramatic example than the collision of India and Eurasia, which has lifted, and continues to lift, the Himalayas and Tibet.
As convergence generates fractures in the crust thrust faulting (with one block pushed up over another) is the dominant mechanism, although normal and strike-slip faulting may also occur depending on the direction of movement and other factors.
In these areas earthquake can occur over a wide and diffuse region – and this week saw tremors at the southern and northern edges of the Himalaya-Tibet region as well as in the interior.
US Earthquakes: The New Madrid Seismic Zone
Minor earthquakes this week in the states of Missouri and Tennessee (magnitudes of just M1.5 to M2.3) may appear insignificant but they draw attention to a region described by the USGS as “the most seismically active in the USA east of the Rockies.”
Continental interiors, even though largely tectonically stable, are typically subject to small earthquakes along old faults.
Close to the junction between Missouri, Arkansas, Tennessee and Kentucky, the existence of a large historic buried rift system (which formed around 500 million years ago) is the source of significant seismic hazard.
Current movement across the area is compressional, causing the faults to be reactivated and earthquakes to occur. This area was the location of three extremely large earth tremors in 1811-12: though they predated seismographic recording, Yeats notes that estimates of their size place the magnitudes at least in excess of M7.0 and possibly as high as M8.4 – making them some of the largest earthquakes to have occurred in the history of the US.
Reactivation of Historical Faults
Though small, the Missouri-Tennessee tremors are a reminder that major earthquakes don’t just occur at subduction zones and that the major processes which we see today in the form of the Himalayas or major subduction zones can be reactivated many millions of years later, to devastating effect.
United States Geological Survey. M2.3 – 3km S of Steele, Missouri: Earthquakes in the New Madrid Seismic Zone. Accessed 8 August 2013
USGS. Real Time Earthquake Map. (2013). Accessed 8 August 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press