Seismically speaking, 2014 crept in quietly with a third successive week in which no tremor of magnitude 6 or larger was recorded on the United States Geological Survey’s real time earthquake map.
The seven day period closing 2013 saw 1,398 tremors of all sizes in the US and its territories, and of at least M4 worldwide, but there were just 24 of M5 or more; the largest being an M5.9 in the Tonga Trench. The distribution of the larger earthquakes is broadly as expected, with the usual focus on the western Pacific Ocean.
The Week’s Largest Earthquake: M5.9, Tonga
The largest earthquake recorded this week was an M5.9 which occurred at the northern end of the Tonga Trench, a subduction zone which marks the boundary between the Pacific and Australian plate in the south western Pacific. Here the Pacific plate is subducting beneath the Australian plate as the two converge at a rate of between 10-20cm per year.
The M5.9 occurred at a depth of around 62km and this, together with the location of its epicentre in the overriding plate, suggests that it is likely to result from movement at or near the interface between the two plates. Typically along subduction zones, earthquakes occur at increasing depth with distance behind the margin itself (i.e. beneath the over-riding plate) and along the Tonga Trench they can occur at great depths: indeed, deep earthquakes are common.
A Volcano-tectonic Earthquake: M5.4, Canary Islands
Although most earthquakes are associated with plate tectonics, they can be generated by other forms of earth movement at great distances from plate margins. Movement of molten rock, or magma, which feeds volcanoes can cause significant fracturing if the crust – so-called volcano-tectonic earthquakes.
Such tremors are often regarded as potential precursors to eruptions and are closely monitored by volcanologists. Earlier this year, earthquakes in the Canary Islands raised concerns about a possible eruption of El Hierro volcano – but despite the occurrence of an M5.4 event on 27 December, the overall seismic activity in the area has decreased and the chances of an eruption are thought to be receding.
US Earthquakes: More From Oklahoma
Although stable continental interiors, far from plate boundaries and from magmatic hotspots, are generally free of major earthquake activity (other than shallow slippage along old faults) there are exceptions. The USGS has for some time been monitoring the occurrence of a large number of relatively low-magnitude tremors which have occurred, and continue to occur, in central Oklahoma: in an October 2013 news release the agency noted that “Since January 2009, more than 200 magnitude 3.0 or greater earthquakes have rattled Central Oklahoma, marking a significant rise in the frequency of these seismic events”.
This week saw the continuation of this pattern, with ten earthquakes of at least M2.5 recorded across the state, the largest of them an M4.1 near the town of Langston. The current earthquake swarm is not, the USGS believes, part of natural seismic variability and may be linked to human activity, such as wastewater disposal.
Quakes This Week: Not Just Plate Margins
The range of earthquakes considered above demonstrates that, while large tremors (usually at subduction zones) are those which make news, earthquakes don’t just occur at plate boundaries but in intra-plate settings too, either as a result of volcanic activity or as movement along fault line – possibly caused by human activity.
USGS Newsroom. Earthquake Swarm Continues in Central Oklahoma. (2013). Accessed January 1, 2014.
USGS. Real time earthquake map. Accessed January 1, 2014.
Volcano Discovery Website. Volcanic unrest at El Hierro (Canary Islands): activity updates. Accessed January 1, 2014.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.