The week of 29 January-4 February 2015 saw many moderate-sized earthquakes (of magnitude 5.0-5.9) and just one large earthquake (of at least magnitude 6, or ≥M6.0).
In total the United States Geological Survey’s real time earthquake map, which includes tremors of all magnitudes in the US and its territories and those of at least M4.0 elsewhere, showed 38 events of ≥M5.0, distributed (as usual) mainly around the margins of the planet’s tectonic plates.
There were plenty of smaller earthquakes on the map, too, including three located at great distances from plate boundaries in usually tectonically-stable continental areas — an M4.7 in Russia (an aftershock of an earlier tremor in the Baikal rift); an M4.2 in central Arabia; and an M3.8 in the English county of Rutland.
The Week’s Largest Earthquake: M6.3, Argentina
This week a tremor of magnitude M.3 struck at a depth of 172km near to the town of La Punta in western Argentina. The tectonic setting in this area is one of subduction: convergence of the Nazca and South American tectonic plates leads to the subduction of the former beneath the latter among the Peru-Chile trench, an area that has been the source of some major earthquakes, including the largest on record (that which struck off Chile in 1960).
Typically, large earthquakes are caused by movement along the interface between the two plates, the depth increasing with distance from the angle of dip. Although geographically the epicentre of the earthquake was to the east of the Andes and therefore appears unrelated to the subduction zone, the fact that Yeats notes that in this area the subduction zone has a very low angle of dip implies that the distance from the trench (well over 400 km) need not rule out subduction as a source.
There’s an alternative scenario. The USGS notes that in South America “Crustal earthquakes result from deformation and mountain building in the overriding South America plate and generate earthquakes as deep as approximately 50 km”.
To complicate matters further, the USGS goes on to note that: “Large intermediate-depth earthquakes (those occurring between depths of approximately 70 and 300 km) … occur within the Nazca plate as a result of internal deformation within the subducting plate”. In other words, deformation, though within the underlying plate, rather than the over-riding one, may have been its cause after all — although without detailed information, it’s impossible to be sure.
M3.9, Leicestershire, England
The residents of the usually-calm English county of Rutland were a-flutter this week, with the occurrence of an earthquake of M3.9.
While such an event might not cause a stir if it occurred in California, Alaska, Japan or many other parts of the world, it’s an unusual occurrence for a country where large earthquakes (≥M6.0) are virtually unknown and where seismic hazard is very low.
Though this earthquake, too, lacks detailed information, the bedrock of the UK is ancient and criss-crossed by many known faults and by many others which are unmapped. The shallow depth of the Leicestershire earthquake (8km) suggests that it was movement along one of these faults which caused the tremor, and set much of the county talking (or so it seems from the response on Twitter).
US Earthquakes: California
The triple junction between the North American, Pacific and Juan de Fuca tectonic plates was the location of this week’s largest earthquake in the US — an M5.7, which was accompanied by numerous aftershocks. The coming together of three plates — and three different types of margin, fracture zone, transform and destructive — causes considerable deformation, as the map of associated faulting shows.
Such earthquakes are by no means unusual in this area, given the tectonics and levels of deformation.
Earthquakes: Many Forces at Play
Earthquakes are strange things. Argentina’s earthquake may have been triggered by subduction (or may not have been); California’s is the result of complex forces in opposition; and even sleepy rural Rutland is at the mercy of ancient tectonics.