This week, the variation in numbers and magnitudes of earthquakes worldwide threw up a relatively high number of intermediate-sized seismic events. The United States Geological Survey’s real time earthquake map, which records tremors of all magnitudes within the US and its territories – and those of at least magnitude 4 (≥M4.0) elsewhere – showed an unusually, though not exceptionally, high number of tremors of ≥M5.0 in the week of 19-25 February 2015.
There were 40 of these seismic events, including four larger than M6.0, in a total of 1,766 on the map.
Most tremors are associated with the planet’s tectonic plate boundaries but with an unusually high number of intermediate sized ‘quakes, it was always going to be likely that there would be one or two anomalies.
This week, unusual quakes included an M5.2 in China (at the edge of the diffuse Indo-Eurasian collision zone); an M5.1 in the East African Rift (where divergent tectonics operate); and and M5.0 in Spain (of which more below).
The Week’s Biggest Earthquake: M6.4 Vanuatu
The western Pacific, where the Australian and Pacific plates come together (catching up a whole load of crustal slivers and microplates in the process) is among the most seismically-active regions on Earth and regularly delivers the largest earthquake in any given week. So it proved again, with an M6.4 in the archipelago of Vanuatu.
Despite the complexity of the overall tectonic setting, this week’s tremor appears to be relatively straightforward. In this area, the Australian plate is subducting beneath the Pacific plate along the New Hebridean Trench. The earthquake, with its epicentre to the east of the boundary in the over-riding plate, occurred at a depth of 10km, suggesting that it was the result of internal deformation rather than movement at the plate interface.
M5.0 Earthquake, Central Spain
Though less prone to major earthquakes (the exception being the Lisbon earthquake of 1755, with its estimated magnitude of M8.6-8.8) the Mediterranean basin is just as complicated. The coming-together of Africa and Eurasia, like that of the Pacific and Australasia, involves complex local tectonics and many slivers of crusts.
This week’s M5.0 in south-central Spain looks unusual but is actually the result of a small and developing subduction zone at the eastern edge of the Mediterranean. Convergence between the continents leads to collisional forces and thrust faulting.
Without access to detailed fault maps, it’s impossible to be completely certain as to the cause – but clues from location and topography, as well as simplified fault maps drawn by Robert Yeats, suggest that the cause of the earthquake is most likely to be movement along a fault zone known as the Betic Cordillera, which is the surface expression of deformation at the northern end of the Gibraltar subduction zone.
US Earthquakes: Colorado
This week Colorado quivered, just a little, with three tremors of M2.5, M3.4 and M3.6. Occurring on the western edge of the Rockies, it’s pretty clear that these are somehow associated with that particular mountain range and so it proves. They occurred in an extensional setting, the Rio Grade rift, and are most likely to be the result of normal faulting.
Although the earthquakes in Colorado and Spain might raise the odd eyebrow, they are, in the greater scheme of things, not unusual. The largest earthquake recorded in Colorado, for example, is estimated to have been as large as M6.6 (although it took place in 1882 and both its magnitude and exact location are uncertain). In Spain, the picture is even less clear – but the written record suggests that over a dozen very large and damaging earthquakes, mostly in the south, have struck since the fourteenth century.