The week of 11-17 July 2013 saw a flurry of smaller earthquakes around the planet – the USGS real-time earthquake map (which shows tremors of all magnitudes in the US and its territories and at least magnitude 4.0 worldwide) recorded almost 1700 – a number which, while not exceptional, is higher than for many weeks past.
The map shows a notable concentration along the central Andes, alongside the usual tremors in the western Pacific. Notably, the was also seismic activity in the southern Atlantic with an M5.7 off the Palmer Peninsular being dwarfed by the week’s largest earthquake, an M7.3 which struck close to the South Sandwich Islands.
The Week’s Largest Earthquake: M7.3, South Sandwich Islands
The South Sandwich Islands are the volcanic island arc formed by a relatively short subduction zone along the junction between the Scotia microplate and the southern edge of the South American plate. The 15 July earthquake occurred at shallow depths (31km) to the south of islands where the situation is complicated by the presence of a triple junction between these two plates and the Antarctic plate.
Location and information on the actual plate movements indicate that the tremor was not, in fact, a subduction zone earthquake but was caused by lateral slip along the boundary between the South American and Antarctic plates. The absence of vertical movement explains why not tsunami occurred, despite despite the magnitude and offshore location. Major earthquakes here are not unknown: the previous largest in the region was an M7.4 in 2006.
M5.0, Canadian Northern Territories
Appearing rather isolated on the map, the M5.0 which occurred on 12 July is likely to be associated with crustal movements in the western part of North America, resulting from the collision of the Pacific and North American plates along a margin from Alaska to British Columbia. Although the plate margin is hundreds of miles to the west, the collision is responsible for the uplift of the Rocky Mountains.
The tremor of 12 July is unusual but not exceptional: information form Natural Resources Canada shows that seismic hazard in the region is low and that although earthquakes of M5.0 or more do occur they are relatively rare and typically cause little or no damage.