An unusually-large earthquake (for its tectonic setting) in the Caribbean topped the list in the week of 13-19 February 2014.
This was the only earthquake of at least magnitude 6 (≥M6.0) to appear on the United States Geological Survey’s real time earthquake map (covering all events in the US and its territories and those of ≥M4.0) elsewhere; but the map shows 23 tremors of ≥M5.0 and 90 of ≥M4.0.
These quakes are mostly concentrated around the Pacific Ocean; in the so-called Pacific Ring of Fire.
The Week’s Largest Earthquake: M6.5, off Barbados
Although not perhaps significant in the wider scheme of things, the M6.5 Caribbean earthquake which struck 170km north of Barbados on 18 February was unusually large for its setting. The Caribbean overall is notably tectonically active; the Caribbean plate is caught between three others (the North American and South American plates to the east and Cocos plate to the west. The result of this is regular regional volcanism along the margins of the plate.
The eastern margin of the Caribbean plate is marked by a subduction zone where convergence between the Caribbean and North American plates leads to subduction of the latter beneath the former. Despite an association between subduction zones and major earthquakes, however, large earthquakes in this particular section of the boundary are rare. The available data suggest that this week’s M6.5 tremor originated through deformation within the overriding plate rather than at the subduction zone itself.
The Scotia Arc
In the extreme south of the Atlantic, a chain of volcanic islands mark the Scotia Arc; the South Sandwich Islands, which represent the eastern margin of the Scotia microplate (which lies between the South American and Antarctic plates).
Like the Caribbean plate, the Scotia micro plate is (broadly) characterised by lateral movement at its northern and southern edges and subduction at the east, although the plates differ in that the western margin of the former is marked by a subduction zone while that of the latter, poorly defined and understood, is probably a fracture zone.
The remoteness and lack of population in the area means that the earthquakes which occur are often overlooked. Nevertheless, this region does experience frequent, sometimes quite significant, tremors; 2013 saw earthquakes ≥M6.0 at both northern and southern margins. This week the most noteworthy was just M5.0 but is worthy of comment because, again like the Caribbean tremor, its intraplate location suggests that it resulted from internal deformation rather than fracturing at the subduction zone.
Earthquakes in the US: South Carolina
The largest tremor in the United States this week was an M5.0 in Alaska; but the most noteworthy was an M4.1 which struck near Edgefield, South Carolina, on 15 February. Although the eastern US is remote from plate boundaries, shallow earthquakes resulting from existing faults are not uncommon; the USGS observes that “moderately damaging earthquakes strike the inland Carolinas every few decades, and smaller earthquakes are felt about once each year or two.”
Indeed, the historic earthquake listings show that an earthquake with an estimated magnitude of M7.3 struck near Charleston in 1886, killing 60 people. More recently, an M4.4 struck the state in 2002.
Atlantic Ocean Subduction Zones
The Atlantic Ocean differs from both the Pacific and Indian Ocean in that it lacks large subduction zones – and, as a result, is still growing. The two short subduction zones which do exist (scientists believe that that a third may be developing off the Straits of Gibraltar) both experienced earthquakes this week – though neither appears to have been a typical subduction zone event.