The week of 25-31 July 2013 saw earthquakes across the globe.
The United States Geological Survey real time earthquake map recorded 1,462 tremors (including all magnitudes in the US and its territories and those of at least M4.0 elsewhere).
Just two of these were greater than M6 (≥M6.0): an M6.2 in the South Sandwich Islands of the South Atlantic and an M6.1 in Vanuatu.
The majority of the other significant tremors (those greater than M4.5) were, as usual, concentrated in the western Pacific Ocean.
The Week’s Biggest Earthquake: M6.2, South Sandwich Islands
The largest tremor recorded this week was an M6.2 which struck in the South Atlantic to the east of the South Sandwich Islands on 26 July. At a depth of 10km the tremor was shallow, but the magnitude of the earthquake was insufficient to generate a tsunami.
The South Sandwich Islands form the volcanic front for a small but active subduction zone in the South Atlantic where the Southern margin of the South American plate is being forced beneath the Scotia microplate as the two converge. To the south the pattern is complicated by a transform plate margin between the Antarctic, South American and Scotia plates.
The past 30 days have seen a flurry of tectonic activity along the length of this arc and the M6.2 is the second largest to occur in this period after the M7.3 of 15 July, which had its epicentre just to the south of the junction between all three plates.
The Eastern Mediterranean: Greece and Turkey
The collision between Africa and the Euarasian continent has caused a complex pattern of tectonic activity with compressional and extensional forces at work within the Mediterranean basin.
The eastern Mediterranean is particularly prone to seismic activity as the remnants of the former Tethys ocean crust are crammed against the continental Anatolian Block which forms the Asian part of Turkey.
The process of subduction operates here along a short subduction zone which curves from the south west of Greece and is responsible for the existence of a group of islands including Crete, Rhodes and Cyprus.
The week saw a scattering of earthquakes along this margin and in the basins of the Aegean Sea behind, including three of M4.5, two long the segment west of mainland Greece and the third in the north-east Aegean.
US Earthquakes: Alaska
Alaska, where the Pacific plate moves northwards against the North American plate, is subject to strong compressional forces. The coming together of the two has created the long curved subduction zone of the Aleutian Trench and the chains of volcanic islands which lie behind it (to the north). These produced the largest tremor in the US this week – an M5.3 north of Amatignak Island. These tremors, however, are small for a region which has produced some of the largest earthquakes on record – including the 1964 M9.2, which caused a tsunami reaching as far as Hawaii.
Small Subduction Zones
Most of the planet’s major earthquakes occur at the largest subduction zones such as Alaska, Sumatra or Japan. The week’s earthquakes show that smaller subduction zones are also highly seismically active, though historically the magnitude of the earthquakes they cause is smaller.
USGS. Real time earthquake map. Accessed 31 July 2013.