Although earthquakes are largely restricted to particular zones of tectonic activity, their frequency and magnitude follow no particular or predictable pattern. The week of 12-18 September 2012 proved relatively quiet in terms of the occurrence of significant earthquakes, with just 33 tremors of magnitude 5.0 or greater (≥M5.0).
Pattern of Larger Earthquakes
The week showed not just relatively few earthquakes of ≥M5.0, but was also noteworthy in that just one of the tremors was greater than M6.0 – an M6.2 event that occurred off the west coast of Sumatra. This tremor occurred in highly active tectonic context, where the Australian Plate is being subducted beneath the Eurasian Plate.
Given the tectonic history of this area, this earthquake is by no means unusual – the data indicate that the subduction zone marked by the Sunda Trench has been subjected to over 70 tremors of ≥M4.0 in the past 30 days, with 14 in the last week. The area is notorious for being the location of some of the largest and most devastating earthquakes in history, most notably the Boxing Day earthquake of 2004 which, with its associated tsunami, was responsible for the loss of over a quarter of a million lives.
The British Virgin Islands Earthquake Cluster
Looking more closely at the earthquake maps, a cluster of almost 120 earthquakes occurred around the British Virgin Islands. Though not marked by an event of any great size or significance (the largest recorded measured just M4.3) the earthquakes are noteworthy in that they clearly illustrate seismic activity along a transform fault zone.
In this region, the Caribbean Plate is caught between the North American Plate in the east and the South American and Cocos Plates to the west. The relative plate movements are accommodated by a combination of lateral movement along strike-slip faults and by thrusting along subduction zones. The cluster of earthquakes observed this week is located on a section of the boundary where plates slide laterally past one another, and are therefore most likely to be caused by lateral, or strike-slip, motion.
The M5.5 in Greece: Mediterranean Earthquakes
Another noteworthy earthquake to occur in the week was the M5.5 tremor just to the south of the island of Crete in the eastern Mediterranean. Like the Caribbean, the eastern Mediterranean basin is tectonically complex – in this case, representing the final stage in an ocean cycle as the northwards-moving African Plate collides with Europe, closing the former Tethys Ocean and leaving its remnants in the form of the Mediterranean basin.
A glance at the map produced by the European-Mediterranean Seismological Centre shows European earthquakes at a more detailed level than the USGS maps for the region. Looking at this map indicates not just a line of earthquakes along the south of Crete, where a subduction zone occurs, but also to the north, where both compression and extension causes earthquakes along fault lines.
Complex Tectonic Settings: Earthquake Summary
Though not marked by any significant earthquake, assessment of the USGS interactive map shows clearly that earthquakes are associated with major plate boundaries – but this week’s earthquakes also draw attention to some of the more complex tectonic settings and the many smaller earthquakes which are associated with them.
European-Mediterranean Seismological Centre. Euro-Med Earthquakes. (2012). Accessed September 18, 2012.
USGS. Interactive real time earthquake map. (2012). Accessed September 18, 2012.
USGS. M4.3 – 137km NNE of Road Town, British Virgin Islands. (2012). Accessed September 18, 2012.