Africa, Alaska and Sumatra: Earthquakes 27 June-3 July 2013


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Earthquakes in the week 27 June-3 July 2013. image credit: USGS

Earthquakes in the week 27 June-3 July 2013. Image credit: USGS

There were just short of 1,500 earth tremors shown on the United States Geological Survey’s real time earthquake map for the week of 27 June- 3 July 2013. In actual fact, the number will be much higher, since the map records only tremors of at least magnitude 4 (≥M4.0) in areas outside the US and its territories.

Nevertheless, the real-time map serves as a useful guide to earthquake activity and, in these terms, represents a quiet week.

Just one tremor, to the west of the island of Sumatra, exceeded M6.0, meaning that for the second week running there was no significant earthquake in the Pacific Ocean – an unusually long period of quiescence.

The focus of earthquake activity overall was, in fact, diverse, with earthquakes of ≥M5.0 occurring in the continental interiors of Africa and Asia as well as at the plate boundaries around the Pacific.

Indonesia M6.1 Quake: The Week’s Biggest Earthquake

The M6.1 of 2 July, which  occurred at a shallow depth (10km) and was followed by two aftershocks exceeding M5.0, may have been the largest of the week but, for its setting it is small. The epicentre of the quake occurred on the northern tip of the island of Sumatra, on an island arc which has been the location of some of the largest earthquakes in recorded history. Here, the Australian plate moves against, and is subducted beneath, the Sunda microplate which lies in the crowded area between the Indian and Pacific plates.

Faulting within the over-riding crust rather than at the plate boundary itself appears to be the cause for the earthquake of 2 July, judging by its location and depth. At M6.1 it was just one-thousandth of the size of the M9.1 Boxing Day earthquake of 2004 which occurred along the subduction zone: since the devastation of 2004 the line of the Sunda Trench, off Sumatra, has in seen tremors of M8.7 (2005), M8.4 (2008) and M8.6 (2012).

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