Like the previous week, the 21-27 November 2013 was dominated by earthquake activity in the usually seismically-quiet South Atlantic as a second major earthquake occurred on the Scotia Sea plate – although there’s no direct evidence to link the two events.
Other than that the week’s earthquakes were, as usual, concentrated upon the major plate boundaries, although in terms of magnitude there was little major activity other than the magnitude 7 (M7.0) in the Scotia Sea and an M6.5 in the Fiji region.
The Week’s Largest Earthquake: M7.0, South Atlantic
November 25 saw a large tremor in the South Atlantic, southwest of the Falkland Islands. The earthquake and its foreshocks and aftershocks occurred close to the boundary between the Scotia Sea plate and the South American plate. This is a conservative margin, so that movement is lateral rather than vertical – a fact which reduces the likelihood of a tsunami even with large-magnitude earthquakes.
The M7.0 tremor is notable for its size and the record shows very little of a similar size in the region, but it’s worth noting that the extreme remoteness of the area and the lack of detailed study compared with other strike-slip faults mean that the record may not be complete. Such faults elsewhere experience major earthquakes – perhaps most notoriously, the 1906 San Francisco earthquake (also a strike slip fault but on land not under sea) may have had a magnitude as high as M8.3.
Quakes in the Western Pacific: M6.5, Fiji
The western Pacific, with its complex jumble of slivers of crust buffering the Australian and Pacific plates and plate boundaries which vary in nature and direction, is regularly the source of earthquakes of at least magnitude 6. This week the region was struck by a tremor of M6.5 between the island groups of Fiji and Tonga.
From the available evidence, it appears that this tremor was directly associated with the subduction of the Pacific plate beneath the Australian plate along the Tonga Trench. Here, the tectonics are relatively straightforward and the depth of the tremor (377km) and its distance from the actual plate boundary suggest that it occurred at or close to the interface between the two plates, rather than as a result of deformation within the over-riding plate.
While the United States was quiet, seismically speaking, the Yellowstone region continued to experience a large number of very small earthquakes. Yellowstone is subject to both tectonic earthquakes (the result of compressive crustal motions) and volcanic earthquakes (resulting from movement of magma underlying the crust). Such events are by no means unusual, with major earthquake swarms in the region in 1985 and 2010 thought to be related to uplift by magmatic movement.
Quakes This Week: What is Unusual?
The M7.0 in the South Atlantic may be remarkable for its size – but the incompleteness of the record may mask other major tremors which have gone unrecorded in the past and have left no physical trace. By contrast, in areas such as Yellowstone, where the smallest earthquakes are recorded and analysed, our knowledge of a long seismic record helps to see that a pattern which may at first appear unusual is not necessarily so.
USGS. Real time earthquake map. Accessed November 27, 2013.
USGS. The Great 1906 San Francisco earthquake: What was the magnitude? Accessed November 27, 2013.
USGS. Repeating earthquakes suggest volcanic and tectonic origins of Yellowstone seismic swarms. Yellowstone Volcano Observatory. Accessed November 27, 2013.