Following two weeks of relative quiet, the week of 5-11 December 2012 saw an upturn in seismic activity around the word, especially in the western Pacific, where all three of the week’s largest tremors occurred.
Overall the United States Geological Survey’s real-time earthquake map showed 120 recorded earthquakes of at least magnitude 4 (≥M4.0), of which 38 were ≥M5.0.
Japan and Indonesia: The Week’s Largest Earthquakes
Two areas particularly vulnerable to seismic activity made the earthquake headlines this week. Both Japan (where four plates jostle for position) and Indonesia (where several small pieces of crust are squeezed between larger plates) have particularly complex tectonic settings and are subject to large and conflicting stresses.
The M7.3 which struck off the east coast of Japan raised memories of the devastating the 2011 earthquake which affected the same area, along the boundary between the Pacific and North American plates. Although it caused initial concern, the tremor, which was followed by several aftershocks of M4.0 or greater, did no damage and generated only a small tsunami of around a metre in height.
Thousands of miles further south, the complicated margin between the Australian and Pacific plates was the location for an M7.1 tremor in the Banda Sea. Although no damage or injuries were reported the tremor was widely felt, rattling windows in the north of Australia according to The Australian newspaper.
M6.3 in North Island, New Zealand
New Zealand is regularly shaken by tremors, with both the North and South Islands of the country affected. As elsewhere, the level of seismic activity varies (New Zealand’s Geonet earthquake service records figures varying from 149 tremors of ≥M4.0 in 1961 to 1,472 in 1995); but large earthquakes are by no means uncommon.
This week saw the North Island experience a quake of M6.3, the epicentre of which lay to the west of the southern extension of the Tonga Trench, where the Pacific Plate subducts beneath the Australian Plate: the island’s characteristic volcanic activity is a product of this. The South Island is affected by a further extension of this margin but the character of the boundary changes from a subduction zone in the north to a transform fault in the south.
West Coast Quakes in North America
As a footnote, it’s interesting to check out earthquakes in the mainland United States. In the previous two weeks, activity has been noticeably less than expected and the period of quiescence continues. The largest tremor recorded in the US (apart from Alaska’s recent quake, and an M4.4 off the Californian coast) was an M3.9 in Colorado. Like most earthquakes in stable tectonic areas, this is unrelated to present major plate movements, but more likely to be linked to local faulting.
Aikman, A. Indonesia earthquake shakes Darwin. (2012). The Australian. Accessed December 11, 2012.
Geonet. Earthquake Facts and Statistics. (2012). Accessed December 11, 2012.
United States Geological Survey. Real-time earthquake map. (2012). Accessed December 11, 2012.
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