The week of 10-16 October saw a total of 32 earthquakes of equal to or greater than magnitude 5 (≥M5.0) across the globe.
The USGS real time earthquake map of earthquakes greater than or equal to M2.5 in the US and its territories (and some other areas) and greater than or equal to M4.0 for the rest of the world shows some 188 events.
Distribution of Seismic Activity and the Week’s Largest Earthquakes
As usual, the map showed that most of the tremors are concentrated around the boundaries of the earth’s tectonic plates, particularly at subduction zones and along transform boundaries (again most notably around the Pacific rim) with a sprinkling along spreading ridges and a few (generally smaller) within the usually more stable intra-plate regions.
Only two earthquakes of greater than or equal to M6.0 were recorded; the largest being an M6.7 event in Indonesia, among the jostling microplates which characterise the collision zone between the Australian and Eurasian plates. The other was an M6.4 in the southern Indian Ocean, along the ridge separating the Australian and Antarctic plates.
Himalayan Earthquakes and Continental Collision
This week’s quake map shows a cluster of earthquakes away from marked plate boundaries in the eastern Himalayas. The tremors, in eastern Afghanistan and western Pakistan, are the product of continental collision as the northwards-moving Indian subcontinent collides with the Eurasian plate at a rate of around 40-50mm per year.
Because both plates are composed of the same buoyant continental crust at the point of collision, there is no subduction – instead, the earth is forced upwards at that point. This upward movement gives rise to the Himalayas, the highest mountain range in the world, along with others such as the Karakoram range and the high Tibetan Plateau which lies to the north. Within this area, therefore, earthquakes are common.
The M3.9 Beloeil Tremor: Canadian Earthquakes
The east of Canada experienced what was, for the area, a significant seismic event with the occurrence of an M3.9 tremor close to Montreal. The earthquake is notable because eastern Canada is distant from tectonic plate boundaries, where most major earthquakes occur, though its size is not remarkable in a worldwide context (the USGS estimates that there are around 130,000 earthquakes of M3.0-M3.9 annually).
The distribution of earthquakes across Canada and the United States is instructive. The vast majority of Canada’s large earthquakes occur in the west, where a two of the earth’s tectonic plates collide, the Pacific Plate being subducted beneath the North American Plate. Here, at the Cascadia subduction zone, major tremors of M7 or greater can occur.
By contrast, the eastern part of the continent experiences smaller tremors (only two, in 1663 and 1929) have reached M7 and the latter, the Grand Banks earthquake which caused a submarine landslide and resulting tsunami costing 28 lives, is eastern Canada’s largest earthquake at M7.2. By comparison, the Cascadia subduction zone has recorded three earthquakes greater than or equal to M8.
Summary of the Week’s Activity
The week’s seismic activity shows that although most earthquakes occur along tectonic plate boundaries, this is by no means universally true. In addition, even tremors of relatively small magnitude may be considered significant within their context.
Natural Resources Canada. Important Canadian Earthquakes. Accessed October 16, 2012.