M7.7 in Western Canada Dominates the Week’s Earthquakes

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A significant proportion of the week’s earthquakes M4.0 or more occurred in western Canada -Image credit USGS

The week of 24-30 November has been dominated by the elements in North America.

As Tropical Storm Sandy was preparing to make landfall across swathes of the continent’s Atlantic seaboard, the west coast of the continent didn’t remain immune – though in the case of Canada’s Haida Gwaii (formerly Queen Charlotte Islands) it was the earth, not the air, which moved.

Were it not for the M7.7 which struck on 28 October, it might have been a quiet week. In total, the United States Geological Survey’s real time earthquake map shows 289 tremors of at least magnitude 4.0 (≥M4.0), again showing a concentration around the Pacific and the north eastern Indian Ocean.

The M7.7 in Haida Gwaii (Princess Charlotte Islands)

Canada’s M7.7 dominated the week seismologically: not only was it the largest, but its 78 accompanying aftershocks of ≥M4.0, including two tremors of M6.2 and M6.3 which, at another time would have been worthy of comment in their own right, made up almost half of the planet’s recorded earthquakes in this size category.  The main shock generated a tsunami which reached Hawaii some hours after the ‘quake, but very little damage was done.

The Princess Charlotte Island tremors took place along a section of the boundary between the North American and Pacific tectonic plates as they move laterally past one another. Typically, movement on this type of fault would be lateral or ‘strike-slip’, reflecting the overall nature of the boundary. In this case, however, there’s an element of oblique movement between the two plates and so the cause of the earthquake may have involved reverse faulting, where compression forces one area of rock upwards over another.

The M7.7 tremor was followed by numerous aftershocks -Image credit USGS

Large earthquakes off the north-western coast of North America are not uncommon, given the existence of the San Andreas Fault zone of the Cascadia subduction zone. Sunday’s earthquake is notable, however, for being the second largest to be recorded on the strike-slip section of the fault, exceeded only by a tremor of M8.1 in 1949.

M5.3 in Southern Italy

Just days after the controversial conviction of six Italian seismologists for failing to predict a damaging earthquake, the south of Italy was again shaken by a significant tremor. The closing of the Mediterranean basin, as Africa collides with Eurasia, has led to a complex system of tectonics with differential movements in different areas.

Although the dominant movement is compressive, Italian earthquakes are largely influenced by extensional processes as new areas of crust are created through a process known as back-arc spreading. The result of this is normal faulting where one area slips downwards relative to another: this appears to have been the case in this week’s event. No significant damage was reported as a result of the tremor: nor is recorded whether any attempts were made to predict it.

Severe Weather and Tectonic Movement

This week, at least, the world has been luckier in seismological terms than meteorologically. Although enormous destructive forces are constantly at work within the earth – and, unlike Tropical Storm Sandy, can strike without warning – the Canadian, Italian and other significant earthquakes have not caused significant damage or loss of life.

Sources

USGS. M5.3 – 6km SE of Mormanno, Italy. (2012). Accessed October 30, 2012.

USGS. M7.7 – 139km S of Masset, Canada. (2012). Accessed October 30, 2012.

USGS. Real time earthquake map. (2012). Accessed October 30, 2012.

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