A tsunami warning was in force for the state of Hawaii after a massive earthquake of magnitude 7.7 (M7.7) struck off Canada’s western seaboard, but no major damage from the waves has been reported.
The tremor, which occurred off the Queen Charlotte Islands at around half past four in the morning (UTC) was followed by over a dozen aftershocks of greater than M4.0, which were continuing at the time of writing.
Early reports indicate that there has been little or no damage as a result of the quake, although the NOAA issued a tsunami warning alert for Hawaii.
Early indications are that a tsunami has been generated and initial measurements given by the Pacific Tsunami Warning Center suggested an amplitude of up to 0.3m – although these measurements are taken in the open ocean and as a result “are generally much smaller than would be coastal measurements at similar locations.”
The reality of the waves, however, caused little damage, according to local reports.
The Causes of the October 28 Queen Charlotte Islands Earthquake
The western coast of the North American continent is part of the so-called Pacific Ring of Fire, a belt of earthquake and volcanic activity surrounding the Pacific Ocean as a result of the relative movements of the earth’s tectonic plate. These zones are varied and complex: in the south of the continent the zone is a major transform zone (the San Andreas Fault) where plates slip past one another, with a subduction zone (the Cascadia subduction zone) further north as the remnant Juan de Fuca plate is subsumed beneath the continent.
The October 2012 earthquake occurred to the north of the Cascadia zone close to the Queen Charlotte Fault in a zone of oblique plate movement (described by Natural Resources Canada as “Canada’s equivalent of the San Andreas Fault”) where plates are moving relative to one another at a rate of around 50mm per year. Early indications from the USGS suggest that an element of reverse thrusting near the plate boundary, to accommodate a degree of oblique movement may have been responsible for the tremor.
Major Earthquakes off Western Canada
Squeezed between the subduction zones of Cascadia to the south and Alaska to the north, it’s unsurprising that western Canada should be subject to major earth movements.
The USGS historic earthquakes list shows 8 events of ≥M6.0 in the west of the country in the last century four of them in the Queen Charlotte Islands region.
Though significant by any standards, the tremor of October 28 is not the largest earthquake to be recorded in the area.
That dubious honour belongs to the 1949 event which occurred along the Queen Charlotte Fault a little way further north. At M8.1 it was around 2.5 times larger than the more recent event (the magnitude scale is logarithmic).
NOAA Pacific Tsunami Warning Center. Tsunami bulletin. (2012). Accessed October 28, 2012.
USGS. M7.7 – 139km S of Masset, Canada. (2012). Accessed October 28, 2012.
Natural Resources Canada. The M8.1 Haida Gwaii (formerly Queen Charlotte Islands) Earthquake of August 22, 1949. (2011). Accessed October 28, 2012.
CBS News. Hawaii Tsunami Waves Smaller Than Expected. (2012). Accessed October 28, 2012.