M6.6 Earthquake off Vancouver Island: 23 April 2014

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Location of the 23 April Vancouver Island earthquake. Image credit: USGS

Location of the 23 April Vancouver Island earthquake. Image credit: USGS

A magnitude 6.6 (M6.6) earthquake struck off Vancouver Island on the evening of 23 April 2014.

Early reports suggests limited damage and no injuries; and despite its offshore location the earthquake didn’t generate a tsunami – but the tremor is significant nevertheless in seismological terms, given its location along a potentially devastating subduction zone.

23 April Earthquake: Tectonic Setting

The coast of western Canada and the northwest United States is marked by a major subduction zone, where one of the Earth’s tectonic plates is forced beneath the North American continent.

The plate in question is the Juan de Fuca plate, thought to be the remnant of a much larger plate which is gradually being consumed. The boundaries to the north and south are both areas of lateral movement, marked by the Queen Charlotte and San Andreas fault zones respectively.

At the time of writing there’s little information available on last night’s Canada earthquake other than its magnitude, location and depth (11.4km). These, taken in its tectonic context, are sufficient to provide certain clues as to its origins.

Looking at fault maps of the area, it seems that the tremor probably occurred along the Nootka fault zone, which divides the main Juan de Fuca plate from its smaller northern neighbour, the Explorer plate. If this is the case then it would explain the relatively shallow depth and would also imply that the fault mechanism is lateral rather than vertical movement at the plate interface.

The Cascadia Subduction Zone

Earthquakes (at least M6.7) around the Juan de Fuca plate since 1900. The Cascadia seismic gap is obvious. Image credit: USGS

Earthquakes (at least M6.7) around the Juan de Fuca plate since 1900. The Cascadia seismic gap is obvious. Image credit: USGS

Subduction zones are the sources of the world’s largest and most destructive earthquakes. The Cascadia zone has been relatively quiet for over two hundred years as far as subduction earthquakes are concerned; but crustal earthquakes, as that of the 23 April appears to be, are not uncommon and can be of significant magnitude.

A map of earthquakes larger than the most recent (drawn from the United States Geological Survey’s earthquake database) shows 28 tremors of M7.6 or greater since 1900. These are clustered along the fracture zones at the southern end of the plate, off California, and along the transform fault zone to the north.

The subduction zone itself shows a clear absence of earthquake activity, providing an example of what’s known as a seismic gap. It’s unclear, however, whether the area is aseismic or whether it is locked and accumulating energy which may in the future be released in a major, potentially disastrous earthquake.

Cascadia Past and Future

The Cascadia Subduction Zone has the potential to trigger very large earthquakes, with devastating results. Although Robert Yeats notes that: “Cascadia has not undergone any plate boundary earthquakes in 200 years of record-keeping” there is evidence from written and oral record in Japan and among the first Nations of a very large (megathrust) earthquake in the area in 1700. This is estimated at around M9, which would make it among the largest known.

On the basis of existing information (subject to change) it appears that the 23 April Canada ‘quake is not an activation of the Cascadia subduction zone. The possibility of a major tremor in this region in the future cannot, however, be ruled out.

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