California is along a transform fault, where the Pacific tectonic plate is sliding past the North American tectonic plate (or the North American plate is sliding past the Pacific plate, but since I’m not so good at standing on water I usually consider land to be the fixed reference point!). This makes for relatively frequent, small, shallow earthquakes, resulting in a local population with a high degree of earthquake awareness.
British Columbia (and Washington and Oregon) are part of a subduction zone, where the Juan de Fuca tectonic plate is diving under the North American tectonic plate. The plates move erratically, getting stuck and building up energy which is released during earthquakes. Subduction zones have the capacity for small shallow earthquakes, but also deeper megathrust earthquakes (like 2004 Sumatra, or 2010 Chile). Several hundred tiny earthquakes occur every year, but the average return period for a major event is approximately every 300 to 500 years. This results in a local population with a low degree of earthquake awareness.
The last major earthquake was around 9 pm on 26 January 1700. (The timing is determined by a combination of geologic evidence, and Japanese tsunami records), and was magnitude 8.5 or larger. Yes, that means we’re within the return period, and are expected another major earthquake soon. (“Soon” is anytime “today” through “a few hundred years”. On geologic scales, that’s downright instantaneous!) ShakeOut BC will be on the anniversary of this event, on 26 January 2011 at 10am.
The consequences of a major earthquake in British Columbia could include liquefaction of wet, unconsolidated sediment (the Fraser River delta), rockfalls, landslides, even a tsunami generated along the west coast of Vancouver Island. It is the responsibility of every resident to be prepared to take care of themselves for at least 72 hours (food, water, medications…). It’s also important to know what to do in an earthquake (we have good building codes: hide under furniture to protect yourself from small falling objects), and afterwards.
So if you’re in the Pacific Northwest, sign up, read up, and participate in ShakeOut BC.