This is a summary of my recent talk, with links to sources and more information.
Plate TectonicsThe surface of the Earth is composed of tectonic plates. They move relative to each other at about the same rate our fingernails grow (a few centimeters per year). Tectonic boundaries are classified by the direction of relative movement:
- Divergent boundary – plates move away from each other. Mid-ocean ridges. African rift zone.
- Convergent boundary – plates move towards each other. Subduction is when an oceanic plate dives under another plate: Sumatra, New Zealand, Japan, Cascadia. When continental plates collide, both stay at the surface forming huge mountain ranges: Himalayas.
- Transform boundary – plates move past each other. California, Haiti.
Earthquakes happen when plates get stuck, and stress builds up along the boundary. Eventually the stress is too much, and the fault ruptures, releasing the built-up energy all at once. Afterward, the stress along the ruptured part of the fault is much lower, but more stress builds up along the ends that didn’t move. (More complications.)
Most earthquakes happen along tectonic boundaries. Of those, the largest earthquakes usually happen along subduction zones, particularly along the Ring of Fire (the subduction zones along the Pacific plate).
Like most disasters, an inverse relationship exists between intensity and frequency. Larger earthquakes happen less frequently than smaller earthquakes, with, on average, approximately 1.3 million M2 earthquakes per year but only 1 M8+.
The two major earthquakes this year New Zealand and Japan) and my local region (Cascadia: Pacific Northwest of US/Canada) share a similar tectonic setting. All three are on triple-plate junctions, all three are subject to subduction-zone earthquakes, and all three have an extensive seismic history.
New Zealand EarthquakesNew Zealand had a M7.1 earthquake on 8 September 2010 near Darfield (Canterbury), and a M6.3 on 21 February 2011 near Christchurch. The first, larger earthquake happened in the middle of the night in the countryside, and caused minimal damage. By pure bad luck, the later, smaller earthquake happened in the middle of the day with the most intense shaking occurring in places with the highest population density. This caused major damage even though the Darfield quake released approximately 16 times the energy of the Christchurch quake.
Locally, cities on the Fraser River Delta (Richmond) are prone to liquefaction during earthquakes. The north shore is at risk to earthquake-triggered landslides. The downtown core (particularly Yaletown & Gastown) have reinforced brick buildings.
Japan Earthquake & TsunamiThe 11 March 2011 Tohoku (Sendai) M9.0 earthquake spawned over 800 aftershocks and a tsunami, and caused major damage. Follow the links for good summaries and compilations of technical data.
Local (Cascadia) Hazards
Although Canada can have earthquakes all over the country, most are on the west coast along the Cascadia subduction zone. From GPS measurements, we know Vancouver Island is uplifting by about 4 mm/year, and shortening by about 10 mm/year. From assorted geologic evidence, we know that the Cascadia zone is subject to megaquakes approximately once every 300 to 500 years, with the latest occurring in 1700.The 11 March 2011 Japan event is a geologic mirror-image of the 26 January 1700 Cascadia earthquake, an estimated M9+ event that sent a tsunami across the Pacific (compared to 2004 Sumatra).
Natural Resources Canada covers the consequences of a modern megaquake as well as a well-populated FAQ. For more technical evaluations of our earthquake risk, check out these 2004 and 2008 papers. If you want to learn more about local hazards beyond earthquakes, I recommend Vancouver: City on the Edge as a clear, well-illustrated popular science book focused on geohazards in the Vancouver region.
Low Intensity, High Frequency Events
- Have a grab & go bag.
- Backup your data (particularly for academics).
- Keep real shoes by your bed.
High Intensity, Low Frequency Events
- Designate an out-of-area contact person.
- Use multiple modes of communication. Sometimes SMS will work when cell networks are busy; sometimes landlines work when everything else is down.
- Have neighbourhood knowledge. Familiarity increases resilience. Neighbours that know the locations of each other’s bedrooms are better at digging for survivors. Communities that know each other handle the stress of disasters better.