Perched beside an earthquake fault line, Vancouver Island must pay close attention to a new study that suggests some of our assumptions about quakes are dangerously wrong.
All our attention has been focused on surviving the initial shock of a quake, but the aftershocks can last for years and affect cities hundreds of kilometres away, U.S. and Japanese seismologists say in the journal Science.
The report means we might have to rethink our disaster planning. Buildings are being designed and upgraded to withstand heavy quakes, but our infrastructure doesn’t take into account repeated aftershocks.
Two years after the magnitude-nine quake that hit Japan, aftershocks in Tokyo are still three times more frequent than they were before the quake; in the immediate aftermath, they were 10 times more frequent. And Tokyo is 400 kilometres from the epicentre.
It is common for aftershocks to be only one magnitude smaller than the initial quake, and some aftershocks can even be larger than the first one.
Planning for earthquake recovery has long assumed that a major quake reduces the risk of more earthquakes because it relieves stress building up between the tectonic plates. The new research suggests that’s not true.
On the Island, we have to be aware that the danger is not only from an earthquake right off our shores. A major quake in California could produce aftershocks in our region for years.
The new research should spur us to take another look at building codes, so we are prepared not only for the Big One, but all the quakes that come after it.