Aftershocks often more dangerous than initial, larger quake

Scott Sutherland
October 29, 2012

On Saturday, just after 8 p.m. Pacific Time (PT), a major earthquake — measured at magnitude 7.7 on the Richter Scale — struck the Queen Charlotte Islands, about 200 km southwest of Prince Rupert, British Columbia. According to some reports, the shaking lasted for 40 seconds, and the quake was felt in the Greater Vancouver area and as far east as Edmonton and Calgary. Although it shook the areas of Prince Rupert fairly badly, luckily no injuries or damages were reported and even the small tsunami that the quake touched off caused no damage.

Since then, nearly 70 aftershocks have been felt, ranging between magnitudes 4.0 (light) and 5.5 (moderate), and one strong, magnitude 6.3 aftershock shook the area just before 10 a.m. PT on Sunday. Light earthquakes and aftershocks were also felt down the west coast, as far as Los Angeles, California.

[ Related: Earthquake off B.C. coast precursor to the Big One? ]

To imagine what causes aftershocks, press your index finger down onto the surface of your desk, and then try to push your finger away from you. The exact moment that your finger overcomes the friction with the desk and slips forward is like the initial quake. All the movement your finger makes after that first moment is like the aftershocks.

The rocks pressing together along fault lines are under an enormous amount of pressure, and they have an equal amount of friction between them. As what happened with your finger and the desk, the pressure eventually overcomes the friction between them and the rocks slip past each other. The amount of pressure between them usually determines the strength of the resulting earthquake. The aftershocks are caused as the rocks continue to slip past each other, until they reach a point when the friction between them builds to the point where they stop moving again, just as your finger stopped moving along the desk.

In some ways, aftershocks are seen as even more dangerous than the initial quake that spawned them. The aftershocks tend to be of lower magnitude, but when there are multiple strong, moderate or even light earthquakes, one after another, possibly for days after the initial quake, any structures that were weakened by the initial quake can completely fail, and even buildings that were unharmed by the initial quake can succumb to the stresses of the aftershocks that follow. Also, more injuries tend to occur because people are usually more out in the open during aftershocks, as they are searching for other survivors or, if the first quake destroyed whatever shelter they had, trying to find other shelter.

[ Related: B.C., earthquake, tsunami warning add fuel to anti-pipeline argument ]

Even though no significant damage was caused by Saturday's quake or the aftershocks, questions are being raised right now about both Vancouver's readiness for earthquakes and the supposedly slow warning time for the tsunami.

Information on what to do during an earthquake can be found here. Information and safety tips regarding tsunamis can be found here.