Shake, Rattle & Roll

What a day! We have active weather moving into the region later this evening (see the blog below) after a moderate size earthquake shook the region this afternoon. At 1:42pm a 5.0 to 5.5 (different reports vary on the magnitude) occurred less than 60km north of Ottawa near Chelsea Quebec. This part of western Quebec is known for mild quake activity but a 5 plus magnitude quake is rare. How rare? The last time a quake this size occurred there was in October of 1990...just 50-60km from the epicenter of today's quake. So what caused it? Earthquakes are not really caused by anything other than stress. Thousands of years ago the Canadian Shield was pushed down by the weight of a massive glacier. That ice has obviously melted and the depression caused by it's weight is now "rebounding." The Earth is rising in this area. The problem is that, unlike squeezing a rubber ball, this rising motion is not fluid. It happens in steps or "slips." As the crust rises it becomes locked along faults as rocks tend to grab one another or bind up (they're jagged not smooth). Pressure builds until finally the rock breaks or moves and a slip occurs. This rapid "readjustment" results in surface waves as all of that energy is released. Where the slip occurs is know as the "hypocenter" and the point directly above the slip at the surface is the "epicenter" which is what you see on the map I've created above.

So we know where the quake occured but how far away was it felt? Earlier I spoke with John Bellini of the U.S.G.S and he told me about reports as far away as Sudbury and as far south as Washington. Wow! So why is this quake such a big deal? While no major damage has been reported this is a very rare event. Peterborough is not located along a tectonic plate boundary (like Los Angeles or Tokyo) so we're simply not used to the ground shaking. That's why a 10-20 second tremor managed to get everybody talking. Full details plus my quick interview with the U.S.G.S. will air tonight on Newswatch @ 6pm. To see a video explanation I gave to the Peterborough This Week newspaper as to why the quake happened click here. Hopefully this video helps answer the question why.