Earthquake

Recent earthquakes as of 2pm EST on June 23. -- Image from USGS Earthquake Hazards Program, http://earthquake.usgs.gov/earthquakes/recenteqsww/

This afternoon I had popped into the shower to rinse off the sweat and grass after mowing the lawn (a concession – it’s easier to pick up dog poops if the grass is short; also, we were getting a lot of ticks this spring and felt the yard should be a tick-free zone) when there was a strange noise and a vibration under my feet. It sounded like it was coming from the water pipes, and my first thought was that the water pump in the basement, which is probably as old as the house (30 years) was finally giving up the ghost. In order to prevent anything disastrous as I assessed the situation, just in case it was about to blow, I turned off the water. But the floor was still vibrating – in fact, it sounded and felt a little like Raven was vigorously itching on the floorboards just outside the washroom. I stepped out of the tub to judge better if it was the dog or actually was the water pump about to have a catastrophic meltdown, and the door began rattling in its latch. And that’s when I thought, this reminds me an awful lot of the quarry blasting we would occasionally feel at the house where I grew up, where the quarry was just 5 km (3 mi) away – except there is no quarry nearby here, and it’s lasting much too long anyway.

Could it be an earthquake?

I stood on the mat, slowly dripping water onto the bathroom floor, my hair lathered up and piled on top of my head, feeling completely indecisive about what to do. While I suspect that those quarry-like tremors I’ve felt in years past might occasionally actually have been earth shakes, I’ve never been in an earthquake before that caused the floor to vibrate and the door to rattle in its frame. This was strange and disconcerting, and, if I’m completely honest, a little bit scary.

It only lasted about ten seconds, and once it was evident that the house wasn’t going to fall apart, nor the water pump in the basement blow up, I climbed back into the shower to finish washing the shampoo out of my hair.

Recent earthquakes as of 10pm EST on June 23. -- Image from USGS Earthquake Hazards Program, http://earthquake.usgs.gov/earthquakes/recenteqsww/

As soon as I was out, however, I popped on to the USGS Earthquake Hazards Program’s realtime earthquake monitoring page, where they display records of all seismic activity over the last seven days. Earthquakes in the last hour are shown in red. There was a giant red square smack over eastern Ontario.

The page, if you click on that red square, gives more details on the event. When they first assessed it they called it a magnitude 5.5 quake, with an epicentre about 61 km (38 mi) north of Ottawa and 19 km (12 mi) deep; this was later revised to a magnitude 5 centred 56 km (35 mi) NNE of Ottawa and 16 km (10 mi) down.

The Richter scale always gets me. It only goes up to 7 or rarely 8, so it seems like 5 should be quite a substantial tremor. However, the scale is logarithmic – that is, each number is to the power of ten times greater than the previous. So a 2 isn’t twice as strong as a 1, it’s ten times as strong. A 3 isn’t three times as strong as a 1, it’s 100 times. And so on. To put it in perspective: the quake that hit Haiti was roughly a 7 on the Richter scale; the one that got Chile shortly after was an 8.8. Our quake was a hundred times weaker than the Haiti quake, and nearly 10,000 times weaker than the Chilean one. Strength here, by the way, is measure in amplitude – side to side movement – and not energy released which follows a slightly different logarithmic principle that I don’t really understand.

A magnitude 5 earthquake is labeled “moderate”. Generally, damage is restricted to poorly-constructed buildings within about 40 km (25 mi) of the epicentre; well-constructed buildings and those farther away should suffer little if any damage. My sister was working in Ottawa at the time of the quake, and reported that it was sufficient only to knock a few glasses off the office shelves and that was all. Out here in Perth there was the door shaking and the floor vibrating, but nothing was displaced. Dan, who was out doing bird surveys in the rock barrens of Frontenac Provincial Park and therefore on very firm ground, didn’t even feel it.

So it was pretty minor, as earthquakes as a whole go. It also wasn’t uncommon, on a worldwide scale. Wikipedia suggests that around the world there are about 800 magnitude 5 earthquakes every year. If that seems like a lot, consider an estimated 6,200 magnitude 4 quakes, 49,000 magnitude 3, 1000 per day magnitude 2, and a crazy 8000 per day magnitude 1. Unsurprisingly, magnitudes 1 and 2 are rarely felt, unless you happen to be directly over the epicentre, and even then if you’re on solid ground you might not notice. Magnitude 3 feels a little like the quarry blasts – might rattle the windows a bit, but could easily pass unnoticed.

Citizens reporting detection of earthquake as of 2:15 pm EST. -- Image from USGS Earthquake Hazards Program, http://earthquake.usgs.gov/earthquakes/recenteqsww/

And yet, a detectable earthquake in northeastern North America is so unusual that within minutes Facebook, Twitter and the local news feeds were all abuzz about it. I commented to one friend that an earthquake here is our equivalent of snow in Florida – we’ve a tendency to the melodramatic because these things just don’t happen here. Tornadoes do the same thing to us, although recently they’ve been becoming a bit more common with a few occurring every year, mostly down in the southwestern part of the province.

The USGS site has a spot where you can report whether you felt the quake and how strong it seemed where you were. Within half an hour, nearly 1700 people had logged on to the site and posted their report. The farthest at that time came from Toronto, a little over 400 km (250 mi) southwest of the earthquake’s epicentre. (For some reason, in Canada the reports are indicated by city, with the circle representing the population size of the city and not the strength of the quake there or the number of people reporting; in the US, presence of a reporting citizen is simply indicated by flood-filling the county.)

Image from USGS Earthquake Hazards Program, http://earthquake.usgs.gov/earthquakes/recenteqsww/

Citizens reporting detection of earthquake as of 10 pm EST. -- Image from USGS Earthquake Hazards Program, http://earthquake.usgs.gov/earthquakes/recenteqsww/

As of this writing (eight hours post-quake), more than 54,000 people have weighed in. The farthest reports now come from beyond the edges of the map. A news article from the Huffington Post, posted to Facebook by a fellow blogger, suggests that the earthquake was detected as far away as Wisconsin and Kentucky.

Earthquakes recorded around Lake Ontario and southern Quebec within the last 20 years. -- Image from USGS Earthquake Hazards Program, http://earthquake.usgs.gov/earthquakes/recenteqsww/

The earthquake occurred within an area known as the Western Quebec Seismic Zone. We typically think of earthquakes as happening where two of the earth’s giant tectonic plates meet and grind together – most well-known to North Americans is probably the meeting of the North American and Pacific plates along the western coast, especially California (who also have a much smaller plate, the Juan de Fuca, tossed into the mix). Since here in Ontario we’re about as far away from a plate edge as you can get in North America, we tend to think of earthquakes as something that happens somewhere else.

However, there are still faults in this area, and many remain “active”. They date back millions of years to when there was a huge mountain chain, as rugged and tall as the Rockies, that ran through what is now southern Quebec and into the United States. In modern times the faults themselves are largely buried and reduced to small cracks, most of them deep and small enough as to be difficult to detect, such that few have official names. They tend to run northwest-southeast, along the same line the prehistoric mountains once stood. Small seismic events, large enough to be noticed but not large enough to do any damage, occur three or four times a year in this seismic zone. Earthquakes of the magnitude recorded today are much rarer, maybe only once a decade or less. There have been only two “large” quakes in this region since Europeans started keeping data: one in 1732 and the other in 1935, both just slightly over magnitude 6. I suspect that quakes here are mostly the result of shifting and settling within the earth’s crust that causes old faults to “pop”, rather than an active building and release of pressure as happens where two plates meet. They occur so far underground that very rarely do they ever rift the surface.

Average number of magnitude 5 earthquakes per year within the region - note that gray is marginally more than zero. -- Image from USGS Earthquake Hazards Program, http://earthquake.usgs.gov/earthquakes/recenteqsww/

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6 responses to “Earthquake

  1. I notice you used the USGS for screenshots; I had to turn to them for information as well, as Natural Resources Canada’s quake site was unavailable for quite some time.

  2. Felt the shaking here in N. Michigan….

  3. I felt it here, too! I was sitting on the sofa when it started to vibrate – just as if an animal was also sitting on it and having a scratching fit. The cat, however, was sound asleep. The piano was also rattling, and the dog, who was lying on the floor (not scratching) was whining. I immediately phoned the office and said “did we just have an earthquake?” They felt nothing, but later my boss called back to confirm that I wasn’t going nuts – we had indeed felt the tremors from Canada!

  4. Very thorough summary, Seabrooke. I’m so glad you did this. I was hoping someone I knew would write about it from both the scientific and personal aspects. And though I know it’s due to population centers, I find that fourth graphic to be very compelling based on the quake’s epicenter and the various reports submitted. Insightful!

    BTW, I was curious as to why you used the USGS data instead of Natural Resources Canada. Any specific reason(s)?

  5. Thanks for link to the earthquake monitoring site. The site sure gives a lot of important information and has great graphics. — barbara

  6. I thought exactly the same as you – quarry blasting. I was in a meeting in Toronto and sort of tuned it out until people looked around at one another and said, “What was that?” and it occurred to me quarry blasting is not a normal thing to feel in the middle of Toronto.

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