There’s something inherently horrifying about earthquakes. Probably it’s because we are accustomed to thinking of the ground beneath our feet as being, well, “rock-solid.” When that ground gets the shakes, it gives us a pretty good case of them, too.
Earthquakes are defined as “a fracture or implosion beneath the surface of the Earth, and the shock waves that travel away from the point where the fracture has occurred.”
They arise because, far from being rock-solid, the ground we stand on, and therefore we ourselves, are afloat on giant stone rafts called “lithospheric plates” that move around on the Earth’s molten interior. Where these plates bump together or split apart, stresses build up in the rocks. Sometimes the stresses are released quickly, in minor tremors, while other times the stresses build and build until the rock gives way in a massive shock that can collapse buildings, slosh rivers out of their banks and create 30-metre tidal waves. There are more than a million earthquakes worldwide every year– 1,000 of them in Canada.
“Rock-solid?” Not hardly.
The magnitude of an earthquake is measured on the Richter Scale, named after the seismologist from California (where else?) who developed it. It’s a logarithmic scale; that is, every whole number represents a quake 10 times more powerful than the one before. Thus, a magnitude 5 earthquake is 10 times as powerful as a magnitude 4 and 100 times as powerful as a magnitude 3.
The biggest earthquakes ever recorded have magnitudes in the range of 8.5 to 9.5, and on average they occur less than once a year worldwide. In this century there have been eight earthquakes of magnitude 7 or higher in Canada, the largest being a magnitude 8.1 quake in the Queen Charlotte Islands in 1949. By comparison, the San Francisco quake of 1903 is estimated at 8.3, the 1964 Alaska quake was 8.5 and the San Francisco quake of 1989, which killed 62 people, caused $6 billion in damage and interrupted the World Series, was “only” a 7.1. (Earthquakes in that range occur 10 to 20 times a year worldwide.)
The local intensity of a quake is another way to measure it, and from an anthropocentric point of view, maybe a better way. The Modified Mercalli Scale ranges from 1 (not felt except by a few) through 5 (felt generally, some plaster falls, dishes and windows broken) to 12 (total destruction; waves seen in the ground, lines of sight and level distorted, objects thrown in the air). The 1989 San Francisco quake reached 8 on this scale.
People who don’t live in California, or in other areas infamous for earthquakes, can sometimes get a little smug. But just because most earthquakes occur where the plates meet doesn’t mean they all do. The rock beneath our feet is full of faults and distortions, and earthquakes can happen almost anywhere. Even Saskatchewan recorded three earthquakes of magnitude 3.5 or higher between 1978 and 1988.
The most powerful earthquake ever in the continental United States didn’t hit in California, or anywhere near a plate boundary; it was a magnitude 8.5 quake in the winter of 1811-1812 along the New Madrid fault, which runs from Arkansas to southern Illinois, that had little effect only because the area was so sparsely settled at the time. (It isn’t anymore.) On the other hand, a magnitude 6.2 earthquake in Charleston, South Carolina, in 1886 levelled the city and killed 60 people.
It’s those kind of quakes, far from the areas where earthquakes are relatively common, that puzzle scientists most. Quakes that occur where two plates are coming together or apart is fairly well understood. Those that occur in the middle of the plates are not. It’s possible that they’re “left-overs” from an earlier phase of plate movement, which left faults deeply buried but not yet fully deactivated.
With a million earthquakes a year, humans have obviously lived with the phenomenon forever. But understanding earthquakes is becoming more and more important because there are more humans– and more human structures– than ever before. Unless everyone in the world moves to the Manitoba prairie (no quakes of magnitude 4 or greater in the last 20 years), there are likely to be people living where the next major earthquake strikes– and dying there.
An earthquake in Shansi Province, China, in 1556 killed 830,000 people. Just 14 years ago hundreds of thousands more died in Tangshan Province. That’s a pretty graphic example of the hazards faced by large populations in earthquake-prone zones.
Understandably, then, scientists would love to be able to predict earthquakes. But the prospects aren’t good. While there are certain pre-quake phenomena that can be observed (increased radon gas emission, tilting of the ground, etc.), they don’t occur with every quake nor on any set timetable. And predicting a quake that doesn’t take place can cause economic disruption, panic, and legal problems for the predictor. About the best scientists can do at this point is (sometimes) give a few minutes warning, just long enough for power to be turned off, which helps prevent fires. Otherwise, they’re stuck with describing the areas that are statistically due for a quake, based on the seismological record, leaving it to society to decide what level of preparedness it’s prepared to live with and pay for.
And lest you think all this applies only to California, remember: southwestern B.C., one of the most populous parts of Canada, records more than 200 small earthquakes a year and is virtually certain to experience a major quake at some point in the future.
Maybe our Saskatchewan winters aren’t so hard to take after all.