Monitoring Earthquakes

By Paula Johanson

There are many sites around the world monitoring seismic tremors from earthquakes and volcanic activity. Scientists are using seismometers to measure when and how much the ground is shaking. The answers are "often" and moderately" on the island where I live in British Columbia!

But data from any one place is not really enough on its own. What makes this information much more useful is when seismic stations share their data in networks. When scientists study and compare measurements from many sites in one region, or even around the world, they learn important things about what's happening under our feet in what looks like solid rock.

Since I live in an earthquake zone, it's really interesting to learn about the Pacific Northwest Seismic Network, which has a website friendly to citizen science fans. This network is run by the University of Washington and the University of Oregon. While most of the other scientific groups contributing to this network are American, there is participation also by Canadian seismic stations. Earth tremors are truly an international issue. Check out their website at this link.

This website's maps of seismic tremors are constantly updating all the time! They use information from seismic sites in Washington State, Oregon, northern California, and the coast of British Columbia. I like to look at their interactive map, which has settings that can be changed to show more information from more sites, or fewer. Also interesting is their Hourly Tremors map, which is simpler -- it can show tremors during the last three hours or up to the last two days.

Whenever I want to know more about current volcanic activity along the nearby Pacific coast, I look on this website. If a local tremor is reported, I can check to see if it's part of a series of tremors moving north or south near the island where I live. And whenever someone tells me they don't have any emergency preparations in case of a big earthquake, this is where I send them to learn more about how many little tremors are often happening around here. Being prepared for an earthquake emergency makes sense, and maps like these help me understand I'm making sensible preparations. They also help me not get too fussy.

How big can an earthquake be?

Last month, a few people thought the world was going to come to an end. There was not scientific basis for their worry whatsoever, but still they raised a few eyebrows.

In one Toronto Star article (that the Star did not share online), geophysicist Qinya Liu discussed ways in which the world, or at least human existence, could come to an end—and systematically ruled them out. Her reasons seemed plausible. We'd know if a meteor the size of the moon were on a collision course with the earth. The biggest volcanic eruption in human history left 10,000 people alive to repopulate the planet (rather successfully, too!). Nuclear weapons have been around for half a century and we're no more likely to use them now than ever before. Alien invasions are the stuff of bad summer movies, not reality (so far). A big enough earthquake isn't possible.

That last one caught my attention. We just saw widespread destruction in Japan from a massive 9.0-magnitude earthquake. That's among the most powerful earthquakes in history and it struck a densely populated area. The carnage was terrible, and more than 15,000 people died. Still, that's nowhere near wiping out all of humanity.

Is there a maximum magnitude for earthquakes?

Earthquake damage to store, California 2010.
Source: FEMA

Even in science fiction, wiping out the human species takes at least a 10.5 earthquake. Nobody knows what it would really take. But why is an earthquake with a magnitude above 10 impossible?

First, you have to remember that the scale we use to measure earthquakes is exponential. A 4.5 is ten times bigger than a 3.5. That means a 5.5 is 100 times bigger than a 3.5.

I remember a 4.5 quake on Vancouver Island that woke me up one morning. It felt like someone was jerking my bed and sounded like a tractor trailer was going through the garden. A 10.5 quake would be one million times more powerful. One million. The biggest quakes in history top out around 9.5, and that's only 100,000 times more powerful. A world-destroyer would have to be at least one-million times the strength of the quake that shook me awake that morning!

It takes a lot of energy to make continents move like that. But if they can hit 9.5, why not 10 or 10.5?

For an answer, I turned to Stephen Johhston, a geology professor at the University of Victoria who specializes in earthquakes in subduction zones (where one plate goes under another—it's where the biggest earthquakes occur and most mountains form). He said it has to do with how strong the rocks are.

"Earthquakes occur when rocks break in response to a buildup of stress," he said. "Imagine taking a long, skinny icicle and you start trying to bend it until it breaks. You will not have to expend too much energy before the icicle breaks, because the ice is fairly brittle and 'weak.' But if you took a similarly shaped wooden stick, it would take you considerably more effort to break the stick. In other words, the earthquake you produced by breaking the stick is greater than the one produced by breaking the icicle."

There is a limit to how much pressure rocks can withstand before they break. So they'd break before enough built up to cause a humanity-destroying quake. Most researchers figure that limit is around 9.6, Johnston said.

How deadly is a big quake?

Big earthquakes can be deadly, causing buildings or bridges to collapse and crush people. But most people who die because of a quake aren't killed directly by the shaking of the ground. Sometimes earthquakes can trigger deadly landslides. But the real carnage comes from tsunamis. These are huge waves, sometimes higher than a house, that are caused when an earthquake happens under the ocean.

In the Japan earthquake this year, much of the destruction came from tsunamis. And earlier this decade, deadly tsunamis killed more than 100,000 people around the Indian Ocean.

Because the big earthquakes, those above 9.0, happen in subduction zones, they tend to happen under water. The shaking can cause big destruction on land, too, but the greatest risk is to people who live in coastal areas.

In other words, even the biggest earthquake possible simply could not kill all the people on Earth.

Craig Saunders is a writer, editor and author of What is the Theory of Plate Tectonics? (Crabtree, 2011). He lives in Toronto.