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.