What causes Earthquakes?
There's no warning. None at all. One minute you're happily walking down the street. The next minute the street seems to be walking all by itself! There's a deafening, rumbling, roaring noise. The buildings start to shake. Brick and glass rains down around you. A huge crack appears in the pavement. Fire hydrants burst open. Cars are crushed by falling masonry. People are screaming. It feels like the end of the world.
There's not much we can do to stop natural disasters like earthquakes: they're an inevitable part of living on a planet like Earth, seething inside with hidden power. What we can do, however, is monitor changes in the ground beneath our feet so we can predict when earthquakes will happen. We can design our buildings much more cleverly so they absorb the power of sudden shocks. And we can prepare ourselves for the inevitable by planning for the time when (and not if) the next quake will strike.
What causes earthquakes?
You might think Earth is a giant lump of rock, but you'd be wrong—it's more like a freshly boiled egg: there's a hot, molten core bubbling away inside a surprisingly thin outer crust. The countries we live in feel like they're safely anchored on solid rocky foundations, but really they're fixed to enormous rocky slabs called tectonic plates that can slide around on the molten rock beneath. Imagine living your life on an eggshell!
Earthquakes happen at places called faults (or fault lines) where the jagged edges of two tectonic plates grind against one another. Most earthquake activity happens in the middles of the oceans where plates are pushing apart on the floor of the sea. Some of the most violent earthquakes happen around the edges of tectonic plates in the Pacific Ocean, forming an intense area of activity known as the Ring of Fire (so-called because there are many active volcanoes there too).
Tectonic plates are constantly moving—in incredibly slow motion—and we don't even notice most of the time. But every once in a while two grinding plates will suddenly jolt into a new position. The energy released by this movement creates an earthquake. It starts at a point inside Earth called the focus where the moving plates are in contact, then travels through the ground as very low-frequency sounds called shock waves or seismic waves. The greatest damage happens at a place called the epicenter, which is the point on Earth's surface above the focus. Earthquakes continue until all the energy released at the focus has been safely dissipated. Even then, there's still a chance that further earthquakes, known as aftershocks, will happen for some hours or even days afterward.
Seismic waves travel in two very different ways. Some of them, known as primary waves (or p-waves), vibrate the ground in the direction in which the waves themselves are moving. They travel in a similar way to ordinary sound waves by alternately squeezing and stretching the ground in patterns known as compressions and rarefactions. Waves like this are called longitudinal waves and travel at incredible speeds of around 25,000 km/h (15,500 mph). There's another kind of seismic wave known as a secondary wave (s-wave) that travels only half as fast. Unlike p-waves, s-waves travel by making the ground vibrate up and down as they move forward. It's because seismic waves travel at such amazing speeds—broadly speaking, as fast as a rocket taking off—that we get so little time to avoid quakes. Earth's diameter is a little under 13,000 km (8,000 miles) at the equator, so a really fast p-wave can theoretically shoot from one side of the planet to the other in less than half an hour!
Non-stop quakes!
Earthquakes are happening all the time. According to the US Geological Survey (USGS),there are several million earthquakes each year (you can find detailed facts and figures on the USGS Statistics page).
We don't notice most of them, because many are really tiny and others release their energy in the middle of the oceans. That's not to say that earthquakes aren't a huge problem. Look at the USGS statistics and you'll see that, most years, around 20,000–30,000 people are killed in earthquake disasters around the world.
Earthquakes don't always happen to someone else. You might be surprised to learn that there are several thousand earthquakes each year in the United States. Californians are well aware that they are long overdue for a major strike from a huge earthquake, because their part of the United States is directly above a major plate boundary known as the San Andreas fault.
Measuring earthquakes
It's important to be able to distinguish between minor earthquakes that cause no damage or fatalities and major earthquakes that result in horrific loss of life. That's why we usually describe earthquakes by giving them a number called magnitude, where a bigger magnitude means a more powerful quake. There are various different ways of calculating magnitude; two of the best known are the the Richter scale (one of the oldest) and the Moment Magnitude Scale (one of the newest). You might also come across the Mercalli intensity scale, which doesn't describe the magnitude of earthquakes but compares their effects on the world around us.
Richter scale
The Richter scale is a scientific way of measuring the strength of earthquakes that was developed in 1935 by US geologist Charles F. Richter. A minor quake that rates less than 2.0 on the Richter scale is known as a micro earthquake and isn't generally strong enough to worry people. A major quake will reach something like 8.0 on the Richter scale and is likely to cause widespread damage and casualties.
You figure out the magnitude of an earthquake on the Richter scale by measuring the logarithm of how much the ground moves (in other words, the logarithm of the amplitude of the seismic waves). Because the scale is logarithmic, every increase of one whole point on the scale means another 10-fold increase in ground movement. In other words, an earthquake that measures 8.0 on the Richter scale involves 10 times more movement than an earthquake that measures 7.0—and 100 times more movement than a quake that measures 6.
MMS (Moment Magnitude Scale)
The Richter scale has now largely been replaced by the MMS (Moment Magnitude Scale), though you still often hear the Richter spoken about in popular science and the news. Earthquake values on the MMS scale are broadly similar to values on the Richter (for bigger quakes, anyway).
Why bigger earthquakes are much more destructive
The Richter scale is logarithmic, not linear. The further up the scale you go, the more significant every new step up the scale becomes. You can see what this means in practice by looking at a graph of how the amplitude (height) of the seismic waves in an earthquake increases as you go up the Richter scale. That's the curved, climbing gray line.
Suppose you start with an earthquake of magnitude 2.0. If the next earthquake you feel is magnitude 3.0, the shock waves have increased in height by an amount shown by the little blue arrow. But if you start with an earthquake of magnitude 7.0 and then increase to magnitude 8.0, the amplitude has increased by a great deal more (shown by the brown arrow) even though you've only gone up one more step on the scale.
So when you hear people talking about earthquakes that measure 8.0 on the Richter scale, they're not four times worse than earthquakes that measure 2.0: the waves they create are ten to the power six or one million times greater in amplitude!
The amplitude of the seismic waves in an earthquake isn't necessarily a good measure of how much damage it will cause. A magnitude 8.0 earthquake releases 32 times more energy than a magnitude 7.0. The higher magnitude earthquakes release hugely more energy than the lower magnitude ones and that's why they cause such immense destruction: it's the energy (which all has to go somewhere) that causes the damage. (There are some interesting calculations on the USGS website.)
Mercalli scale
Although the Richter scale is the most common way of comparing earthquakes, you might also see quakes described using the older Mercalli scale. While the Richter scale has no upper limit, the Mercalli has a fixed range from I ("Instrumental"—barely even noticed) to XII ("Catastrophic"—with almost total destruction). The Richter scale is based on a scientific measurement of an earthquake's amplitude, while the Mercalli scale is more of a description based on the apparent damage that an earthquake causes. That makes the Mercalli more like the Beaufort scale of wind measurement.
Shaking all over
Scientists who study earthquakes are called seismologists. They use instruments called seismometers to record earth tremors and draw charts on graph paper known as seismographs. A seismometer is little more than a weighted pen on a spring suspended over a piece of paper that's slowly wound underneath it at constant speed by an electric motor. If an earthquake (or something like a mining explosion or a building collapse) makes the ground vibrate, the pen jiggles about. The greater the earth movement, the more the pen moves. Seismometers are usually set up with three separate traces so they can record ground movements in all three directions at once. (Note the similarity between seismometers and accelerometers.)
Protecting buildings
We can't stop earthquakes and we can't prevent their energy from traveling through the Earth. So how can we protect buildings and people in areas where earthquakes are common? Although it's impossible to secure a building completely, it is possible to reduce the chances of earthquake damage by designing the structure in such a way that it can absorb and dissipate the energy from seismic waves. One way to do this is to separate the building from its foundation using what are known as base isolators. In effect, instead of making the building a rigid extension of the foundations, you stand the upper part of the building on lots of very sturdy rubber feet so it can move about more freely. The University of Southern California (USC) University Hospital in Los Angeles is an eight-story building supported by 149 of these isolators. When a 6.7 magnitude earthquake struck the building in 1994, scientists found that the isolators helped to reduce shaking of the building by about two thirds, and stopped it from collapsing.
Other buildings are mounted on giant hydraulic devices called dampers, a bit like car shock absorbers, that soak up the ground's shaking motion before it can be transmitted to the upper floors. Some dampers work like gas springs; others rock back and forth like pendulums in clocks; the most complex ones are like pendulums mounted on top of one or two other pendulums to provide huge amounts of shock-absorbing. Possibly the world's most famous building shock absorber is a giant, 660-tonne ball inside the Taipei 101 skyscraper in Taiwan. If an earthquake (or high wind) makes the tower sway in one direction, the ball (mounted on hydraulic rams) moves the other way, effectively canceling out the movement.
Saving people
We don't always have to take such elaborate steps to protect ourselves against the power of earthquakes: sometimes very simple precautions are equally effective. If you live in a small apartment in California, you probably don't have the means to install a huge mass damper in your home. But you could still prepare your family for what to do in the event of an earthquake.
You could make sure at least one person is trained in basic first aid and ensure that a first-aid kit is easily accessible in your home. You could check that any tall pieces of furniture (such as bookcases or dressers) are screwed securely to the wall so they won't fall on people in a quake. How about putting safety fittings on gas appliances and ensuring that everyone knows how to turn off the gas, electricity, and water in an emergency? You could also swap any ordinary windows for toughened or laminated glass (a bit like bulletproof glass) to reduce the risk of people being cut by huge falling shards if windows break. Simple precautions like these can help to reduce the risk of people losing their lives or being injured both by earthquakes themselves or the fires and other problems that inevitably follow them. If you live in an earthquake zone, why not spend a few minutes now considering how well prepared you are?
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