by Chris Woodford. Last updated: August 17, 2014.
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.
Photo: Our classic idea of what an earthquake looks like.
The Earth has literally split apart in this quake, because ground shaking made the fine-grained soil behave like a liquid
that drained away, leaving the road above unsupported. This quake, which happened in 1989 at Loma Prieta in San Francisco, California, measured 6.9 on the Richter scale. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
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
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 a huge tectonic plate 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
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!
Artwork: As s-waves travel forward, they shake the Earth up and down or from side to side (at right angles to the direction of motion). P-waves shake the Earth back and forth in the same direction in which they're moving. An s-wave is
an example of a transverse wave; a p-wave is an example of a longitudinal or compression wave.
Earthquakes are happening all the time. According to the
US Geological Survey (USGS),
there are several million earthquakes each year.
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.
Photo: Right: Housing damage caused by the 1989 Loma Prieta earthquake in San Francisco, California.
Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
Photo: Left: The 1,200-km (800-mile) San Andreas fault is
one of the longest in the United States. It's some 15km (9.3 miles) deep and 20 million years old.
A major quake in the fault in 1857 (known as the Fort Tejon earthquake) was equivalent to magnitude 8.0 on the Richter scale and caused this striking, 350-km (220-mile) surface rupture in the Carrizo Plain. Photo by courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).
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
on either the Richter scale or the Mercalli 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
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
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.)
Photo: A seismometer (left) drawing three charts (one of them shown in closeup, right) of the Loma Prieta earthquake in San Francisco, California as it happened on October 17, 1989.
Photo by courtesy of US Geological Survey Photo Library.
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.
Photo: The mass damper in the Taipei 101 skyscraper in Taiwan.
Photo © Guillaume Paumier by courtesy of Wikimedia Commons published under a Creative Commons Attribution ShareAlike 3.0 licence. See more of Guillaume's photos on Flickr.
Animation: How the Taipei 101 mass damper works: The hydraulic rams (red and gray) that support the damper (yellow ball) help to slow the skyscraper's sway in a similar way to the dampers (shock absorbers) in a car's suspension. If the wind or an earthquake wobbles
the building one way, the damper tries to stay where it is, dragging on the hydraulics and effectively pulling the building back again. Note that the sway of the building is greatly exaggerated in this artwork.
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?
Photo: Are you prepared? Earthquakes can destroy everything you own
in a matter of seconds. This quake, which measured 8.1 on the Richter scale, happened at
Gizo in the Solomon Islands in 2007. Photo by Andrew Meyers courtesy
of US Navy.
Find out more
On this website
On other sites
- BBC News: Animated guide: Earthquakes has a very clear animation showing you how earthquakes are formed and travel through Earth.
- US Geological Survey Earthquakes website
includes superb live maps showing exactly where earthquakes are currently happening!
- Earthquakes for Kids, from the USGS,
includes pictures of quakes, activities, ideas for science fair projects, puzzles, games,
and loads more great stuff. It even tells you how to build your own seismometer!
For older readers
- Earthquakes by Bruce A. Bolt. W.H. Freeman, 2005. A classic introduction suitable for older teenagers, college students, and adults.
- Earthquake Seismology by Hiroo Kanamori. Elsevier, 2009. A detailed, comprehensive guide to the physics of earthquakes.
- Predicting the Unpredictable: The Tumultuous Science of Earthquake Prediction by Susan Elizabeth Hough. Princeton University Press, 2009. A very readable, popular-science account of the scientists who try to predict when and where quakes will happen next. Hough is, herself, a scientist at the USGS and well-qualified to write the book.
For younger readers
- Earthquakes by Sally Walker. Lerner, 2007. A good introduction for young readers living (or having family members) in earthquake zones. Explains the geological processes inside Earth that lead to quakes, what quakes are like when they happen, and how they impact people's lives. Suitable for ages 6–8.
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