An earthquake is one of the most terrifying phenomena that nature can dish up. We generally think of the
ground we stand on as "rock-solid" and completely stable. An earthquake can shatter that perception
instantly, and often with extreme violence.
Up until relatively recently, scientists only had unsubstantiated guesses as to what actually caused
earthquakes. Even today there is still a certain amount of mystery surrounding them, but scientists have a
much clearer understanding.
There has been enormous progress in the past century: Scientists have identified the forces that cause
earthquakes, and developed technology that can tell us an earthquake's magnitude and origin. The next
hurdle is to find a way of predicting earthquakes, so they don't catch people by surprise.
In this article, we'll find out what causes earthquakes, and we'll also find out why they can have such a
devastating effect on us.
An earthquake is a vibration that travels through the earth's crust. Technically, a large truck that rumbles
down the street is causing a mini-earthquake, if you feel your house shaking as it goes by, but we tend to
think of earthquakes as events that affect a fairly large area, such as an entire city. All kinds of things can
underground explosions (an underground nuclear test, for example)
collapsing structures (such as a collapsing mine)
But the majority of naturally-occurring earthquakes are caused by movements of the earth's plates.
We only hear about earthquakes in the news every once in a while, but they are actually an everyday
occurrence on our planet. According to the United States Geological Survey, more than three million
earthquakes occur every year. That's about 8,000 a day, or one every 11 seconds!
The vast majority of these 3 million quakes are extremely weak. The law of probability also causes a good
number of stronger quakes to happen in uninhabited places where no one feels them. It is the big quakes that
occur in highly populated areas that get our attention.
Earthquakes have caused a great deal of property damage over the years, and they have claimed many lives.
In the last hundred years alone, there have been more than 1.5 million earthquake-related fatalities.
Usually, it's not the shaking ground itself that claims lives -- it's the associated destruction of man-made
structures and the instigation of other natural disasters, such as tsunamis, avalanches and landslides.
The biggest scientific breakthrough in the history of seismology -- the study of earthquakes -- came in the
middle of the 20th century, with the development of the theory of plate tectonics. Scientists proposed the
idea of plate tectonics to explain a number of peculiar phenomenon on earth, such as the apparent movement
of continents over time, the clustering of volcanic activity in certain areas and the presence of huge ridges at
the bottom of the ocean.
The basic theory is that the surface layer of the earth -- the lithosphere -- is comprised of many plates that
slide over the lubricating athenosphere layer. At the boundaries between these huge plates of soil and rock,
three different things can happen:
Plates can move apart - If two plates are moving apart from each other, hot, molten rock flows up
from the layers of mantle below the lithosphere. This magma comes out on the surface (mostly
at the bottom of the ocean), where it is called lava. As the lava cools, it hardens to form new
lithosphere material, filling in the gap. This is called a divergent plate boundary.
Plates can push together - If the two plates are moving toward each other, one plate typically
pushes under the other one. This subducting plate sinks into the lower mantle layers, where it
melts. At some boundaries where two plates meet, neither plate is in a position to subduct under
the other, so they both push against each other to form mountains. The lines where plates push
toward each other are called convergent plate boundaries.
Plates slide against each other - At other boundaries, plates simply slide by each other -- one
moves north and one moves south, for example. While the plates don't drift directly into each
other at these transform boundaries, they are pushed tightly together. A great deal of tension
builds at the boundary.
Where these plates meet, you'll find faults -- breaks in the earth's crust where the blocks of rock on each side
are moving in different directions. Earthquakes are much more common along fault lines than they are
anywhere else on the planet. One of the best known faults is the San Andreas fault in California. The fault,
which marks the plate boundary between the Pacific oceanic plate and the North American continental plate,
extends over 650 miles (1,050 km) of land.
Scientists identify four types of faults, characterized by the position of the fault plane, the break in the rock
and the movement of the two rock blocks:
In a normal fault, the fault plane is nearly vertical. The hanging wall, the block of rock positioned
above the plane, pushes down across the footwall, which is the block of rock below the plane.
The footwall, in turn, pushes up against the hanging wall. These faults occur where the crust is
being pulled apart, due to the pull of a divergent plate boundary.
The fault plane in a reverse fault is also nearly vertical, but the hanging wall pushes up and the
footwall pushes down. This sort of fault forms where a plate is being compressed.
A thrust fault moves the same way as a reverse fault, but the fault line is nearly horizontal. In
these faults, which are also caused by compression, the rock of the hanging wall is actually
pushed up on top of the footwall. This is the sort of fault that occurs in a converging plate
In a strike-slip fault, the blocks of rock move in opposite horizontal directions. These faults form
when the crust pieces are sliding against each other, as in a transform plate boundary
In all of these types of faults, the different blocks of rock push very tightly together, creating a good deal of
friction as they move. If this friction level is high enough, the two blocks become locked -- the friction keeps
them from sliding against each other. When this happens, the forces in the plates continue to push the
rock, increasing the pressure applied at the fault.
If the pressure increases to a high enough level, then it will overcome the force of the friction, and the blocks
will suddenly snap forward. To put it another way, as the tectonic forces push on the "locked" blocks,
potential energy builds. When the plates are finally moved, this built-up energy becomes kinetic. Some fault
shifts create visible changes at the earth's surface, but other shifts occur in rock well under the surface, and
so don't create a surface rupture.
The initial break that creates a fault, along with these sudden, intense shifts along already formed faults, are
the main sources of earthquakes. Most earthquakes occur around plate boundaries, because this is where
the strain from the plate movements is felt most intensely, creating fault zones, groups of interconnected
faults. In a fault zone, the release of kinetic energy at one fault may increase the stress -- the potential energy
-- in a nearby fault, leading to other earthquakes. This is one of the reasons that several earthquakes may
occur in an area in a short period of time.
Every now and then, earthquakes do occur in the middle of plates. In fact, one of the most powerful series of
earthquakes ever recorded in the United States occurred in the middle of the North American continental
plate. These earthquakes, which shook several states in 1811 and 1812, originated in Missouri. In the 1970s,
scientists found the likely source of this earthquake: a 600-million-year-old fault zone buried under many
layers of rock.
When a sudden break or shift occurs in the earth's crust, the energy radiates out as seismic waves, just as
the energy from a disturbance in a body of water radiates out in wave form. In every earthquake, there are
several different types of seismic waves.
Body waves move through the inner part of the earth, while surface waves travel over the surface of the
earth. Surface waves -- sometimes called long waves, or simply L waves -- are responsible for most of the
damage associated with earthquakes, because they cause the most intense vibrations. Surface waves stem
from body waves that reach the surface.
There are two main types of body waves.
Primary waves, also called P waves or compressional waves, travel about 1 to 5 miles per
second (1.6 to 8 kps), depending on the material they're moving through. This speed is greater
than the speed of other waves, so P waves arrive first at any surface location. They can travel
through solid, liquid and gas, and so will pass completely through the body of the earth. As they
travel through rock, the waves move tiny rock particles back and forth -- pushing them apart and
then back together -- in line with the direction the wave is traveling. These waves typically arrive
at the surface as an abrupt thud.
Secondary waves, also called S waves or shear waves, lag a little behind the P waves. As these
waves move, they displace rock particles outward, pushing them perpendicular to the path of the
waves. This results in the first period of rolling associated with earthquakes. Unlike P waves, S
waves don't move straight through the earth. They only travel through solid material, and so are
stopped at the liquid layer in the earth's core.
When P and S waves reach the earth's surface, they form L waves. The most intense L waves radiate out
from the epicenter.
Both sorts of body waves do travel around the earth, however, and can be detected on the opposite side of
the planet from the point where the earthquake began. At any given moment, there are a number of very faint
seismic waves moving all around the planet.
Surface waves are something like the waves in a body of water -- they move the surface of the earth up and
down. This generally causes the worst damage because the wave motion rocks the foundations of manmade
structures. L waves are the slowest moving of all waves, so the most intense shaking usually comes at
the end of an earthquake.
The exact speed of P and S waves varies depending on the composition of the material they're traveling
through, the ratio between the speeds of the two waves will remain relatively constant in any earthquake. P
waves generally travel 1.7 times faster than S waves.
Using this ratio, scientists can calculate the distance between any point on the earth's surface and the
earthquake's focus, the breaking point where the vibrations originated. They do this with a seismograph, a
machine that registers the different waves. To find the distance between the seismograph and the focus,
scientists also need to know the time the vibrations arrived. With this information, they simply note how much
time passed between the arrival of both waves and then check a special chart that tells them the distance the
waves must have traveled based on that delay.
If you gather this information from three or more points, you can figure out the location of the focus through
the process of trilateration. Basically, you draw an imaginary sphere around each seismograph location, with
the point of measurement as the center and the measured distance (let's call it X) from that point to the focus
as the radius. The surface of the circle describes all the points that are X miles away from the seismograph.
The focus, then, must be somewhere along this sphere. If you come up with two spheres, based on evidence
from two different seismographs, you'll get a two-dimensional circle where they meet. Since the focus must be
along the surface of both spheres, all of the possible focus points are located on the circle formed by the
intersection of these two spheres. A third sphere will intersect only twice with this circle, giving you two
possible focus points. And because the center of each sphere is on the earth's surface, one of these possible
points will be in the air, leaving only one logical focus location.
Whenever a major earthquake is in the news, you'll probably hear about its Richter Scale rating. You might
also hear about its Mercalli Scale rating, though this isn't discussed as often. These two ratings describe the
power of the earthquake from two different perspectives.
The Richter Scale is used to rate the magnitude of an earthquake -- the amount of energy it released. This is
calculated using information gathered by a seismograph. The Richter Scale is logarithmic, meaning that
whole-number jumps indicate a tenfold increase. In this case, the increase is in wave amplitude. That is, the
wave amplitude in a level 6 earthquake is 10 times greater than in a level 5 earthquake, and the amplitude
increases 100 times between a level 7 earthquake and a level 9 earthquake. The amount of energy released
increases 31.7 times between whole number values.
The largest earthquake on record registered an 9.5 on the currently used Richter Scale, though there have
certainly been stronger quakes in Earth's history. The majority of earthquakes register less than 3 on the
Richter Scale. These tremors, which aren't usually felt by humans, are called microquakes. Generally, you
won't see much damage from earthquakes that rate below 4 on the Richter Scale. Major earthquakes
generally register at 7 or above.
Richter ratings only give you a rough idea of the actual impact of an earthquake. As we've seen, an
earthquake's destructive power varies depending on the composition of the ground in an area and the design
and placement of manmade structures. The extent of damage is rated on the Mercalli Scale. Mercalli ratings,
which are given as Roman numerals, are based on largely subjective interpretations. A low intensity
earthquake, one in which only some people feel the vibration and there is no significant property damage, is
rated as a II. The highest rating, a XII, is applied only to earthquakes in which structures are destroyed, the
ground is cracked and other natural disasters, such as landslides or Tsunamis, are initiated.
Richter Scale ratings are determined soon after an earthquake, once scientists can compare the data from
different seismograph stations. Mercalli ratings, on the other hand, can't be determined until investigators
have had time to talk to many eyewitnesses to find out what occurred during the earthquake. Once they have
a good idea of the range of damage, they use the Mercalli criteria to decide on an appropriate rating.
In some areas, severe earthquake damage is the result of liquefaction of soil. In the right conditions, the
violent shaking from an earthquake will make loosely packed sediments and soil behave like a liquid. When a
building or house is built on this type of sediment, liquefaction will cause the structure to collapse more easily.
Highly developed areas built on loose ground material can suffer severe damage from even a relatively mild
earthquake. Liquefaction can also cause severe mudslides.
We understand earthquakes a lot better than we did even 50 years ago, but we still can't do much about
them. They are caused by fundamental, powerful geological processes that are far beyond our control. These
processes are also fairly unpredictable, so it's not possible at this time to tell people exactly when an
earthquake is going to occur. The first detected seismic waves will tell us that more powerful vibrations are on
their way, but this only gives us a few minutes warning, at most.
Scientists can say where major earthquakes are likely to occur, based on the movement of the plates in the
earth and the location of fault zones. They can also make general guesses of when they might occur in a
certain area, by looking at the history of earthquakes in the region and detecting where pressure is building
along fault lines. These predictions are extremely vague, however -- typically on the order of decades.
Scientists have had more success predicting aftershocks, additional quakes following an initial earthquake.
These predictions are based on extensive research of aftershock patterns. Seismologists can make a good
guess of how an earthquake originating along one fault will cause additional earthquakes in connected faults.
Another area of study is the relationship between magnetic and electrical charges in rock material and
earthquakes. Some scientists have hypothesized that these electromagnetic fields change in a certain way
just before an earthquake. Seismologists are also studying gas seepage and the tilting of the ground as
warning signs of earthquakes. For the most part, however, they can't reliably predict earthquakes with any
Preparing for Earthquakes
So what can we do about earthquakes? The major advances over the past 50 years have been in
preparedness -- particularly in the field of construction engineering. In 1973, the Uniform Building Code, an
international set of standards for building construction, added specifications to fortify buildings against the
force of seismic waves. This includes strengthening support material as well as designing buildings so they
are flexible enough to absorb vibrations without falling or deteriorating. It's very important to design structures
that can take this sort of punch, particularly in earthquake-prone areas.
Another component of preparedness is educating the public. The United States Geological Survey (USGS)
and other government agencies have produced several brochures explaining the processes involved in an
earthquake and giving instructions on how to prepare your house for a possible earthquake, as well as what
to do when a quake hits.
In the future, improvements in prediction and preparedness should further minimize the loss of life and
property associated with earthquakes. But it will be a long time, if ever, before we'll be ready for every
substantial earthquake that might occur. Just like severe weather and disease, earthquakes are an
unavoidable force generated by the powerful natural processes that shape our planet. All we can do is
increase our understanding of the phenomenon and develop better ways to deal with it.
Unlike many of nature's deadly forces, earthquakes almost always strike without warning. These destructive
and devastating forces can topple cities in seconds, leaving behind rubble and tragedy in their wakes.
Earthquakes are not limited to any one area of the world or any one season of the year. Although most
earthquakes are just small tremors, it only takes one to cause millions of dollars in property damage and
thousands of deaths. For this reason, scientists continue to pursue new technologies to limit the destruction
that earthquakes can dish out.
At Lord Corporation's labs in Cary, N.C., researchers believe they have developed, in cooperation with
University of Notre Dame researchers, the latest product that can reduce the damage caused by earthquakes.
Lord is one of the largest producers of a unique substance, called magnetorheological fluid (MR fluid),
which is being used inside large dampers to stabilized buildings during earthquakes. MR fluid is a liquid that
changes to a near-solid when exposed to a magnetic force, then back to liquid once the magnetic force is
During an earthquake, MR fluid inside the dampers will change from solid to liquid and back as tremors
activate a magnetic force inside the damper. Using these dampers in buildings and on bridges will create
smart structures that automatically react to seismic activity. This will limit the amount of damage caused by
What is MR Fluid
Looking at it in a beaker, MR fluid doesn't seem like such a revolutionary substance. It's a gray, oily liquid
that's about three times denser than water. It's not too exciting at first glance, but MR fluid is actually quite
amazing to watch in action.
A simple demonstration by David Carlson, a physicist at the North Carolina lab, shows the liquid's ability to
transform to solid in milliseconds. He pours the liquid into the cup and stirs it around with a pencil to show it's
liquid. He then places a magnet to the bottom of the cup, and the liquid instantly turns to a near-solid. To
further demonstrate that it's turned to a solid, he holds the cup upside down, and none of the MR fluid drops
Typical MR fluid consists of these three parts:
Carbonyl Iron Particles -- 20 to 40 percent of the fluid is made of these soft iron particles that are
just 3 to 5 micrometers in diameter. A package of dry carbonyl iron particles looks like black flour
because the particles are so fine.
A Carrier Liquid -- The iron particles are suspended in a liquid, usually hydrocarbon oil. Water is
often used in demonstrating the fluid.
Proprietary Additives -- The third component of MR fluid is a secret, but Lord says these
additives are put in to inhibit gravitational settling of the iron particles, promote particle
suspension, enhance lubricity, modify viscosity and inhibit wear.
Carbonyl iron particles
When a magnet is applied to the liquid, these tiny particles line up to make the fluid stiffen into a solid. This is
caused by the dc magnetic field, making the particles lock into a uniform polarity. How hard the substance
becomes depends on the strength of the magnetic field. Take away the magnet, and the particles unlock
While scientists have just recently discovered many new applications for MR fluid, it has actually been around
for more than 50 years. Jacob Rabinow is credited with discovering MR fluid in the 1940s while working at the
U.S. National Bureau of Standards.
Until about 1990, there were few applications for MR fluid because there was no way to properly control it.
Today, there are digital signal processors and fast, cheap computers that can control the magnetic field
applied to the fluid. Applications for this technology include Nautilus exercise equipment, clothes washing
machine dampers, shock absorbers for cars and advanced leg prosthetics.
Buildings and Bridges
Skyscrapers and long bridges are susceptible to resonance created by high winds and seismic activity. In
order to mitigate the resonance effect, it is important to build large dampers into their design to interrupt the
resonant waves. If these devices are not in place, buildings and bridges can be shaken to the ground, as is
witnessed anytime an earthquake happens.
Dampers are used in machines that you likely use every day, including car suspension systems and clothes
washing machines. If you take a look the How Stuff Works article on washing machines, you'll learn that
damping systems use friction to absorb some of the force from vibrations. A damping system in a building is
much larger and is also designed to absorb the violent shocks of an earthquake. The size of the dampers
depend on the size of the building. There are three classifications for dampening systems:
Passive -- This is an uncontrolled damper, which requires no input power to operate. They are
simple and generally low in cost but unable to adapt to changing needs.
Active -- Active dampers are force generators that actively push on the structure to counteract a
disturbance. They are fully controllable and require a great deal of power.
Semi-Active -- Combines features of passive and active damping. Rather than push on the
structure they counteract motion with a controlled resistive force to reduce motion. They are fully
controllable yet require little input power. Unlike active devices they do not have the potential to
go out of control and destabilize the structure.
Inside the MR fluid damper, an electromagnetic coil is wrapped around three sections of the piston.
Approximately 5 liters of MR fluid is used to fill the damper's main chamber. During an earthquake, sensors
attached to the building will signal the computer to supply the dampers with an electrical charge. This
electrical charge then magnetizes the coil, turning the MR fluid from a liquid to a near-solid. Now, the
electromagnet will likely pulse as the vibrations ripple through the building. This vibration will cause the MR
fluid to change from liquid to solid thousands of times per second, and may cause the temperature of the fluid
to rise. A thermal expansion accumulator is fixed to the top of the damper housing to allow for the
expansion of the fluid as it heats up. This accumulator prevents a dangerous rise in pressure as the fluid
Depending on the size of the building, there could be an array of possibly hundreds of dampers. Each damper
would sit on the floor and be attached to the chevron braces that are welded into a steel cross beam. As the
building begins to shake, the dampers would move back and forth to compensate for the vibration of the
shock. When it's magnetized, the MR fluid increases the amount of force that the dampers can exert.
Earthquake-Proof Buildings (ball-bearings)
The new San Francisco International Airport uses many advanced building technologies to help it withstand
earthquakes. One of these technologies involves giant ball bearings.
The 267 columns that support the weight of the airport each ride on a 5-foot-diameter (1.5-meter) steel ball
bearing. The ball rests in a concave base that is connected to the ground. In the event of an earthquake, the
ground can move 20 inches (51 cm) in any direction. The columns that rest on the balls move somewhat less
than this as they roll around in their bases, which helps isolate the building from the motion of the ground.
When the earthquake is over, gravity pulls the columns back to the center of their bases.