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.
Photo courtesy USGS A section of Interstate 880 in Oakland,
California, damaged by the magnitude 7.1 earthquake that
shook the San Francisco area in
1989.
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 edition of How Stuff
Works, we'll find out what causes earthquakes, and
we'll also find out why they can have such a devastating
effect on us.
Shaking Ground 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 cause earthquakes:
underground explosions (an underground nuclear test, for
example)
collapsing structures (such as a collapsing mine)
But the majority of naturally-occuring earthquakes
are caused by movements of the earth's plates, as we'll
see in the next section.
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!
Photo courtesy FEMA Residential damage caused by the 1994
earthquake in Northridge,
California.
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 manmade structures and the instigation of other
natural disasters, such as tsunamis, avalanches and
landslides.
Photo courtesy NGDC Residential damage in Prince William Sound,
Alaska, due to liquefaction caused by a 1964
9.2-magnitude
earthquake.
In the next section, we'll examine the powerful forces that
cause this intense trembling and find out why earthquakes
occur much more often in certain regions.
Sliding Plates
Photo courtesy USGS 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.
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.
In the next section, we'll look at some different types of
faults and see how their movement creates earthquakes.
Faults 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 (see animation below), 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.
Normal fault
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 boundary.
Reverse fault
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
Strike-slip fault
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.
Photo courtesy USGS Crop rows offset by a lateral strike slip
fault shifting in the 1976 earthquake that shook El
Progresso,
Guatemala.
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.
Photo courtesy USGS Railroad tracks shifted by the 1976 Guatemala
earthquake
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.
The vibrations of one earthquake in this series were so
powerful that they actually rang church bells as far away as
Boston! In the next section, we'll examine earthquake
vibrations and see how they travel through the ground.
Making Waves 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.
Photo courtesy USGS Structural damage caused by vibrations
from the 1964 Alaska
earthquake
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.
Click the play button to start the
earthquake. 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.
In the next section, we'll see how scientists can calculate
the origin of an earthquake by detecting these different
waves.
Pinpointing the Earthquake's Origin
Photo courtesy USGS A fence along a strike slip fault that
shifted in the 1906 San Francisco
earthquake.
We
saw in the last section that there are three different types
of seismic waves, and that these waves travel at different
speeds. While 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.
For a more thorough discussion of trilateral calculation,
check out How
GPS Receivers Work.
Rating Magnitude and Intensity 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.
Photo courtesy NGDC Destruction caused by a (Richter) magnitude
6.6 earthquake in Caracas, Venezuela. The 1967
earthquake took 240 lives and caused more than $50
million worth of property
damage.
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. For more
information about the Richter Scale and seismographs, check
out this Question
of the Day.
Photo courtesy NGDC Damage to a school in Anchorage, Alaska,
caused by the 1964 Prince William Sound earthquake. The
earthquake, which killed 131 people and caused $538
million of property damage, registered an 9.2 on the
Richter
Scale.
Liquefaction
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, like the ones that took so many lives in the
recent earthquake that shook Central America. In this
case, in fact, mudslides were the most significant
destructive force, claiming hundreds of lives.
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.
Photo courtesy NGDC Damage from a magnitude 7.4 earthquake
that hit Niigata, Japan, in
1964.
Dealing with Earthquakes 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.
Photo courtesy USGS Damage in downtown Anchorage, Alaska, caused
by the 1964 Prince William Sound
earthquake.
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
precision.
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. See this article on How
Smart Structures Will Work for more on how scientists are
creating new ways to protect buildings from seismic activity.
Photo courtesyUSGS Bridge columns cracked by the Loma
Prieta, Calif. earthquake of
1989.
Another component of preparedness is educating the public.
The United
States Geological Society (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. To find out what you should
do to prepare yourself, check out this
online guide from the Red Cross.
Photo courtesy USGS The great San Francisco fire of 1906 was
initiated by a powerful earthquake. The earthquake
vibrations and catastrophic fire destroyed most of the
city, leaving 250,000 people
homeless.
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. To learn more about earthquakes,
check out the USGS
Web site, or any of the other sites listed in the Links
section.
