Download Shaking Ground

EARTHQUAKES
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:




volcanic eruptions
meteor impacts
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.
Sliding Plates
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
Photo courtesy USGS
is called a divergent plate boundary.
One of the best known faults is the San
Plates can push together - If the two plates are moving toward
Andreas fault in California. The fault,
which marks the plate boundary between
each other, one plate typically pushes under the other one. This
the Pacific oceanic plate and the North
subducting plate sinks into the lower mantle layers, where it
American continental plate, extends over
650 miles (1,050 km) of land.
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, 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 boundaryIn 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
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.
Pinpointing the Earthquake's Origin
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.
Photo courtesy USGS
If you gather this information from three or more points, you can figure out
A fence along a strike slip fault that
shifted in the 1906 San Francisco
the location of the focus through the process of trilateration. Basically,
earthquake.
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.
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.
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.
Photo courtesy NGDC
Damage from a magnitude 7.4 earthquake that
hit Niigata, Japan, in 1964.
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.
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.
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.
Photo courtesy USGS
Bridge columns cracked by the
Loma Prieta, Calif. earthquake of 1989.
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. 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.