Earthquakes
In-Depth
Keep reading to find out:
How are earthquakes formed?
What happens when an earthquake starts?
What happens to the waves earthquakes form?
How do you measure seismic waves?
How do you locate an earthquake?
How is its size measured?
How frequently do earthquakes occur?
Can you predict an earthquake?
How are earthquakes formed?
When Earth was first formed, radioactive materials, such as uranium,
potassium, and thorium, began to decay naturally in the planet's interior.
This process of radioactive decay slowly heated up the rocky material
deep inside the planet. Since heat attempts to rise, this heated material
has been moving slowly toward the Earth's surface. The movement is very
slow � less than an inch or so each year. But over hundreds of millions
of years, this slow movement adds up to distances of thousands of miles.
Deep inside the earth, where temperatures are very high, the heated rocky
material is flexible, so it moves slowly and steadily. Nearer the earth's
surface, however, the rocky material becomes cooler and more brittle,
and it cannot move so easily. The slow, continual movement of material
deep in the interior builds up and exerts stronger and stronger forces
on the brittle rocks near the surface, making them move as well. For an
earthquake to occur, the rocks nearer the surface have to break suddenly
� just like a stick that breaks suddenly when it is bent too far.
You don't usually think of the ground as something that can break. But
if you take a small piece of rock and squeeze it strongly enough in a
metal vise, it will crack or crumble. An earthquake is like that cracking
rock, but on a much larger scale. Often a break occurs in a place that
has broken before, on what scientists call a fault � a break in the earth's
surface between two blocks of rock that have moved past each other.
When rock breaks along a fault, the pieces of rock will move by sliding
past each other if the force is great enough. The motion may occur in
a series of jerks, each one of which is an earthquake. In a large earthquake,
the rock along the fault may move several feet in only a few seconds.
Over tens of millions of years, such geologic forces have caused massive
changes in the earth's landscape, as millions of earthquakes have moved
large chunks of the planet's surface hundreds of miles. This process has
changed the shape and location of the continents and oceans over Earth's
long history, and it continues to do so today.
There are thousands of earthquakes every year —
most of which are so minor we don't even feel them. In fact, there are
approximately 8,000 earthquakes EVERY DAY, but they hardly register in
intensity.
What happens when an earthquake starts?
The sudden release of energy from an earthquake sends out several
different shaking movements, or seismic waves. The word "seismic"
comes from a Greek word for "shaking." Some of these seismic
waves travel over the surface of the earth and are called surface waves.
Others, called body waves, travel down through the earth's deep interior
before returning to the surface.
Surface Waves
If you throw a stone into a calm pond, it creates a short splash
and then ripples spread outward for a much longer time. Something similar
happens in the earth when there is an earthquake. When rocks slip past
each other along a fault, the initial movement is over very quickly. However,
the energy that is released causes ripples — called surface waves
— that spread outward. These waves gradually get weaker the farther
they travel, until they eventually die out. These waves travel at speeds
ranging from about 1 to 3 miles per second. The surface waves from small
earthquakes usually go unnoticed except by sensitive instruments. But
people who experience a large earthquake often describe a swaying or a
rolling motion, which is the characteristic motion of seismic surface
waves.
Body Waves
A seismic wave that travels through a material rather than over
its surface is called a body wave. There are two basic types of seismic
body waves: sound waves and a type of shaking called shear waves. Everyone
knows that sound travels through the air, but did you know that sound
can also travel through water and even through solid rock? Sound waves
are the fastest type of wave. Shear waves travel slower than sound but
a little faster than surface waves. Because sound waves and shear waves
are usually the first two types of seismic waves to arrive at any point
after an earthquake, they are called P-waves and S-waves, from the Latin
for "first" (primus) and "second" (secundus).
P-Waves
The faster of the two types of body waves is the P-wave, or sound
wave. When an earthquake occurs, sound waves travel through the interior
of the Earth rather than over the surface. A sound wave is created in
air, water, or rock as particles of material press rapidly together and
then pull apart. The sound wave travels through the material as a series
of squeezes and stretches. At the earth's surface, sound waves travel
through rock at about 3 miles (4.8 kilometers) per second. Deep inside
the earth, however, the speed can be almost three times faster. P-waves
also travel at different speeds depending on the type of material they
pass through. Within the earth they travel fastest in the region at the
bottom of the earth's mantle. Within the fluid core they travel much more
slowly. P-waves reach everywhere around the Earth after about 20 minutes.
S-Waves
The slower type of body wave, the S-wave, or shear wave, can be
described in this way. Imagine that you have tied one end of a rope to
a post and are holding the other end. When you shake your end of the rope
from side to side, a shaking motion, like a wriggling snake, travels along
the rope. S-waves are a similar type of shaking motion. With S-waves,
material in the earth is actually moving sideways to the direction that
the wave is traveling. S-waves cannot travel in liquid; they can travel
only in solid material. Therefore, when S-waves reach the fluid part of
the earth's outer core they stop or are reflected back, or they convert
to P-waves in the core. The shaking motions of S-waves are usually much
larger than the motions of P-waves.
In 1663 the European settlers experienced their first
earthquake in America.
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