Earthquakes
By Murray Legge
The natural disaster I picked is earthquakes. Reason why I picked this
natural disaster is because I am interested on how and why they happen.
While reading this research paper you should get a good understanding
about earthquakes. I know I did.
Earthquakes have always been occurring on earth. With the help of
man-made devices, like bombs, buildings, even cars have put great stress on
the earth. Thus creating more movement of plates and vibrations that make
earthquakes occur more frequent. Earthquakes usually occur somewhere in
the world almost every day, but not sever so you cannot really feel them.
Severe earthquakes can occur every few months to many years depending on
the amount of stress of the plates and the geographic surroundings of an
area (sand, rock, clay).
An earthquake is the vibration that occurs beneath the earth’s
surface crust and upper mantle. That is caused by pieces of the earth’s
crust that suddenly moves, or shift. Beneath the earth are twelve large
plates and their boundaries. These boundaries are compressed very tightly
together. The force that compresses the large plates together also causes
them to move very slowly. When two of these boundaries move against each
other they get stuck and create extreme amount of friction making it hard
to move. Eventually the force will break them apart and move, with this
great break also causes the rock around it to shake, made by seismic waves.
Creating an earthquake. The point where the rock first brakes apart is
called the focus. The point directly above the focus is called the epicenter.
(Resource found at http://pbs.usgs.gov/gip/earthq1/measure.html visited on
April 25, 2003)
The Earth is made up of twelve major plates, which make up the
theory of plate tectonics. These plates can move slide over, under and past
each other. This is because these plates lye on molten rock beneath the
earth. These plates make up the earths outer layer or crust. Which is 70km
thick. At the end of the plates are boundaries. These boundaries consist of
three types. Spreading zones, transform faults, and seduction zones. At
spreading zones molten rock rises and pushes two plates apart, adding new
material at their edges. This type of, boundaries are found in oceans.
Transform faults are common in areas where plates slide past one
another. The San Andreas Fault is an example of this type of boundary.
These earthquakes occur at shallow depths.
Subduction Zones happen when one plate slides over another plate
causing the other one to slide downward into the mantle where it melts.
(Reference at http://pbs.usgs.gov/gov/earthq1/where.html visited on April
27, 2003)
http://www.germantown.k12.il.us/html/earthquakes.html
Within these large plates are cracks called faults. These faults are
put into three main groups, depending on how they move.
Normal faults happen when one block moves down,
caused by pulling or tension. The remaining block
moves up.
Thrust faults, caused by
compression. One block moves up in
the fault plan. While the other
moves down.
Strike-slip faults are caused by stress. The blocks
move horizontally past one another.
Faults are areas of weakness in the earth’s crust or plates. Thus
making it common for earthquakes to occur. After stress from an
earthquake has been released in a fault does not mean all the stress has
been realeased, this can create another earthquake. The most common fault
is the San Andreas Fault. 2/3 of the world’s earthquakes occur at this
Southern California fault.
The focal depth of an earthquake is measured from the surface to
the focus (where earthquake originates). A focal depth of 70km is shallow
and a focal depth of 700km or more is deep.
90% of all earthquakes occur at plate boundaries. While the other
10% occur within plates. When these large plates move against each other
they move extremely slow creating more and more stress. When they
eventually break apart the stress is greatly lowered. This process is called
elastic rebound affect. Which then causes an earthquake.
With the release of stress from the boundaries give off great
vibrations called seismic waves, which can move in many directions from the
focus. These waves are categorized by the way they move.
Primary waves move the fastest. These waves are also known as “P”
waves. They arrive at a point before any other seismic wave. “P” waves can
travel through solids, liquids, and gases. The speed of these waves, depend
on the density of the material they are traveling through. However when
they move deeper into the earth, they speed up through dense material. “P”
waves push rock particles into particles ahead creating them to compress.
Secondary waves are much slower then “P” waves. Secondary waves
are also known as “S” waves. They arrive at a point after “P” waves. “S” waves
can travel through solids but not through liquids and gases. When traveling
through dense material they speed up. This causes rock particles to move
side to side.
Surface waves are the slowest of the seismic waves. Also known as
“L” waves. These waves originate at the epicenter. They travel along the
surface of the earth. However they cause more damage then “S” and “P”
waves.
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay http://www.uwgb.edu/dutchs/202ovhds/quakes.htm
http://www.germantown.k12.il.us/html/earthquakes.html
These seismic waves can help predict the time, epicenter, focal depth, type
of fault and the energy released, through a seismograph.
Another way of measuring an earthquake is using a Richter scale
measures the magnitude of an earthquake. The scale goes from 0-10. 0 being
low, and 10 being high. (Dangerous) Highest recorded was 9.5.
Richter Scale
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay http://www.uwgb.edu/dutchs/202ovhds/quakes.htm
The first indication of an earthquake is often a sharp thud from the “P”
wave, then “S” waves. Followed by the ground shaking caused by surface
waves.
With these destructive seismic waves the earth shakes and rattles.
These seismic waves not only create earthquakes but landslides through the
process of liquefaction. Liquefaction happens when loose, wet sediments lose
their strength due to strong shaking. Thus creating the earth to slide (like
an avalanche). During an earthquake, landslides often cause the most damage.
When the ocean floor is shaken creating extreme surface waves that
create a tsunami. A tsunami is a wave of water traveling in all directions
from the epicenter with great height (15m) and speed (960km).
http://www.germantown.k12.il.us/html/tsunami.html
U.S. Earthquakes, 1973-2002
Areas where earthquakes are very common
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay http://www.uwgb.edu/dutchs/202ovhds/quakes.htm
Eastern North America Earthquakes 1534-1994
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay http://www.uwgb.edu/dutchs/202ovhds/quakes.htm
Pictures of Earthquake destruction
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay http://www.uwgb.edu/dutchs/202ovhds/quakes.htm
Has Earthquakes increased over the years
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay