Earthquakes
Earth's crust is broken into seven major tectonic plates and many smaller ones.
Tectonic plates are huge slabs of rock. The largest plates are named after the
continents and oceans that sit on them. Plates beneath the oceans, called oceanic
plates are thinner and younger compared to continental plates. Continental plates are
located under continents, and are much older and thicker. Plates are constantly moving.
Scientists call this movement "creep." At the areas where plates meet, four things can
happen. One layer may slide beneath another and form a volcano and cause an
earthquake. The plates may crash into one another, and scrunch up to form mountains.
The plates may slide roughly past one another, triggering earthquakes. The plates may
move apart in a process called separating.
I. broken tectonic plates
1. plates constantly moving
2. layer slide beneath
3. convergent boundaries toward
4. slide below subduction
At convergent boundaries, plates move toward each other. When they meet, one plate
can slide below another in a process called subduction. Subduction usually happens
where ocean plates slide under continental plates. Subduction created the Andes
Mountains at the boundary of the Nazca Plate and the South American Plate. When two
continental plates converge, the plates scrunch up to form mountains, such as the
Himalaya Mountain range. Earthquakes at convergent boundaries begin deep in the
crust and are strong and destructive.
The spot underground where an earthquake occurs is called the focus or hypocenter.
The spot on the surface of the Earth above the focus is called the epicenter. The
strongest shaking is usually felt near the epicenter of an earthquake. When a fault
breaks it sends out waves of energy called seismic waves. Seismic waves are the
vibrations we feel during an earthquake. An earthquake is usually a group of seismic
waves. Foreshocks are smaller tremors that come before a large earthquake.
Sometimes these tremors are felt as a slight vibration and shaking. A large earthquake is
called a mainshock. Aftershocks are tremors that follow. Aftershocks can happen weeks,
months, or years after a large earthquake. Usually the larger the mainshock, the more
numerous the aftershocks there will be.
II. deep strong destructive
1. earthquake occurs epicenter
2. strongest shaking near
3. foreshocks smaller before
4. aftershocks tremors follow
5. Rayleigh waves roll 3 km. sec.
6. creep slow movement
7. laser beams track
8. seismograph measure waves
9. Richter Scale 7.9 serious
Surface waves cause the most damage during an earthquake, even when they are far
from the epicenter. Love waves are surface waves that travel through the earth's
surface horizontally and move the ground from side to side. Love waves usually travel at
about three kilometers per second. Rayleigh waves make the surface of Earth roll like
waves on the ocean. They are the slowest of the seismic waves and they move at about
the same speed as Love waves.
Seismologists cannot predict the exact date and location of an earthquake. They use the
information they have to monitor areas that have many earthquakes in order to estimate
when one might occur. Creep is the slow and steady movement along faults caused by
moving plates. Seismologists use tools to keep track of creep, but they cannot
distinguish between creep and foreshock until after an earthquake. Seismologists use
laser beams to track changes in fault lines. They beam light across a fault and can tell by
the way the light reflects back whether the ground has moved. The laser beams record
very small movements. A seismograph is used to measure seismic waves. It is made up
of a heavy weight that hangs from a spring or wire. When the ground shakes, the weight
appears to jiggle and bounce up and down or back and forth. Attached to the other end
of the spring or wire is a pen that records the bumping of the weight onto a piece of
paper. This is called a seismogram. When there is no movement in the ground the pen
draws a straight line. Powerful earthquakes have high peaks and low valleys on their
seismographs. In the 1930's, American seismologist Charles F. Richter measured the
peaks and valleys on a seismogram to determine the strength, or magnitude, of an
earthquake. From his findings he developed the Richter Scale to rate earthquakes. For
every increase of one number on the scale, the earthquake is ten times more powerful.
A quake that rates from 5.5 to 6.0 will do slight damage to buildings and other structures.
A quake measuring 6.1 to 6.9 may cause a lot of damage in very populated areas. A 7.0
to 7.9 quake would cause serious damage. A quake measuring 8.0 or larger would
totally destroy communities near the epicenter.
Most of the damage an earthquake causes is not from the earthquake itself, but from its
after-effects: tsunamis, liquefaction, landslides, mudslides, avalanches and fires.
Earthquakes beneath the ocean floor cause huge ocean waves called tsunamis. When
the ocean floor moves up or down during an earthquake, a large amount of water is
displaced, creating the wave. Tsunamis travel thousands of miles across the ocean at
speeds of 960 kilometers per hour or more. At sea these waves are not very large, but
as tsunamis near shorelines and shallower waters, the waves slow down and increase in
height. Liquefaction occurs in areas where the soil is loose or sandy. Strong quaking
shakes the soil apart into individual grains. This turns the soil into a wet, slushy
quicksand. Buildings on the soil are no longer supported and topple. Once the shaking
stops, the ground returns to its firm condition. A landslide is the rapid movement of soil,
rocks, trees and plants down a slope. the shaking of the surface generated by
earthquakes loosens the soil and debris on the slope. Some landslides are slow-moving,
while other landslides move so rapidly that they can destroy cities and kill people without
warning. Mudslides are fast moving landslides that start on steep hillsides. As the mud
forms, the slide can reach speeds up to 56 kilometers per hour. Mudslides can be a
watery mixture or thick, rocky debris. Mudslides are strong enough to wash away trees,
cars and homes. An avalanche is a rapid slide of snow and ice down a mountainside or
slope. They are often triggered by earthquakes. Avalanches can reach speeds of more
than 322 kilometers per hour. During an earthquake power lines may be knocked down
and natural gas lines sometimes crack causing fires. Often water lines are also damaged
during the earthquake. Without water to put out the fires, cities can burn to the ground.
III. liquefaction soil sandy
1.
2.
3.
4.
5.
6.
shakes soil individual
wet slushy quicksand
buildings No longer supported
stops returns firm
power knocked down
water lines damaged
IV. people trapped rubble
1. long without food
2. days without water
3. successful within hours
4. planning practice react
5. items power out
6. safe spot doorway
During an earthquake people can be trapped under the rubble of buildings. Earthquake
rescue teams must work quickly and carefully to extract people from the rubble. Humans
can live a long time without food, but only days without water. Most successful
extractions occur within hours or days of a quake. A number of disaster relief agencies
such as the Red Cross work in earthquake zones as soon as an earthquake hits. Many
countries depend on relief agencies to help rescue people, provide hospital supplies and
infrastructure, such as roads, railways, communications, power and water supply.
People who live in areas where earthquakes commonly occur need to earthquake proof
their homes. They do this by imagining what would happen if their houses started to
shake and planning for it. By planning and practicing what to do before an earthquake
occurs, people can learn to react quickly when shaking begins. It is a good idea to have
some items on hand in case the power goes out. When severe earthquakes hit, an area
is often left without electricity, running water and heat for a couple of hours or even days.
Remember to stay calm. If you are inside, stay inside. Go to a safe spot in a doorway,
under a desk, or on the inside corner of the house or building. Stay away from windows
or glass doors. If on the road, drive away from bridges, stop in a safe area and stay in
the vehicle.