Download Earthquakes The theory of plate tectonics states that Earth`s crust is

Earthquakes
The theory of plate tectonics states that Earth’s
crust is made of sections called “plates”. These
plates are solid, but float on top of partially liquid
mantle. The plates move in different directions—
toward each other, away from each other, or past
each other.
When the plates move, rocks layer are bent. At the surface, the rock
layers are cold and break when they are moved. Below the surface,
under heat and pressure, the rock layers can bend and store energy
before breaking. When the rock layers do break, energy is released
as earthquake waves. A fault is a break in rock layers where the rocks
have moved. All faults cause earthquakes when they move.
The focus is where the earthquake starts.
The epicenter is the point on the ground directly above the focus.
Four different types of waves are produced by earthquakes: P, S, L, and R waves.
1. P Waves. P (or Primary waves are
compressional waves. P waves travel the fastest
of all the earthquake waves and arrive first. P
waves are able to travel through the Earth to
locations around the globe. P waves can travel
through solids and liquids. P waves travel at
speeds between 6 and 13 km/sec.
2. S Waves. S (or secondary) waves are transverse waves. S waves arrive second of all the earthquake waves. S
waves can travel through the body of the Earth, like P waves, but S waves cannot travel through liquid. S waves
can only travel through solids. S waves travel at
speeds between 3.5 and 7.5 km/sec.
Finding Earthquake Distance.
Scientists use the time difference between the
arrival of the P and S waves to find out how far
away an earthquake happened. Because S waves
travel slower than P waves, the longer the waves travel, the farther out in front the P waves get.
3. L Waves. L (or Love) waves are transverse
waves the travel on the surface, like R waves. L
waves are like R waves turned on their side. L
waves are very damaging to buildings, also.
4. R Waves. R (or Rayleigh) waves are
transverse waves which move on the surface of
the Earth. R waves have the same type of motion
as water waves, the particles of the Earth move in
circular orbits as the wave passes. R waves are
slower than S waves, but are much more
damaging to buildings.
Structure of the Earth:
P and S waves both travel through the body of the Earth, but S waves do not travel through liquids. When
scientists study the pattern of earthquake waves that travel through the Earth, they notice that there are
“shadows” where the P and S waves do not pass through the Earth. The largest shadow is the S wave shadow.
Since S waves do not travel through liquids, scientists think the outer core of the Earth is liquid. The liquid
blocks the S waves and causes a shadow. There is a second, P wave shadow, which scientists think is caused by
refraction around the inner core. Based on the speed of the P waves, scientists think the inner core of the Earth
is solid--either iron or iron and a mixture of nickel.