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EARTHQUAKES
CHAPTER 8
Earthquake – a vibration in the earth caused
by a rapid release of energy, usually a
slippage along a fracture in the Earth’s
crust
• Focus – the point within the earth where
the earthquake starts
• The energy released from an earthquake
spreads out in all directions in the form of
waves
• Epicenter – the location on the surface
directly above the focus
• Elastic Rebound Theory – Forces within the
Earth slowly deform the crustal rocks on both
sides of the fault. When rocks are deformed,
rocks first bend storing elastic potential energy.
The resistance caused by the internal friction
that holds the rocks together is overcome and
then the rocks slip at the weakest point releasing
stored energy.
• Most earthquakes are produced by the rapid
release of elastic energy stored in rock that has
been subjected to great forces. When the
strength of the rock is exceeded, it suddenly
breaks, causing the vibrations of an
earthquakes.
• Aftershocks – smaller much weaker
earthquakes that occur for several days
after the main earthquake
• Foreshocks – small earthquakes that often
come before a major earthquake
• The San Andreas fault system is the most
studied fault system in the world. Each
segment behaves slightly different than
the other segments.
• Seismographs –
instruments used to
measure earthquake
waves
• Seismogram –
recorded trace of a
vibration
• Earthquake waves – there are two main types of
seismic waves, surface waves and body waves
(P waves and S waves)
• Surface waves – seismic waves that travel along
the ground, move up and down or side-to-side.
Te most destructive of all earthquake waves.
• P waves – push-pull waves, they compress and
expand rocks in the direction the waves travel.
This type of wave is called compressional or
longitudinal waves. Travel through solids liquids
and gases.
• S waves – shakes the rock at right angles to the
direction of travel. S waves are transverse
waves. Gases and liquids will not transmit S
waves because they do not rebound elastically.
• P waves travel faster than S waves. Surface
waves travel the slowest.
• Locating an Earthquake – earthquakes are located
by the differences in velocities of P and S waves
from three seismic stations
• Earthquake distance can be determined by the
difference in travel time. See chart on figure 8
page 225.
• How far is the epicenter if the difference in travel
time is 3 seconds?
• Earthquake direction – a circle is drawn around
each seismic station. The epicenter is where the
three circles overlap.
• Pacific ring of Fire – most earthquakes occur
around the outer edge of the Pacific Ocean
• Richter Scale – measures the magnitude
of earthquakes on a scale from 1 to 10.
• Moment magnitude – the amount of
displacement along the fault, estimates the
energy released
• Earthquake destruction – the damage to
buildings and other structures from
earthquake waves depends on the
vibrations, the nature of the material on
which the structure is built, and the design
of the structure
• Liquefaction – stable soil is turned into a
liquid that is unable to support buildings
and other structures
• Tsunamis – tidal
waves triggered by an
earthquake where a
slab of ocean floor is
displaced vertically
along the fault.
Tsunamis are also
generated by
vibrations from
underwater
landslides.
• Landslides – violent shaking of an
earthquake can cause the soil and rock on
slopes to slide downhill or collapse
• Fire – gas and electric lines are cut
starting fires
• Predicting Earthquakes – accurate shortand long-term predictions when
earthquakes will occur are not accurate.
• Earth’s Structure
• Moho boundary – the boundary that
separates the crust from the underlying
mantle. His is where the velocity of
seismic waves increases abruptly.
• Seismic P waves are refracted or bent as
they travel through the earth. The
refraction is explained by the different
composition of the crust, mantle, and core.
This deflection creates a shadow zone.
• Seismic S waves are stopped at the outer
core, therefore it must be liquid
• Crust
•
Thin, rocky outer layer
•
Divided into oceanic and continental
•
Ocean crust is 7 km thick, density of
3.0 g/cm3, younger 180 million years or
less
•
Continental crust, 8-75 km thick,
density of 2.7 g/cm3, some over 4
billion years
•
Made of lighter, granitic rocks
Mantle
•
82% of the Earth’s volume
•
Density 3.4 g/cm3
•
Basaltic composition
Core
•
Iron-nickel alloy
•
Extreme pressure and high
temperature
•
Density 13 g/cm3
• Lithosphere – crust and uppermost
mantle, cool rigid shell
• Asthenophere – rocks close to melting
temperature and are easily deformed
• Lower mantle – rocks are very hot and
capable of plastic flow
• Outer core – liquid layer, the flow of
metallic iron generates the Earth’s
magnetic field
• Inner core – the material is compressed
into solid iron-nickel rock
Earthquake Destruction