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Earthquakes
Chapter 8
Earthquakes
• Def: vibration of Earth
produced by the rapid
release of energy
• Focus- point within
Earth where EQ starts
• Epicenter- location on
the surface directly
above the focus
• Fault- fractures in
Earth where
movement has
occurred
Smaller Quakes
• Aftershocks- smaller EQs following larger
EQ
– usually weaker but can destroy structures
partially damaged during first EQ
• Foreshocks- small EQs that come before
major EQ
Elastic Rebound Theory
Measuring EQs
• Seismographinstrument that
records EQ
waves
• Seismogrampaper record of
EQ waves
Seismic Waves
• Surface waves- travel along outer layer; most
destructive
– Love waves- side-to-side motion
– Rayleigh waves- circular motion
• Body waves- travel through Earth’s interior
– Primary waves• Compressional wave
• Fastest seismic wave
• Travels through solid or liquid
– Secondary waves• Transverse wave
• Slower of the two body waves
• Travels only through solid
Seismic Waves cont…
Locating an EQ
• In order to find the exact location of an
EQ, there must be three seismic stations
• Using the triangulation method, the exact
location of an EQ can be found
Measuring
Distances to
Epicenters
Measuring EQs
• Intensity- a measure of the amount of EQ
shaking at a given location based on the amount
of damage
• Magnitude- a measure of the size of seismic
waves or the amount of energy released at the
source of the EQ
– Richter Scale- based on the amplitude of the largest
seismic wave
• Increases tenfold in magnitude with an increase of 1 on the
Richter scale
• Moment Magnitude- the amount of displacement
that occurs along a fault zone
• Modified Mercalli Scale- scale the represents the
intensity of shaking
Destruction from EQs
• The damage to buildings and other structures
from EQ waves depends on several factors:
– Intensity and duration of vibrations
– Nature of the material on which the structure is built
– The design of the structure
• Building Design
– Wood and steel frames withstand vibrations better
than un-reinforced stone or brick buildings
• Liquefaction- a phenomenon in which soils and
other unconsolidated materials saturated with
water are turned into a liquid that is not able to
support buildings
• Fires- broken gas lines can ignite
Dangers from EQs
• Tsunamis(Japanese for
harbor wave)- giant
waves produced
from a large
displacement of
water
Faulting
• Faults are fractures along which there has
been displacement of the material on
either side of the fault.
• Faults are classified based on:
1) the sense of movement (the direction in
which the blocks on either side of the fault
move) - this is controlled by the type of
stress that is applied.
2) the orientation of the fault surface (the
angle of the plane of fracture)
Fault Terminology
• Fault Plane - the plane along which the
rock or crustal material has fractured.
• Hanging Wall Block - the rock material
which lies above the fault plane.
• Footwall Block - the rock material which
lies below the fault plane.
Normal
Fault
• Tension Stress
• Hanging wall moved down relative to
footwall
• Because of the downward motion of the
hanging wall block, normal faults result in
the lengthening, or extension, of the crust
Reverse
Fault
• Compression Stress
• Hanging wall moved up relative to
footwall
• Reverse faults result in a shortening of
the crust
Lateral
Fault
•
•
•
•
Known as strike-slip
Shearing Stress
2 plates move horizontally not vertically
Crushed and broken rocks produced during
faulting are more easily eroded, often
producing linear valleys or troughs that mark
the locations of strike-slip faults.
Reverse
Thrust
Fault
• Compression Stress
• Hanging wall moved up and over footwall
• Thrust faults result in a shortening of the
crust