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Earthquakes
Earthquakes
- The movement of the ground, caused by
waves of energy released as rocks move along
faults
Fault – a large fracture in rocks, from several
meters to many kilometers long, where rocks not
only crack but also move along either side of the
break
Types of Stresses; tension, compression, & shear
Types of Faults; Normal, Reverse, Strike-Slip
Earthquake Mechanics (cont.)
Epicenter – the point on Earth’s surface directly
above the earthquake’s focus
Focus – the point in Earth’s interior or where
movement releases energy to cause the
earthquake
Seismic waves:
Waves of vibration caused by the energy
released during an earthquake.
• Types of waves produced: Surface and body
waves
• Seismograph: is the instrument used to
detect, measure, and record seismic waves
produced by earthquakes.
• Magnitude - A measure of the strength of an
earthquake or strain energy released by it, as
determined by seismographic observations.
• Seismogram: the recording of an earthquake
made by a seismograph
Body Waves
P waves – Primary Wave:
compression & expansion - push-pull seismic waves
• sound waves (too low to be heard)
• fastest (~8 to 9 km/sec) depending on material
• first to be recorded at a seismograph
• causes rock particles to vibrate in the same
direction the wave is traveling
• denser the material, the faster the P-wave travels
http://www.youtube.com/watch?v=2rYjlVPU9U4
Body Waves
S waves – Secondary Wave:
• shearing (lateral; side to side) seismic waves
• travels through solid only (3 to 5 km/sec) - NOT
liquid or gasses
• second to reach and be recorded at a
seismograph
• causes the rock particles to vibrate at right angles
to the direction of travel
•
http://www.youtube.com/watch?v=gl4FvHKzAlU
Surface Waves
Surface Waves – waves of energy, released
during an earthquake, that reach Earth’s
surface and travel outward from the epicenter
in all directions ON THE SURFACE ONLY
Travels only through solid
Surface Waves
L wave – Love wave:
• shearing (lateral; side to side) seismic waves
• speed is 2 to 6 km/sec
• has the greatest height of the ‘waves’
recorded on a seismogram
• has a horizontal (side to side) motion that is
perpendicular to the direction of travel
• causes the most destruction
• (named after A.E.H. Love, the English
mathematician who discovered it.)
Surface Waves
Rayleigh wave (Surface wave):
• Slowest of the waves 1 to 5 km/sec
• Most complex of earthquake waves; elliptical
motion at the Earth's surface.
• They are usually felt as a rolling or rocking
motion
• In the case of major earthquakes, can be
seen as they approach
• Often the largest and most destructive, of the
wave types caused by an earthquake.
• Named after Lord Rayleigh, the English
physicist who predicted its existence.
Surface Waves
• Love waves – L-waves
• Rayleigh waves
Earthquake Machines
Seismograph – instrument that detects &
records earthquake seismic waves
Seismogram – the recording data sheet of
an earthquake made by a seismograph
http://www.leo.lehigh.edu/projects/seismic/graphics/012601_SIndia.jpg
Earthquake Intensity
• Modified Mercalli Scale – intensity scale
(modified for North American conditions);
composed of 12 increasing levels of intensity
that range from imperceptible shaking to
catastrophic destruction.
http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
• Seismic moment - measures the earthquake’s
strength, or the energy released based the
amount of displacement along a fault.
Earthquake Intensity
• Richter Scale – measures the strength of an earthquake
– increase of one unit of magnitude (for
example, from 4.6 to 5.6) represents a 10-fold
increase in wave amplitude on a seismogram
– or approximately a 32-fold increase in the
energy released.
Therefore a magnitude 6.7 earthquake
releases over 900 times (32 times 32) the
energy of a 4.7 earthquake.
Creation of a Tsunami
Essential Questions -- Earthquakes
• Name and describe the 3 different seismic waves
• Explain how the structure of the earth’s interior
affects seismic waves
• Describe how seismographs are used to
determine the epicenter of an earthquake
• List the different scales used to measure the
magnitude and intensity of an earthquake
• Discuss the relationship between earthquakes and
tsunamis
• Identify the three types of stress and the resultant
features (faults/folds) associated with each.