Earthquakes and Earth’s Interior
1.
2.
3.
What are Earthquakes
How do we locate and measure EQ
Use EQ to understand Earth’s Structure (next lecture)
What is an Earthquake?
• Earthquake = ground shaking by waves
• Waves are caused by the passing of ENERGY
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Definition of Earthquake:
Motion of earth caused by transmission of energy (wave)
released from breaking strained rocks (in lithosphere)
EQ focus vs. epicenter
Focus = location in Earth where breaking occurs
Epicenter = projection of Focus onto surface
Earthquakes & Faults
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•
•
EQs generated on faults
Faults = failure by stress (more on faults later)
The rupture/failure of a fault is caused by stresses
(tectonic or loading) in the lithosphere
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Earthquakes and Plate Tectonics:
• Thus, most Earthquakes are located
at plate margins where tectonic
stresses are greatest.
• Recall: EQs define lithospheric
plate boundaries
The Stress Map
•
Stresses focused at plate boundaries
•
Types of plate boundary and related stresses:
– Convergent = compressional stress
– Divergent = tensional stress
– Transform = shear stress
Stresses and their resultant Faults
•Tension = Normal Fault
•Compression = Reverse
and Thrust Faults
•Shear = Strike-slip Fault
Animation
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The Elastic Rebound Theory
• How faults create earthquakes
• Rocks deform or strain due to tectonic stresses
(elastic deformation)
• Failure occurs (rupture strength exceeded) at one
spot/point (focus) in rock when elastic deformation
can no longer accommodate stress.
• Strained rocks around focus rebound, releasing
energy in the form of waves (Earthquake!)
• Draw on board…
Elastic Rebound Theory
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Types of Seismic Waves
P-wave
animation
S-wave
Why don’t S-Waves travel through liquids?
A.
B.
C.
Liquids cannot sustain shear
stresses
Liquids are not compressible
S-waves are too slow to travel
through liquid
Locating Earthquakes:
Seismographs and difference in wave speeds
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Principle ideas behind locating earthquakes
P-wave velocity > S-wave velocity
Thus there is a lag time in the arrival of the P and S waves at a
seismometer.
Given the velocity of these waves through the Earth and the lag
time, one can calculate a distance.
Seismograph
Animation
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What causes Earthquakes?
A.
Faults
B.
C.
D.
Volcanoes
Stress focused at plate margins
Mountains
How many seismic stations are required to locate
the focus of an earthquake?
Local or Richter Magnitude: ML = log 10 (A/T) + f(d,D)
Or
Moment magnitude: M w=2/3log10 µAu – 6.0
A = wave amplitude, T = wave period, d =
distance, D = depth
Note M L is function of depth and Distance
to Focus - not desirable.
Mw is ‘complex’ but really just = amount of
slip * length of rupture * depth of rupture *
strength of rock
Mw better than Richter because M w does
not ‘saturate’ (give low values) at high
magnitude – BETTER measurement of
energy.
Note these are log 10 scales:
For one unit increase, 10x wave amplitude
increase
For one unit increase 32-time energy
increase
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December, 2005 Earthquake and Tsunami: M = 9, fourth largest since
1900. Earth rang like a bell for more than a month afterward.
EQ lasted 7 minutes, displacement of up to 20 m along a 1200 km rupture.
Released strain = to sum of all EQs between 1976 and 1990.
>220,000 people killed by Tsunami (most lethal disaster in human history).
Bbc Animation
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Review Questions:
• What is an earthquake?
• How and where are earthquakes generated (what causes
earthquakes)?
• How are faults related to earthquakes?
• How are the types of faults (e.g. normal, reverse, strike-slip)
related to stress and types of plate boundaries?
• What property of seismic waves allows us to locate earthquake
epicenters?
• Contrast S-waves with P-waves.
• What is the difference in energy released by a Magnitude 9
compared to a Magnitude 7 earthquake?
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