Download Lecture 7

Earthquakes and Earth’s Interior
1. What are Earthquakes
2. Locate and Measure EQ
3. Use EQ to understand
Earth’s structure
4. Plate Tectonics and EQs
What is an Earthquake?
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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
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Earthquakes & Faults
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EQs generated on faults
Faults = brittle failure by stress
The rupture/brittle failure of a fault is caused by
stresses (tectonic or loading) in the lithosphere
Earthquakes and Plate Tectonics:
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Thus, most Earthquakes are located
at plate margins where tectonic
stresses are greatest.
Recall: EQs define lithospheric
plate boundaries
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The Elastic Rebound Theory
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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 ELASTIC REBOUND (stress-strain diagram)
and TYPES OF WAVES on board…
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Elastic Rebound Theory
Types of Seismic Waves
P
S
P-wave
animation
S-wave
Draw stress vs. strain, elastic moduli and Velocity equations.
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Locating Earthquakes:
Seismographs and difference in wave speeds
First Seismometer (132 AD) invented by Chinese
mathematician Chang Heng during Han Dynasty
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Principle ideas behind locating earthquakes
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P-wave velocity > S-wave velocity
Thus the 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
Draw Lag Time
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How many seismic stations are required
to locate the focus of an earthquake?
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Local or Richter Magnitude: ML = log10 (A/T) + f(d,D)
Or
Moment magnitude: Mw=2/3log10µAu – 6.0
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A = wave amplitude, T = wave period, d =
distance, D = depth
Note ML is function of depth and Distance
to Focus - not desirable.
Mw is ‘complex’
n A = area of fault, u = displacement along fault
µ= shear modulus, Seismic Moment = µAu
Reall just amount of slip * length of rupture *
depth of rupture * strength of rock
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Mw better than Richter because Mw does not
‘saturate’ (give low values) at high
magnitude – BETTER measurement of
energy.
Note these are log10 scales:
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one unit increase = 10x amplitude increase
one unit increase = 32-time energy increase
Illustrate difference between Richter and Moment Magnitudes
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