4. Global Seismology William Wilcock

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4. Global SeismologyWilliam Wilcock
OCEAN/ESS 410
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Lecture/Lab Learning Goals

• Understand the distribution of earthquakes on the
  Earth and their relationship to plate tectonics
  (see also Lab 1)
• Know what an earthquake is, how earthquake sizes
  are classified, and the different types of body
  waves.
• Understand how seismic waves propagate through
  the earth along many different paths and how this
  constrains the internal structure of the earth.
• Be able to identify seismic body wave arrivals
  for a teleseismic earthquake, interpret a seismic
  travel time curves, and locate an earthquake
  using S-wave minus P-wave arrival times and
  P-wave arrival times - LAB

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Tectonic Plates
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Global Seismograph Network
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What is an Earthquake

• An earthquake is a sudden and sometimes
  catastrophic movement of a part of the Earth's
  surface. Earthquakes result from the dynamic
  release of elastic strain energy that radiates
  seismic waves. Earthquakes typically result from
  the movement of faults, planar zones of
  deformation within the Earth's upper crust. The
  word earthquake is also widely used to indicate
  the source region itself. - Wikipedia
• Earthquakes radiate two types of seismic waves -
  body waves that travel through the earth and
  surface waves that travel over it. There are two
  types of body waves - P waves and S waves

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Body Waves P-waves
Primary Wave P wave is a compressional (or
longitudinal) wave in which rock (particles)
vibrates back and forth parallel to the direction
of wave propagation. P-waves are the first
arriving wave and have high frequencies but their
amplitude tends not to be very large
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Body Waves S-waves
Secondary Wave S wave is a slower, transverse
wave propagated by shearing motion much like that
of a stretched, shaken rope. The rock (particles)
vibrate perpendicular to the direction of wave
propagation. They tend to have higher amplitudes
and lower frequencies than P-waves. S-waves
cannot travel through liquids (i.e., the outer
core, the oceans)
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Surface Waves
Surface waves travel over the surface of the
earth. They travel more slowly than body waves
but tend to have higher amplitudes and often are
the most damaging waves from an earthquake
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Surface wave
P-wave
S-wave
aftershock
S-P
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10
20
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Time (min)
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Velocity Structure of the Earth
Velocity (km/S)
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8
12
0
40

• Upper mantle
• P waves 8-10 km/s
• S-waves 4-6 km/s
• Lower mantle
• P-waves 12-14 km/s
• S-waves 6-7 km/s
• Outer Core
• P-waves 8-10 km/s
• S-waves - Do not progagate
• Inner Core
• P-waves 11 km/s
• S-waves 5 km/s

670
2900
Depth (km)
5155
6371
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How do waves propagate through the earth

• Refraction - Snells Law
• Waves bend back towards the surface when
  traveling through regions where the velocity
  increases with depth
• Interfaces
• When a seismic P-wave propagates across a sharp
  boundary a portion of the wave will be reflected
  as P-wave and a portion will be converted to
  transmitted and reflected S-waves. The same
  applies to an S-wave. 1 incoming wave gives rise
  to 4 outgoing waves.

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Seismic Phase Names
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Seismic Travel Time Curve
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S minus P travel times constrain the Earthquake
Distance
This Figure is wrong in one respect - the
seismograms do not show clearly that the S-waves
are much lower frequency than P waves. You will
see this in the exercise.
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S-minus-P travel-times will constrain the
distance from station to earthquake Note the
lower frequency for S-waves compared to
P-waves Surface waves are low-frequency and high
amplitude arrivals Travel-times are relative to
P-wave arrival time
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Earthquake Location Exercise
In the next lab we are going to be doing an
earthquake location exercise which is courtesy of
Professor Larry Braile at Purdue University.
Professor Braile has developed an impressive
array of earth science education activities. His
web site is. http//web.ics.purdue.edu/braile/