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Seismic WavesAn introduction
Walter D. Mooney,
USGS Menlo Park, CA.
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What is an Earthquake?
• Instrumentally recorded (or felt) ground shaking, normally a result of underground movement on a fault
San Francisco 1906 (USGS)Seismogram of the 1906 earthquake recorded in Germany
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Seismic waves
Faulting
USGS
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Types of Seismic Wave
Three-components of a seismometer record proportional to ground velocity of the P and S waves from a local aftershock of the Killari-Latur EQ, India (1993), at a hypocentral distance of 5.3 km
P. Bormann. 2002. New Manual of Seismological Observatory Practice (NMSOP)
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Body Waves
Copyright 2004. L. Braile.
3/42
pv
sv 3s
p
v
v
Bulk modulus = P / (V/ V)
Shear modulusor „rigidity“ = (F/A) / (L/L)
Young´s or „stretch“modulus E = (F/A)/ (L/L) and Poisson ratio = (W/W) / (L/L)
Deformation of material samples for determining elastic moduli
P. Bormann, NMSOP
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Note: The incidence angle is 59.5° for the long-period P- wave oscillation and 47.3° for the high-frequency P-wave group.
3-component records at station MOX (top traces) and related plots of particle motion in the horizontal (N - E) plane and two vertical planes (Z - N and Z – E, respectively) of the P- wave onset from seismic event (mining collapse) in Germany (1989; Ml = 5.5; epicentral distance D = 112 km, back-azimuth BAZ = 273°). Left: broadband recording (0.1 – 5 Hz); right: filtered short-period recording (1 – 5 Hz).
4.5 s
1s
P. Bormann (NMSOP)
Particle motion of body waves
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Surface Waves
– Form at the free surface
– Amplitude decays exponentially with depth.
Copyright 2004. L. Braile.
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January 26, 2001 Gujarat, India Earthquake (Mw7.7)
Recorded in Japan at a distance of 57o (6300 km)
Love Waves
vertical
radial
transverse
Rayleigh Waves
Courtesy J. Mori
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Wave Period and Wavelength
Velocity 6 km/s
x
t
wavelength
period
Space
Time
period 50 sfrequency = 1/period= 0.02 Hz
Velocity = Wavelength / Period
wavelength 300 km
Courtesy J. Mori
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Body waves
0.01 to 50 sec 50 m to 500 km
Surface waves 10 to 350 sec 30 to 1000 km
Free Oscillations 350 to 3600 sec 1000 to 10000 km
Static Displacements
-
Period Wavelength
Courtesy J. Mori
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Other phases
Digital broadband record of the Seattle Mw = 6,8 earthquake on 28 February 2001 at the station Rüdersdorf (RUE) in Germany (epicentral distance D = 73°). Note the detailed interpretation of secondary phase onsets.
P. Bormann (NMSOP)
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Ray theory• Seismic waves can be represented as rays
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1
2
1 < 2
Ray Paths in a Layered Medium
1
2
1 > 2
slower
Faster
Faster
Slower
velocity of seismic energy in the layer
sin 1 / 1 = sin 2 / 2 = s1 sin 1 = s2 sin 2
Courtesy J. Mori
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1
2
3
Ray Paths in a Layered Medium
1/1
1/2
1/3
Distance
Time
Courtesy J. Mori
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Andrija Mohorovicic (1857-1936)
Found seismic discontinuity at 30 km depth in the Kupa Valley (Croatia).
Mohorovicic discontinuity or ‘Moho’
Boundary between crust and mantle
The Moho
The MohoThe Moho
Copywrite Tasa Graphic Arts
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Structure in the Earth results in complicated paths Lowrie, 1997, fig 3.69
Bolt, 2004, fig 6.3
USGS
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Propagation of Seismic Waves In the Earth; M. Wysession
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Courtesy R. Mereu
Courtesy J. Mori
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Courtesy J. Mori
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Forward Branch
Backward Branch
Courtesy J. Mori
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Forward Branch
Backward Branch
Forward Branch
Shadow Zone
Courtesy J. Mori
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Forward Branch
Backward Branch
Forward Branch
Shadow Zone
PcP
・ 1912 Gutenberg observed shadow zone 105o to 143o
・ 1939 Jeffreys fixed depth of core at 2898 km (using PcP)
ForwardBranch
BackwardBranch
ForwardBranch
PPcP
PKP
Shadow Zone
Courtesy J. Mori
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PcP
Core Reflections
Courtesy J. Mori
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P Mantle P
S Mantle S
K Outer core P
I Inner core P
c Reflection from the outer core
i Reflection from the inner core
diff Diffracted arrival
IASP91, Kennett and Engdahl, 1991
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Stacked broadband seismograms for shallow earthquakes. Seismic phases are shown in different colors:Blue = verticalGreen = radial horizontalRed = transverse horizontal
P. Bormann. 2002. New Manual of Seismological Observatory
Practice (NMSOP)
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Amplitude and Intensity
Seismic waves lose amplitude with distance traveled - attenuation A(t) = A0e -ω0t/2Q
So the amplitude of the waves depends on distance from the earthquake. Therefore unlike magnitude intensity is not a single number.
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Normal Modes Normal Modes
Useful for studies of ・ Interior of the Earth ・ Largest earthquakes
l=1 m=1 l=1 m=2 l=1 m=3
Houseman http://earth.leeds.ac.uk/~greg/?Sphar/index.html
Liberty Bell(USA)
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Toroidal and Spheroidal Modes
ToroidalSpheroidal
Dahlen and Tromp Fig. 8.5, 8.17
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Natural Vibrations of the Earth
Shearer Ch.8.6Shearer Ch.8.6Lay and Wallace, Ch. 4.6Lay and Wallace, Ch. 4.6