Seismology Lecture 3: Understanding Seismic Waves and Their Dispersion, Slides of Geology

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• Seismology-­‐ lecture

ALPHABET SOUP FOR SEISMIC WAVES

mantle core

Reflected waves on the CMB and the ICB

Surface waves

• Two types
  • Rayleigh waves (R)
  • Love waves (G) Appear on different seismometer components

From Stein and Wysession, 2003 Shallow earthquake

Surface wave dispersion

• Group velocity (U) – km/s
  • Velocity of travel of energy at a parRcular frequency
• Phase velocity (C) -­‐ km/s
  • Velocity of travel of a parRcular phase (peak or trough)

• Consider the sum of two harmonic waves with slightly different angular frequencies and wavenumbers:
• We obtain:

u ( x , t ) = cos(!

1 t! k 1

x ) + cos(!

2 t! k 2 x ) with :

1

1

2

k 1

= k + " k ; k

2

= k! " k ; k << k

u ( x , t ) = 2 cos(! t! kx )cos( "! " t! " k " x )

U = d! dk C = ! k => U = C + k dC dk = C +! dC d! (layer over half-­‐space) RelaRon between group and phase velocity:

Mongolia earthquake recorded in Japan

Models based solely on surface wave dispersion are non-­‐unique