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Unformatted text preview: PX266 Geophysics (2010/11)
Lecture 9 Handout – Seismology
Dr. Gavin Bell
Seismology: study of elastic waves (seismic waves) propagating through the Earth’s
interior and on the Earth’s surface.
Elastic Wave: periodic, propagating distortion of an elastic medium (solid or fluid).
Classification of Seismic Waves
Body Waves propagate in bulk medium (3D, point source → spherical wavefront).
Wave speed S-waves
Wave speed Surface Waves propagate on a surface (2D, point source → circular wavefront).
No transverse motion
Only transverse motion
Body waves are also classified by the region of the Earth in which they propagate.
The figure shows selected seismic body wave paths from an earthquake at focus F:
compressional P and shear S waves in the crust and mantle (behaving as an elastic
solid) and compressional K waves only in the outer core (behaving as a liquid).
Rayleigh (LR) and Love (LQ) surface waves are not shown; nor are pP, sS and
similar waves reflected from the surface close to F. From Earthquakes by B.A. Bolt. Top: direct compressional
wave P from focus F to
seismometer S and
compressional wave pP which
is reflected from the surface
close to the epicentre E. The
focal depth EF is exaggerated
on the figure but can be
derived from the arrival time
difference between P and pP. Bottom: strong refraction of Pwaves at the core-mantle
boundary gives rise to a
‘shadow zone’ in which no Pwaves can arrive from F.
Waves travelling in the liquid
outer core are denoted K,
hence the waves to the right of
the shadow zone shown are
denoted PKP. The angular size
of the shadow zone (about 40°)
tells us about the depth of the
core-mantle boundary. Simple seismogram – we will only deal with easy-to-interpret seismograms with these
features. It shows P then S wave arrival followed by large-amplitude surface waves
(Love wave groups usually arrive before Rayleigh waves). Further study
Check you understand the P-wave “shadow zone” effect and why it does not depend
on the fluid nature of the outer core.
Revise Fermat’s principle and Snell’s Laws if needed.
You don’t need to learn all the interior seismic wave classifications (PKP, SP, etc.)
but must know the four basic wave types. ...
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- Spring '11