Geophysics, Lecture Notes- Physics - Prof Gavin Bell 10

Geophysics, Lecture Notes- Physics - Prof Gavin Bell 10 -...

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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). P-waves Compressional Wave speed S-waves Shear Wave speed Surface Waves propagate on a surface (2D, point source → circular wavefront). Rayleigh Wave No transverse motion Love Wave 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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