Solid Earth Geophysics-Geop503  Ali Oncel [email_address] Department of Earth Sciences, KFUPM Seismic Waves and Earth’s In...
Mysteries of the Inner Earth  http://ourworld.compuserve.com/homepages/dp5/inner1.htm
<ul><li>Solid Earth  Processes  may contribute to the weakness of the  a sthenosphere . They  include  some   phase change...
Seismic  Discontinuities <ul><li>Note that velocities increase gradually within layers (phase changes) but  jump at  disco...
 
<ul><li>Ray Paths for PKIKP </li></ul><ul><li>Movie </li></ul>Nature of the Core The direct P-wave passing through the man...
<ul><li>Probing the Earth’s Interior with seismic waves </li></ul>Constant Velocity Variable Velocity Constant Velocity Va...
Seismic waves - uniform density <ul><li>In a homogeneous planet with  uniform density   seismic waves are neither refracte...
Seismic waves - increasing density <ul><li>Because, In a planet where  density increases gradually  with depth, seismic wa...
Ray Paths and Travel Times <ul><li>As the angle of incidence at the hypocenter decreases, the rays descend more steeply an...
<ul><li>PKIKP  is a  P  wave which has traveled through the mantle and both the inner and outer cores, whilst  PKiKP  is r...
<ul><li>Seismic waves (wave fronts shown by dotted lines) and ray paths through the Earth’s interior that indicate interio...
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ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS

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Seismic Waves and Earth’s Interior

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  • Lehman 5144 Fe solid against FeO, FeS fluid (inner/outer core boundary) Gutenberg: 2885, it was discovered by Oldham but corrected depth for this discontinuity was carried out by Gutenberg. D&apos;&apos; 2870 thin, mixing of mantle and core material? (D”=D double-prime) 670 km 670 worldwide, no earthquakes deeper, debates over whether a composition, phase, or viscosity change 400 km 400 worldwide LVZ 50-200 regionally variable depth Moho 4-55 (abbreviation of Mo-ho-RHO-vi-chik) - sharp compositional change Conrad ? 5-30 mafic to felsic crust, often absent
  • Richard Dixon Oldham Richard Dixon Oldham ( July 31 , 1858 – July 15 , 1936 ) was a British geologist who, in 1906 , argued that the Earth must have a molten interior as S waves were not able to travel through liquids nor through the Earth&apos;s interior. Inge Lehmann Inge Lehmann ( May 13 , 1888 – February 21 , 1993 ), Fellow of the Royal Society ( London ) 1969 , was a Danish seismologist who, in 1936 , argued that the Earth must not only have a molten
  • Ray paths for PKIKP, the direct P-wave passing through the mantle, outer core and inner core (1959). pp.254-Richter-1959 The surface of the core, when finally found by Gutenberg, proved to be halfway down to the center. Nevertheless, the core is large; it is larger than the planet Mars, and its radius is a little greater than diameter of the moon.
  • As the angle of incidence at the hypocenter decreases, the rays descend more steeply and to greater depth and emerge at the surface at increasing distance. When the distance corresponds to a central angle of about 103 between hypocentre and station (for ordinary shallow earthquakes) the P ray grazes the core. P waves are refracted at the Mantle-Outer Core interface S waves are stopped at this interface Both create shadow zones and S wave attenuation implies a fluid outer core
  • After Bolt (1982)
  • Richter-1959-pp.256 As the angle of incidence at the hypocenter decreases, the rays descend more steeply and to greater depth and emerge at the surface at increasing distance. When the distance corresponds to a central angle of about 103 between hypocenter and station (for ordinary shallow earthquakes) the P ray grazes the core. Beyond this distance the amplitude of recorded P decreases rapidly; but there is not a sharp shutting off; and P-waves, especially of long periods, continue to be recorded up to angular distances of at least 130. This is a wave phenomenon not explained in terms of geometrical optics; it corresponds to diffraction of the P waves round the core boundary, analogous to the diffraction of light into a shadow. The effect for S is even stronger, but not so easy to observe. PKP: At the angle of incidence for which P grazes the core, the first refracted wave enters the core and the resulting ray is the first for PKP (P’). It emerges at the central angle about 10 deg . In excess of 180. Because of symmetry, a wave of this type appears at all points distant 10 deg from the antipodal “ on the opposite side or sides of the earth ” point, or at 170 degrees.
  • ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS

    1. 1. Solid Earth Geophysics-Geop503 Ali Oncel [email_address] Department of Earth Sciences, KFUPM Seismic Waves and Earth’s Interior Reading: Fowler Chapter 8- Section 8.1
    2. 2. Mysteries of the Inner Earth http://ourworld.compuserve.com/homepages/dp5/inner1.htm
    3. 3. <ul><li>Solid Earth Processes may contribute to the weakness of the a sthenosphere . They include some phase change s which can be listed as: </li></ul><ul><li>dehydration or partial melting </li></ul><ul><li>change in composition of mineralogy </li></ul><ul><li>increase in stress or dislocation density </li></ul><ul><li>very rapid in increase of the temperature gradient . </li></ul>U pper M antle Reference: Anderson, D. L., 1995. Lithosphere, asthenosphere and perisphere, Reviews of Geophysics, 33, 125-149.
    4. 4. Seismic Discontinuities <ul><li>Note that velocities increase gradually within layers (phase changes) but jump at discontinuities, which are thought mostly to be compositional changes. </li></ul><ul><li>LVZ (“ low velocity zone ”) is uppermost part of asthenosphere, with low Vp due to less viscous nature (partial melts). Plates ride on the LVZ </li></ul>YOU DON’T HAVE TO REMEMBER THESE!
    5. 6. <ul><li>Ray Paths for PKIKP </li></ul><ul><li>Movie </li></ul>Nature of the Core The direct P-wave passing through the mantle, outer core and inner core (1959).
    6. 7. <ul><li>Probing the Earth’s Interior with seismic waves </li></ul>Constant Velocity Variable Velocity Constant Velocity Variable Velocity
    7. 8. Seismic waves - uniform density <ul><li>In a homogeneous planet with uniform density seismic waves are neither refracted or reflected </li></ul><ul><li>Seismic ‘rays’ drawn perpendicular to the wave fronts follow straight lines </li></ul><ul><li>But early seismic observations showed the Earth is not like this </li></ul>
    8. 9. Seismic waves - increasing density <ul><li>Because, In a planet where density increases gradually with depth, seismic waves are refracted , i.e. they are bent </li></ul><ul><li>Seismic velocities would increase steadily with depth and rays would follow a curved path </li></ul><ul><li>This is similar to the paths of seismic waves in the Earth - up to about 11,000 km from the epicenter </li></ul>Why do seismic waves generally curve upward in Earth’s mantle? Ray Theory Ray Wave front
    9. 10. Ray Paths and Travel Times <ul><li>As the angle of incidence at the hypocenter decreases, the rays descend more steeply and to greater depth and emerge at the surface at increasing distance. When the distance corresponds to a central angle of about 103 between hypocenter and station (for ordinary shallow earthquakes) the P ray grazes the core. </li></ul>Stein, 2003
    10. 11. <ul><li>PKIKP is a P wave which has traveled through the mantle and both the inner and outer cores, whilst PKiKP is reflected back from the surface of the inner core. Similarly an S wave reflected at the core-mantle boundary is indicated by ScS , and if conversion occurs in reflection we have ScP. </li></ul>Body wave Phases After Bolt (1982)
    11. 12. <ul><li>Seismic waves (wave fronts shown by dotted lines) and ray paths through the Earth’s interior that indicate interior structure (crust, mantle, outer core, inner core, etc.) </li></ul>Seismic phases in the Earth <ul><li>PKP – The direct P-wave passing through the mantle and outer core. </li></ul><ul><li>C – Mantle P-wave which has grazing incidence on the core has an epicentral distance of 103. </li></ul><ul><li>C’ – The PKP ray with the shallowest angle of incidence on the outer core is refracted and finally at an epicentral distance of 188. </li></ul>A C E F D C’ B Figure modified after Gutenberg and Richter, 1939

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