Global geophysics and wave propagation

Transcription

1 Global geophysics and wave propagation Reading: Fowler p76 83 Remote sensing

2 Geophysical methods Seismology Gravity and bathymetry Magnetics Heat flow Seismology: Directly samples the physical properties of the Earth s interior Earthquakes indicate the stress regime Geophysical methods Seismology Gravity and bathymetry Magnetics Heat flow Gravity and bathymetry Connected through isostasy Tell us about density variations

3 Geophysical methods Seismology Gravity and bathymetry Magnetics Heat flow Magnetics Changes in the Earth s magnetic field recoded within surface rocks tell us about earth history Geophysical methods Seismology Gravity and bathymetry Magnetics Heat flow Surface heat flow Related to mantle temperature variations

4 Seismological techniques Global seismology Earthquake sources Global ray paths Imaging 3D structure of the Earth s interior Refraction & Reflection seismology Controlled sources Crustal and uppermost mantle ray paths Crustal structure Body waves and surface waves Point source seismic disturbance: Wavefront expands out from the point Body waves: sphere Surface waves: circle Rays: perpendicular to wavefront

5 Body waves Energy traveling through the interior of the earth Rays bend and reflect due to variations in physical properties of the Earth s interior Energy sources Earthquakes Globally distributed Primarily along tectonic plate boundaries

6 P-waves P for primary or push-pull Compression and rarefaction, no rotation Causes volume change as the wave propagates Similar to sound waves traveling through air S-waves S for secondary or shear and shake Shearing and rotation No volume change as the wave propagates

7 P and S-waves Waves a reminder v = fλ Terms: Velocity, v Wavelength, λ Frequency, f Period, T = 1/f λ

8 Seismic waveform Seismometer Use inertia of a mass to measure ground motion

9 P and S-velocities P-velocity κ + 4 µ ρ 3 α = V P = change of shape and volume S-velocity = S β V = µ ρ change of shape only Density, ρ = mass / volume Bulk modulus, κ Ratio of increase in pressure to associated volume change Always positive Shear modulus, µ Force per unit area to change the shape of the material µ of a liquid = 0, therefore α = 0 in fluid Velocity and density Birch s Law A linear relationship between velocity and density v = aρ + b Crust and mantle rock observations

10 Velocity and density Nafe-Drake curve V P igneous and metamorphic rocks V S sediments and sedimentary rocks Surface waves Energy traveling around the surface of the earth They sample the physical properties of the near-surface rocks Deep earthquakes do not excite surface waves as well as shallow ones

11 Amplitude Larger amplitude than body waves: conservation of energy, sphere versus cone Love and Rayleigh Waves Ground roll in plane of propagation direction Ground shake in horizontal direction

12 Velocity sensitivity The amplitude of surface motion decreases with depth Related to depth/wavelength Longer wavelengths sample deeper Seismic velocity generally increases with depth. Therefore, longer wavelengths arrive first Oceanic lithosphere Summary Geophysical remote sensing of the Earth s interior Seismology: direct sampling and stress regime Gravity and bathymetry: density variations Magnetics: rocks act as magnetic tape recording Earth history Heat flow: show temperature gradients within the mantle Seismic wave propagation Body waves: P and S; surface waves: Love and Rayleigh Propagation paths dependent on the physical properties of rocks