Solid Earth Geophysics Ali Oncel [email protected] .sa Department of Earth Sciences KFUPM Today’s class: Seismology Measuring the Interior Reading: Fowler Chapter 4
Jun 19, 2015
Solid Earth Geophysics
Ali [email protected].
saDepartment of Earth SciencesKFUPM
Today’s class: Seismology Measuring the InteriorReading: Fowler Chapter 4
Body and surface waves
Earthquake’s energy is transmitted through the earth as seismic waves according to the wave equation.
Two types: Body waves - transmit energy through earth’s interior
Primary (P) wave- rocks vibrate parallel to direction of wave; compression and expansion (slinky example)- irrotatational waves
Secondary (S) wave- rocks move perpendicular to wave direction; rock shearing (rope-like or ‘wave’ in a stadium)- rotational waves
Surface waves - transmit energy along earth’s surface Love waves: rock moves from side to side like snake Rayleigh waves: rolling pattern like ocean wave
Primary WavesPrimary Waves Secondary WavesSecondary Waves
Body Waves
Dilatation and No Rotation Rotation and no change in volume
P-waveP-wave S-waveS-wave
Animations: Body Waves
Movement of EarthMovement of Earth
3D Components of Waves
3D Components
6.27P-waveP-wave S-waveS-wave 6.35
Snell's law
When velocity increases in next layer, ray gets bent away from normal on layer
In earth, velocities increase throughout the mantle
Types of Surface Waves
Rayleigh Waves
Typical velocity: ~ 0.9 that of the S wave
Behavior: Causes vertical together with back-and-forth horizontal motion. Motion is similar to that of being in a boat in the ocean when a swell moves past.
Arrival: They usually arrive last on a seismogram.
Love Waves
Typical velocity: Depends on earth structure (dispersive), but less than velocity of S waves.
Behavior: Causes shearing motion (horizontal) similar to S waves.
Arrival: They usually arrive after the S wave and before the Rayleigh wave.
Amp ~ 1/x
Amplitudes of waves
Body WaveBody Wave Surface WaveSurface Wave
Amp ~ x-1/2