Solid Earth Geophysics

Ali Onceloncel@kfupm.edu.

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