Seismic Wave Propagation

What are Seismic Waves

• Seismic waves are the vibrations from earthquakes that travel through the Earth

• They are the waves of energy suddenly created by rock fracture within the earth or an explosion.

• They are the energy that travels through the earth and is recorded on seismographs

History

Seismology - the Study of Earthquakes and Seismic Waves

Dates back almost 2000 years

• Around 132 AD, Chinese scientist Chang Heng invented the first seismoscope, an instrument that could register the occurrence of an earthquake.

• Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument.

• Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.

• The time, location and magnitude of an earthquake can be determined from the data recorded by seismograph stations.

Seismometers and Seismographs

• Seismometers are instruments for detecting ground motions

• Seismographs are instruments for recording seismic waves from earthquakes.

• Seismometers are based on the principal of an “inertial mass”

• Seismographs amplify, record, and display the seismic waves

• Recordings are called seismograms

Types of Seismic Waves

•Body waves Travel through the earth's interior

•Surface Waves Travel along the earth's surface -

similar to ocean waves

Seismic Waves

Body waves

Surface Waves

“Ground Roll” Rayleigh

Love

P

S

P-Wave(Body Wave)

Primary or compressional (P) waves

The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave.

The P wave can move through solid rock and fluids, like water or the liquid layers of the earth.

It pushes and pulls the rock it moves through just like sound waves push and pull the air.

Highest velocity (6 km/sec in the crust)

P-Wave

Particle motion consists of alternating compression and dilation. Particle motion is parallel to the direction of propagation (longitudinal). Material returns to its original shape after wave passes.

Particle motion

Deformation propagates

Secondary Wave (S Wave)

Secondary or shear (S) waves The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake.

An S wave is slower than a P wave and can only move through solid rock. (3.6 km/sec in the crust)

This wave moves rock up and down, or side-to-side.

S-Wave

Particle motion

Particle motion consists of alternating transverse motion. Particle motion is perpendicular to the direction of propagation (transverse). Transverse particle motion shown here is vertical but can be in any direction. Material returns to its original shape after wave passes.

Deformation propagates

L-Wave

Love Waves• The first kind of surface wave is called a

Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911.

• It's the fastest surface wave and moves the ground from side-to-side.

L-Wave

Particle motion

Particle motion consists of alternating transverse motions. Particle motion is horizontal and perpendicular to the direction of propagation (transverse). Particle motion is purely horizontal, focus on the Y axis (black lines) as the wave propagates through it. Amplitude decreases with depth (yellow lines). Material returns to its original shape after wave passes.

Deformation propagates

Rayleigh Waves

Rayleigh Waves

• The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885.

• A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.

• Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving.

• Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

Rayleigh Waves

Particle motion

Particle motion consists of elliptical motions (generally retrograde elliptical) in the vertical plane and parallel to the direction of propagation. Amplitude decreases with depth. Material returns to its original shape after wave passes.

Deformation propagates

Seismic Wave Speeds

= shear modulus

K = modulus of compressibility (bulk modulus)

= density

The bulk modulus (K) of a substance essentially measures the substance's resistance to uniform compression.

It is defined as the pressure increase needed to effect a given relative decrease in volume.Shear modulus, μ, sometimes referred to as the modulus of rigidity, is the ratio of shear stress to the shear strain.

L

F

L

F = E * L/L (Hooke’s Law)

E = Young’s modulus

Rand quartzite

strain

stress

Elastic Materials

Young's modulus (E) is a measure of the stiffness of a given material.

E predicts the amount a wire will extend under tension, or to predict the load at which a thin column will buckle under compression

Seismic Wave SpeedsMaterial P wave Velocity (m/s) S wave Velocity (m/s)

Air 332

Water 1400-1500

Petroleum 1300-1400

Steel 6100 3500

Concrete 3600 2000

Granite 5500-5900 2800-3000

Basalt 6400 3200

Sandstone 1400-4300 700-2800

Limestone 5900-6100 2800-3000

Sand (Unsaturated) 200-1000 80-400

Sand (Saturated) 800-2200 320-880

Clay 1000-2500 400-1000

Glacial Till (Saturated) 1500-2500 600-1000

= modulus of rigidity

K = modulus of compressibility (bulk modulus)

= density

Seismic Velocities

Why are seismic waves important? Some things seismic waves are good for include·       Mapping the Interior of the Earth·       Monitoring the Compliance of the Comprehensive Test Ban Treaty·       Detection of Contaminated Aquifers·       Finding Prospective Oil and Natural Gas Locations

IRIS Earth’s Interior Structure Poster – Seismic waves through the Earth

Earth’s interior structure and seismic raypaths that are used to determine the Earth structure.