Earthquakes and earthquake (or seismic) waves

Pages 332-333, 338-341, 358-361

EARTHQUAKE: Vibrations caused by the sudden release of energy stored in the rocks (page 332)

SEISMIC WAVES or EARTHQUAKE WAVES : Energy that moves through the earth because of an earthquake (page 332)

PROPERTIES OF WAVES In a uniform medium, a wave

radiates outwards as concentric spheres at constant velocity

Velocity of a wave depends on the nature of the medium

When a wave passes from one medium to another, it refracts (bends) and/or reflects (page 358)

There are two broad groups of seismic waves (page 338)

Body wavesCan travel through the interior of the earth

Surface wavesTravel along the surface of the earth

Body waves are used to study the internal structure of the earth (pages 358-359)

Two different types of body waves (page 338)

PRIMARY or COMPRESSIONAL waves (P-waves)

SECONDARY or SHEAR waves (S-waves)

Compressional Wave (P-Wave) Animation

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

http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm

Shear Wave (S-Wave) Animation

Deformation propagates. 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. However, Earth’s layers tend to cause mostly vertical (SV; in the vertical plane) or horizontal (SH) shear motions. Material returns to its original shape after wave passes.

http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm

• P- waves travel faster through solids than through liquids

•P-waves are faster than S-waves

•Both P-and S- waves travel faster through denser material

•Only P-waves can travel through liquids

S-waves do not pass through the outer core

Figure on page 359

P-waves can pass through parts of the core

Figure on page 359

The instrument used to record an earthquake

Seismograph (page 339)

Seismogram The actual record of an earthquake

(Page 341)

Seismograms record the AMPLITUDE (height of the peaks) of the seismic waves against TIME along the horizontal axis

Amplitude

What is the maximum amplitude recorded on this seismogram?

What is the maximum amplitude recorded on this seismogram?

RICHTER MAGNITUDE SCALE

Specifies the amplitude of the largest ground motion generated by the earthquake at a seismograph station located 100 km from the epicenter

• The amplitude can be measured directly from a seismogram (more quantitative)

• The scale is logarithmic (page 341)

• An increase of one magnitude on the Richter scale means approximately 30- fold increase in energy released (page 341)

Characteristics of Richter Scale