Seismic Waves

Question 1

What are the two main seismic waves?

Answer 1

Earth radiates seismic energy by both body and surface waves:

• Body Waves (P and S)- can travel through the Earth’s inner layers (its ‘body’). Body waves travels through the interior of the earth, body waves arrive before surface waves. Also has a higher frequency than surface waves.
• Surface Waves (Love & Raleigh)- can only move along the Earth’s surface- like waves on water.

Question 2

Body Waves- Describe P waves.

Answer 2

P (primary) Wave:

• P wave or primary wave- fastest seismic wave
• First to ‘Arrive’ at a seismic station
• Move through solid rock & fluids
• Particles move in the same direction as wave propagation
• P waves are compressional and longitudinal

Question 3

Body Waves- Describe S waves.

Answer 3

S (secondary) Wave or shear wave:

• Slower than a P wave
• Only moves through solid rock, NOT through liquid
• This property of S waves led seismologists to conclude that Earth’s outer core is liquid.
• Particles move up and down, or side to side, i.e. perpendicular to direction of wave propagation
• Secondary waves are shear and transverse

Question 4

Describe (generally) Surface Waves

Answer 4

• Travel only through the Earth’s crust
• Lower frequency than body waves- easily distinguished on a seismogram
• Arrive after body waves
• Are almost entirely responsible for damage associated with earthquakes
• Damage and strength of surface waves reduced in deeper earthquakes

Question 5

Surface Waves- Love Waves

Answer 5

• Fastest surface waves
• Move the ground from side to side- entirely horizontal motion
• Confined to the surface of the crust

Question 6

Surface Waves- Rayleigh Waves

Answer 6

• Roll along ground and move ground up & down, and side to side in the same direction as wave propagation
• Most shaking felt from an earthquake is due to the Rayleigh wave

Question 7

Measuring Earthquakes

Answer 7

Seismograph units of measurement = the modified Richter Scale

Measures Energy.

Richter scale has no upper limit- so far, earthquakes have not exceeded 9.5 on the Richter scale.

Scale is logarithmic, log base 10 (powers of 10)… magnitude 7.0 created 10 times the ground motion of a magnitude 6.0 earthquake; the total energy release is about 30x greater.

Question 8

Modified Mercalli; (MM)

Answer 8

• Rates the amount of intensity, shaking and damage caused.
• Measures damage
• Uses Roman numerals: 1=a barely detectable tremor, and XII= total damage