Transcript Unit 3: Earthquake Waves Introduction

EARTHQUAKES
What is an Earthquake?
• Sudden movement in the Earth’s crust that
releases energy (vibrations, waves)
• Movement occurs along areas of
weakness in the earth’s crust
Place on earth’s
surface directly
above the focus
Location where the
actual slippage or
displacement occurs
Crack in the
crust along
which movement
occurs
How is energy transmitted?
• Energy is transmitted through seismic
(earthquake) waves or vibrations
• Types of earthquake waves:
• P- waves: Primary waves
–Travel fastest, arrive first
• S- waves: Secondary waves
–Travel more slowly than p-waves,
arrive after P-wave
Types of Waves
• Longitudinal:
– Particles move
parallel to the
direction wave
travels
Primary waves (p-waves)
•
•
•
•
Longitudinal wave
Make the ground vibrate back and forth
Arrive at seismic recording stations first
Can travel through solids, liquids and
gases (Solids and Fluids)
• Transverse waves
– Particles move perpendicular to direction of
wave motion
Secondary waves (s-waves)
• Transverse wave
• Travel about ½ as fast as p-waves
• Make the ground vibrate from side to side,
up and down
• Can only travel through solids
Velocities:
• in the same medium, P waves travel at a
greater velocity (speed) than S waves.
However, the velocities of seismic waves
depend upon the physical properties of the
material through which the waves travel
(medium). Higher density media result in a
higher velocity of wave propagation.
Fill in the chart and graph below:
Wave
P
S
Type
Compressional
Or
Longitudinal
Shear
Or
Transverse
Faster
Slower
Solids AND Fluids
Solids ONLY
Relative
Velocity
It go
Like dis
Transmitted
through
Types of Seismic Waves
• Body waves: travel through the earth
– P-waves
– S-waves
• Surface waves:
– Form when p and s waves reach the surface
– Move slowly
– Cause the most damage
How are earthquakes detected?
Seismograph:
• an instrument attached to the Earth
designed to detect crustal shifts
Seismogram:
• recording of the earthquake waves by the
Seismograph machine
seismograph
Seismogram
How is the strength of an
earthquake measured?
• Mercalli scale (un-scientific):
– Based upon reports from people in affected area
including damage and what was felt
– Also known as Mercalli Intensity
• Richter Magnitude Scale:
– Measured using the wave size on seismogram
– Magnitude scale (ex. An earthquake of 5 has 10
times as much energy released as a 4)
Indian Ocean December 26 , 2004
9.0