Earthquakes Booklet

stress -a force that acts on rock to change its shape or volume.

3 Types of Stress

tension -pulls on crust, stretching rock so it becomes thinner in the middle

compression -squeezes rock together until it folds or breaks

shearing-pushes rock in twoopposite directions

Forces in Earth’s Crust

Type of Fault:◦ Reverse Fault

Stress Force:◦ Compression

Movement Along Fault:◦ The hanging wall is

moving up relative to the footwall

Fault I

Type of Fault:◦ Normal Fault

Stress Force:◦ Tension

Movement Along Fault:◦ The hanging wall is

moving down relative to the footwall

Fault II

Type of Fault:◦ Strike-Slip Fault

Stress Force:◦ Shearing

Movement Along Fault:◦ Opposing forces

cause the rock to break and move horizontally

Fault III

Movement Fault Type of Stress

Boundary

Hanging wall moves down relative to footwall

Normal Fault Tension Divergent Boundary

Hanging wall moves up relative to the footwall

Reverse Fault

Compression Convergent Boundary

Opposing forces cause the rock to break and move horizontally

Strike – Slip Fault

Shearing Transform Boundary

Deformation – the process by which the shape of a rock changes because of stress

Elastic Rebound – the sudden return of elastically deformed rock to its undeformed shape

What is the direct cause of earthquakes?◦ Elastic rebound

What Causes Earthquakes?

Seismology – the study of earthquakes

Seismic Waves – a wave of energy that travels through the Earth, away from an earthquake in all directions

  Seismograph – an instrument that records vibrations in the ground and

determines the location and strength of an earthquake  Seismogram – a tracing of earthquake motion that is created by a

seismograph   Seismic Gap – an area along a fault where relatively few earthquakes have

occurred recently but where strong earthquakes have occurred in the past 

And some other important terms… Epicenter – the point on Earth’s surface directly above an

earthquake’s starting point, or focus  Focus – the point along a fault at which the first motion of an

earthquake occurs

“SEISMO” ….means “to shake”

Body Waves P waves (pressure or primary waves)

arrive first, travel through liquid S Waves (secondary waves) arrive later,

can not travel through liquid

 

Earthquake Waves

P Waves

S WavesSurface Waves

Earthquake Waves

The S-P Time Method◦ Describe how a seismograph is used to determine

the epicenter of an earthquake.  p 231

Measuring Earthquakes

What does the Richter Scale measure?◦ The strength of earthquakes – measures the ground motion from an

earthquake and adjusts for distance to find its strength

  What is the magnitude of an earthquake?

◦ The strength

How are magnitude and ground motion related in the Richter scale?◦ Each time the magnitude increases by 1 unit, the amount of ground

motion increases by 10 times

  What does the Modified Mercalli Intensity Scale measure?

◦ The degree to which an earthquake is felt by people and the amount of damage caused b the earthquake

  Why might an earthquake have more than one intensity value?

◦ Because the effects of an earthquake vary from place to place, the intensity values are usually higher near the earthquakes epicenter

Strength & Intensity