Earthquakes

1
Earthquakes

• Earthquakes are natural vibrations of the ground
  caused by movement along gigantic fractures in
  Earths crust, or sometimes, by volcanic
  eruptions.
• These fractures form when the stress on the rocks
  exceed the strength of the rocks.
• Stress force per unit area

2
Earthquakes

• Three types of stress that can act on Earths
  rocks
• Compression
• Stress that pushes a material together
• Tension
• Stress that pulls a material apart
• Shear
• Stress that causes a material to twist

3
Earthquakes

• Faults
• When stress between to rocks becomes too great or
  is applied too quickly, a fracture is formed
• This fracture or series of fractures, along which
  movement occurs, is called a fault.
• Fault plane
• The surface along which the movement takes place

4
Earthquakes

• Types of faults
• Reverse faults
• Fractures that form as a result of horizontal
  compression
• Normal faults
• Fractures that form as a result of horizontal
  tension
• Strike-slip faults
• Fractures caused by horizontal shear

5
Earthquakes

• Seismic waves
• The vibrations of the ground during an earthquake
• Types of seismic waves
• Primary waves
• Also called P-waves
• Squeeze and pull rocks in the same direction as
  the wave is travelling
• Travel through Earths interior

6
Earthquakes

• Secondary waves
• Also called S-waves
• Cause rocks to move at right angles to the
  direction of the wave
• Travel through Earths interior
• Surface waves
• Move in two directions as they pass through rock.
• Both up-and-down and side-to-side movement occurs
• Travel along Earths surface

7
Earthquakes

• Focus
• The origination point of an earthquake
• The point where the rocks suffer failure
• Epicenter
• Point on the Earths surface directly above the
  focus.

8
Earthquakes

• Seismology the study of earthquake waves
• Seismometer sensitive instrument that detects
  and records seismic waves
• Seismogram record produced by a seismometer

9
Earthquakes

• Travel-time curves
• Over many years, the arrival times of seismic
  waves from countless earthquakes at seismic
  facilities all over the globe have been
  collected.
• Using this data, seismologists have been able to
  construct global travel-time curves for the
  initial P-waves and S-waves of an earthquake.

10
Earthquakes

• These general curves have provided the average
  travel times of all seismic waves for different
  distances, no matter where on Earth an earthquake
  occurs.
• P-waves always arrive first at a seismic facility
• With increasing travel distance, the separation
  between P and S-waves increases.
• Waves recorded on seismograms from more distant
  facilities are farther apart

11
Earthquakes

• Measuring and locating earthquakes
• Magnitude
• The amount of energy released by an earthquake
• Richter Scale
• Devised by seismologist Charles Richter
• Based on the size of the largest seismic waves
  generated by an earthquake
• Each successive number represents an increase in
  wave amplitude of a factor of 10

12
Earthquakes

• Energy vs. Size
• Each increase in magnitude (i.e. 8 to 9 on
  Richter Scale) corresponds to about a 32-fold
  increase in seismic energy
• Example a magnitude 8 earthquake releases about
  32 times more energy than a magnitude 7
  earthquake and over 1000 times more energy than a
  magnitude 6.
• Moment magnitude scale
• Similar to Richter, but takes into account the
  size of the fault rupture, amount of movement
  along the fault, and the rocks stiffness.

13
Earthquakes

• Modified Mercalli scale
• A measurement of the amount of damage done by an
  earthquake to the structures involved.
• Uses Roman numerals to designate the degree of
  intensity