Whats an Earthquake

1
Whats an Earthquake?

• Its the vibration of Earth produced by a rapid
  release of energy.
• Most often caused by slippage along a fault.
• The energy radiates from its source called the
  focus.
• The location directly above the focus on the
  surface is called the epicenter.

2
The Economics and Societal Impacts of EQs
Damage in Oakland, CA, 1989

• Building collapse
• Fire
• Tsunami
• Ground failure

3
Where Do Earthquakes Occur and How Often?

• 80 of all earthquakes occur in the
  circum-Pacific belt
• most of these result from convergent margin
  activity
• 15 occur in the Mediterranean-Asiatic belt
• remaining 5 occur in the interiors of plates and
  on spreading ridge centers
• more than 150,000 quakes strong enough to be felt
  are recorded each year

4
Seismic Waves

• There are three basic types of seismic waves
• P- waves or Primary waves
• S- waves or Secondary waves
• L- waves or Surface waves

5
Lets see what they look like.

• Animations of different seismic waves
• http//www.edumedia.fr/a89_l2-the-types-seismic-wa
  ves.html 

6
Primary waves

• Cause rocks to move back forth in the same
  direction or compress stretch.
• Change the volume of intervening material.
• Travel the fastest of the seismic waves
• and travel through solids and liquids.

7
Secondary waves

• Cause rocks to move at right angles to the
  direction of the wave or in a side to side,
  shearing motion.
• Temporarily change the shape of material that
  transmits them
• and they travel through solids only.

8
Surface waves

• Cause rocks to move in a elliptical, up-down,
  side to side motion.
• Travel slowest of the seismic waves.
• Cause damage to structures during an earthquake.

9
What happens when seismic waves
collide?Constructive Interference

• When waves have a displacement in the same
  direction they meet and the resultant
  displacement is increased or amplified.
• (The green wave is the resultant wave)

10
Destructive Interference

• When waves have a displacement in opposite
  directions they meet and the resultant
  displacement is cancelled out or destroyed.
• (The green wave is the resultant wave)

11
Animation of Wave Interference

• http//www2.biglobe.ne.jp/7Enorimari/science/Java
  Ed/e-wave2.html
• http//www2.biglobe.ne.jp/7Enorimari/science/Java
  Ed/e-wave3.html

12

• The Focus and Epicenter of an Earthquake

• The point within Earth where faulting begins is
  the focus, or hypocenter
• The point directly above the focus on the surface
  is the epicenter

13

• Seismographs record earthquake events

At convergent boundaries, focal depth increases
along a dipping seismic zone called a Benioff zone
14
How is an Earthquakes Epicenter Located?

• Seismic wave behavior
• P waves arrive first, then S waves, then L and R
• Average speeds for all these waves is known
• After an earthquake, the difference in arrival
  times at a seismograph station can be used to
  calculate the distance from the seismograph to
  the epicenter.

15
How is an Earthquakes Epicenter Located?

• Time-distance graph showing the average travel
  times for P- and S-waves. The farther away a
  seismograph is from the focus of an earthquake,
  the longer the interval between the arrivals of
  the P- and S- waves

16
How is an Earthquakes Epicenter Located?

• Three seismograph stations are needed to locate
  the epicenter of an earthquake
• A circle where the radius equals the distance to
  the epicenter is drawn
• The intersection of the circles locates the
  epicenter

17
How are the Size and Strength of an Earthquake
Measured?First, we need to know the difference
between Earthquake Intensity and Earthquake
Magnitude

• Intensity
• subjective measure of the kind of damage done and
  peoples reactions to it
• isoseismal lines identify areas of equal
  intensity
• depends on distance to epicenter, building
  materials and design, type of ground
• Modified Mercalli Scale - ranges from I to XII

• Modified Mercalli Intensity Map
• 1994 Northridge, CA earthquake, magnitude 6.7

18
How are the Size and Strength of an Earthquake
Measured?

• Magnitude
• Richter scale measures total amount of energy
  released by an earthquake independent of
  intensity
• Amplitude of the largest wave produced by an
  event is corrected for distance and assigned a
  value on an open-ended logarithmic scale
• Each interval represents a tenfold difference and
  involves an energy release about 32 times as
  great

19
Japanese Shindo Scale

• more commonly used in Japan than the Richter
  scale - Shindo refers to the intensity of an
  earthquake at a given location, i.e. what people
  actually feel at a given location varies from
  place to place
• ranges from shindo one, a slight earthquake felt
  only by people who are not moving, to shindo
  seven, a severe earthquake. Shindo two to four
  are still minor earthquakes that do not cause
  damage, while objects start to fall at shindo
  five, and heavier damage occurs at shindo six and
  seven.

20
Shindo Scale in more detail

• 7 In most buildings, wall tiles and windowpanes
  are damaged and fall. In some cases, reinforced
  concrete-block walls collapse.
• 6 In many buildings, wall tiles and
  windowpanes are damaged and fall. Most
  unreinforced concrete-block walls collapse.
• 6- In some buildings, wall tiles and
  windowpanes are damaged and fall. 5 In many
  cases, unreinforced concrete-block walls collapse
  and tombstones overturn. Many automobiles stop
  due to difficulty in driving. Occasionally,
  poorly installed vending machines fall.
• 5- Most people try to escape from danger, some
  finding it difficult to move.
• 4 Many people are frightened. Some people try
  to escape from danger. Most sleeping people
  awake. 3 Felt by most people in the building.
  Some people are frightened. 2 Felt by many
  people in the building. Some sleeping people
  awake.
• 1 Felt by only some people in the building. 0
  Imperceptible to people.

21
Using P- and S- waves to reveal the Earths
Internal Structure
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)