Earthquakes

1
Earthquakes

• Chapter 5

2
An Earthquake is

• The movement of Earth's plates produces strong
  forces that squeeze or pull the rock in the
  crust. We feel this as shaking and trembling.
• Stress is a force that acts on rock to change its
  volume or shape

3
3 Types of Stress

• There are three different types of stress that
  occur on the crust, shearing, tension, and
  compression
• These forces cause some rocks to become fragile
  and they snap
• Some other rocks tend to bend slowly like road
  tar softened by the suns heat

4
Faults

• A fault is a break in the crust where slabs of
  crust slip past each other. The rocks on both
  sides of a fault can move up or down or sideways
• When enough stress builds on a rock, the rock
  shatters, creating faults
• Faults usually occur along plate boundaries,
  where the forces of plate motion compress, pull,
  or shear the crust too much so the crust smashes

5
Strike-Slip Faults

• Shearing creates this fault
• In this fault, rocks on both sides of the fault
  slide past each other with a little up and down
  motion
• When a strike-slip fault forms the boundary
  between two plates, it becomes a transform
  boundary

6
Normal Faults

• Tension forces in Earth's crust causes these
  types of faults
• Normal faults are at an angle, so one piece of
  rock is above the fault, while the other is below
  the fault
• The above rock is called the hanging wall, and
  the one below is called the footwall
• When movement affects along a normal fault, the
  hanging wall slips downward
• Normal faults occur along the Rio Grande rift
  valley in New Mexico, where two pieces of Earth's
  crust are diverging

7
Reverse Faults

• Compression forces produce this fault
• This fault has the same setup as a normal fault,
  but reversed, which explains its name
• Just like the normal fault, one side of the
  reverse fault is at an angle of the other
• This fault produced part of the Appalachian
  Mountains in the eastern United States

8
How Do Mountains Form?

• The forces of plate movement can build up Earth's
  surface, so over millions of years, movement of
  faults can change a perfectly flat plain into a
  gigantic mountain range
• Sometimes, a normal fault uplifts a block of
  rock, so a fault-block mountain forms
• When a piece of rock between two normal faults
  slips down, a valley is created

9
Mountains Formed by Folding

• Sometimes, under current conditions, plate
  movement causes the crust to fold
• Folds are bends in rock that form when
  compression shortens and thickens part of Earth's
  crust
• The crashing of two plates can cause folding and
  compression of crust
• These plate collisions can produce earthquakes
  because rock folding can fracture and lead to
  faults

10
Anticlines and Synclines  

• Geologists use the terms syncline and anticline
  to describe downward and upward folds in rock
• An anticline is a fold in a rock that arcs upward
  
• A syncline is a fold in a rock that arcs downward
• These folds in rocks are found on many parts of
  the earths surface where compression forces have
  folded the crust

11
Plateaus

• The forces that elevate mountains can also raise
  plateaus, a large area of flat land elevated high
  above sea level
• Some form when a vertical fault pushes up a large
  flat piece of rock
• Like a lasagna, a plateau consists of many
  layers, so it is wider than it is tall

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How Earthquakes Form

• Everyday, about 8,000 earthquakes hit Earth, but
  most of them are too little to feel
• Earthquakes will always begin in a rock beneath
  the surface
• A lot of earthquakes begin in the lithosphere
  within 100 km of Earth's surface
• Focus the point beneath Earth's surface where
  rock that is under stress breaks
• The focus triggers an earthquake

13
Seismic Waves

• Seismic Waves vibrations that travel through
  Earth carrying the energy released during an
  earthquake
• an earthquake produces vibrations called waves
  that carry energy while they travel out through
  solid material
• During an earthquake, seismic waves go out in all
  directions to the focus
• They ripple like when you through a stone into a
  lake or pond

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Seismic Waves Ctd.

• There are three different types of seismic waves
  Primary waves, Secondary waves, and surface waves

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Primary Waves

• Also known as P Waves
• The first waves to come are these waves
• P waves are earthquake waves that compress and
  expand the ground like an accordion
• P waves cause buildings to expand and contract

16
Secondary Waves

• Also known as S Waves
• After p waves, S waves come
• S waves are earthquake waves that vibrate from
  one side to the other as well as down and up
• They shake the ground back and forth
• When S waves reach the surface, they shake
  buildings violently
• Unlike P waves, which travel through both liquids
  and solids, S waves cannot move through any
  liquids

17
Surface Waves

• When S waves and P waves reach the top, some of
  them are turned into surface waves
• Surface waves move slower than P waves and S
  waves, but they can produce violent ground
  movements
• Some of them make the ground roll like ocean
  waves
• Other surface waves move buildings from side to
  side

18
Detecting Seismic Waves

• Geologists use instruments called seismographs to
  measure the vibrations of seismic waves
• Seismographs records the ground movements caused
  by seismic waves as they move through the Earth

19
Mechanical Seismographs

• Until just recently, scientists have used a
  mechanical seismograph
• a mechanical seismograph consists of a heavy
  weight connected to a frame by a wire or spring
• When the drum is not moving, the pen draws a
  straight line on paper wrapped around the drum
• Seismic waves cause the drum to vibrate during an
  earthquake
• the pen stays in place and records the drum's
  vibrations
• The higher the jagged lines, the more severe
  earthquake

20
Measuring Earthquakes

• There are many things to know about the measures
  of an earthquake
• There are at least 20 different types of measures
• 3 of them are the Mercalli scale, Richter scale,
  and the Moment Magnitude scale
• Magnitude is a measurement of earthquake strength
  based on seismic waves and movement along faults

21
The Mercalli Scale

• Developed in the twentieth century to rate
  earthquakes according to their intensity
• The intensity of an earthquake is the strength of
  ground motion in a given place
• Is not a precise measurement
• But, the 12 steps explain the damage given to
  people, land surface, and buildings
• The same earthquake could have different Mercalli
  ratings because of the different amount of damage
  in different spots

• The Mercalli scale uses Roman numerals to rank
  earthquakes by how much damage they cause

22
The Richter Scale

• The Richter scale is a rating of the size of
  seismic waves as measured by a particular type of
  mechanical seismograph
• Developed in the 1930s
• All over the world, geologists used this for
  about 50 years
• Electric seismographs eventually replaced the
  mechanical ones used in this scale
• Provides accurate measurements for small, nearby
  earthquakes
• Does not work for big, far ones

23
The Moment Magnitude Scale

• Geologists use this scale today
• Its a rating system that estimates the total
  energy released by an earthquake
• Can be used for any kind of earthquakes, near or
  far
• Some news reports may mention the Richter scale,
  but the magnitude number they quote is almost
  always the moment magnitude for that earthquake

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Locating the Epicenter

• Sine the P waves travel faster than the S waves,
  scientists can use the difference in arrival
  times to see how far away the earthquake
  occurred.
• It does not tell the direction however.

25
Determining Direction

• One station can only learn how far away the quake
  occurred.
• They would draw a circle at that radius.
• If three stations combine their data, the quake
  occurred where the three circles overlap.

26
How Earthquakes Cause Damage

• The severe shaking provided by seismic waves can
  damage or destroy buildings and bridges, topple
  utility poles, and damage gas and water mains
• With their side to side, up and down movement, S
  waves can damage or destroy buildings, bridges,
  and fracture gas mains.