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Earthquakes
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  • 1. Earthquakes
  • 2. An Earthquake is…
    • the shaking and trembling that results from the movement of rock beneath Earth's surface
    • The movement of Earth's plates produces strong forces that squeeze or pull the rock in the crust
    • This is an example of stress, a force that acts on rock to change its volume or shape
  • 3. 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 it’s 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. 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
    • The focus triggers an earthquake
    • Focus : the point beneath Earth's surface where rock that is under stress breaks
  • 12. 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
  • 13. Seismic Waves Ctd.
    • There are three different types of seismic waves: P waves, S waves, and surface waves
    • An earthquake sends out two of those waves, P and S waves
    • When they reach the top of the epicenter, surface waves form
  • 14. 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
  • 15. 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
  • 16. 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
  • 17. 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
  • 18. 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
  • 19. 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
  • 20. 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
  • 21. 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 1930’s
    • 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
  • 22. The Moment Magnitude Scale
    • Geologists use this scale today
    • It’s 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
  • 23. 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.
  • 24. 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.
  • 25. 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.