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 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. 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

12. 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

14. Seismic Waves Ctd.
There are three different types of seismic waves: Primary waves, Secondary waves, and surface waves

15. 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 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

23. 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

24. 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.