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 produce 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. - Forces in Earth’s Crust
Types of Stress
Stress that pushes a mass of rock in two opposite directions is called shearing.

5. - Forces in Earth’s Crust
Types of Stress
The stress force called tension pulls on the crust, stretching rock so that it becomes thinner in the middle.

6. - Forces in Earth’s Crust
Types of Stress
The stress force called compression squeezes rock until it folds or breaks.

7. 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

8. Kinds of Faults
- Forces in Earth’s Crust
Tension in Earth’s crust pulls rock apart, causing normal faults.

9. Kinds of Faults
- Forces in Earth’s Crust
A reverse fault has the same structure as a normal fault, but the blocks move in the opposite direction.

10. Kinds of Faults
- Forces in Earth’s Crust
In a strike-slip fault, the rocks on either side of the fault slip past each other sideways, with little up and down motion.

11. 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

12. Mountains Formed by Folding
Folds are bends in rock that form when compression shortens and thickens part of Earth's crust
MOUNTAINS FORM
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

13. 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

14. How Earthquakes Form
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

15. 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

16. Seismic Waves There are three different types of seismic waves:
P waves,
Secondary or 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

17. 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
Travel through BOTH liquids and solids

18. Secondary Waves Also known as S Waves After P waves, come S waves
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

19. 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

20. 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

21. 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

22. Instruments used to detect movement
In trying to predict earthquakes, geologists have developed instruments to measure changes in elevation, tilting of the land surface, and ground movements along faults.

23. Measuring Earthquakes
There are at least 20 different types of measures
Main 3: 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
Mercalli – INTENSITY, based on effects

24. 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

25. 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

26. Locating the Epicenter
Since 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.

27. 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.

28. Locating the Epicenter
- Earthquakes and Seismic Waves
TRIANGULATION is used to detect the location of the epicenter

29. 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.

30. Earthquake Risk
- Earthquake Safety
Geologists can determine earthquake risk by locating where faults are active and where past earthquakes have occurred.

31. How Earthquakes Cause Damage
- Earthquake Safety
How Earthquakes Cause Damage
In addition to the DANGERS of ground shaking, earthquakes can cause tsunamis
A tsunami spreads out from an earthquake's epicenter and speeds across the ocean.

32. Designing Safer Buildings
- Earthquake Safety
To reduce earthquake damage, new buildings must be made stronger and more flexible.