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Definition: the shaking /trembling that results from the movement of rock beneath Earth's surface

Occur along fault lines.
Faults: a break in the crust where slabs of crust slip past each other.

Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the crust too much so the crust smashes

The focus triggers an earthquake
Focus: the point beneath Earth's surface where rock that is under stress breaks

The point at the surface of the earth directly above the focus is called the epicenter

Seismic Waves

Seismic Waves

• vibrations that travel through Earth carrying the energy released during an earthquake
• During an earthquake, seismic waves go out in all directions to the focus

Types of Waves

• Primary (P) Waves
• Secondary (S) Waves
• 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

Waves

• An earthquake sends out two of those waves, P and S waves When they reach the top of the epicenter, surface waves form
• Use a seismograph to measure

Primary (P) Waves

• The first and fastest waves to come
• compress and expand the ground like an accordion
• cause buildings to expand and contract
• travel through liquids and solids

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Secondary (S) Waves

• vibrate from one side to the other as well as down and up Unlike P waves, which travel through
• shake the ground back and forth
• When reach the surface, they shake buildings violently
• cannot move through any liquids

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Surface Waves

• When S waves and P waves reach the top, some of them are turned into surface waves
• Slower than other waves but can produce violent ground movement
• can move side to side and up and down

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What do we use them for?

• P and S waves were used to determine the composition of the Earth's Layers
• Lag time between p and S waves determine distance to the epicenter

Determining epicenters

• Data of the lag time from 3 seismograph stations to calculate distances to epicenter
• epicenter is the point that the three circles connect

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Measuring Seismic Waves

How long does a "typical" quake last?
A. 30 sec
B. 60 sec
C. 90 sec
D. 100 sec

Seismograph

• Measures vibrations
• Records ground movements
• larger the line, the stronger the quake

What can we measure?

• about 20 different types of measurement

Measure:

• Intensity: the effects of a quake on a particular location
• Magnitude: strength or energy released during an earthquake

3 most common: Mercalli, Richter, and Moment Magnitude scale

Modified Mercalli Scale

• estimate of intensity
• based on observable damage
• subjective - based on opinion
• not precise
• can change depending on structure of buildings

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Richter Scale

• measures amplitude
• uses seismograph
• scale from 1-10
• each number is 10 times the amplitude of the number below
• not accurate for large or far quakes

Moment Magnitude Scale

• measures total energy
• use it today
• accurate for large and far quakes

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