Earthquakes

1. Tension

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   • Tension stretches or pulls on the crust. As an result the rock in the middle becomes thinner.
2. Compression

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   • Compression squeezes or pushes rocks together until they fold or break.
3. Shearing

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   • Shearing is stress that pushes rock in two opposite directions.

1. Normal Faults

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   • A normal fault is cause by tension. Its structure includes a footwall and a hanging wall.
2. Reverse Faults

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   • Reverse faults are caused by compression. Its structure is the same as a normal fault except the reverse fault's blocks move in opposite directions.
3. Strike-Slip Faults

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   • Strike-slip faults are caused by a type of stress called shearing.

1. Folding Earth's Crust

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   • Compression causes Earth's crust to either fold upward, an anticline, or fold downward, a syncline.
2. Stretching Earth's Crust

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   • The type of stress that causes this is called tension.
3. Uplifting Earth's Crust

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   • The forces that uplift mountains can also uplift plateaus, which are elevated high, flat, and large land.

1. P Waves

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   • P waves are a type of seismic wave that compress and expand land and can travel through both liquids and solids.
2. S Waves

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   • S waves are seismic waves that vibrate, and can travel through only solids.
3. Surface Waves

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   • Sometimes, when P waves or S waves reach the surface, the waves become surface waves.

1. The Mercalli Scale

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   • The Mercalli scale measures earthquakes based on their intensity on a scale from 1, the least intensity, and 12, the most.
2. The Richter Scale

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   • The Richter scale rates earthquakes by giving them a magnitude number based on the size of seismic waves.
3. The Moment Magnitude Scale

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   • The moment magnitude scale measures how much energy is released during an earthquake.
4. Comparing Earthquakes

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   • Geologists compare earthquakes using the earthquake's magnitude. An earthquake's magnitude tells how much energy was released during an earthquake.

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• Geologists find the earthquakes epicenter by measuring the difference between the arrival of the P waves and S  waves. The greater the time interval between the two waves, the further away the epicenter.

1. Measuring Seismic Waves

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   • You measure seismic waves with a seismograph. The pen on the seismograph is heavy so it resists the vibration of an earthquake. The rest of the seismograph vibrates along with the ground when the seismic waves reach it.
2. Reading a Seismograph

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   • The jagged lines that a seismograph makes is called seismogram. The higher the lines are, the closer or stronger the earthquake is.

1. Tiltmeters

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   • Tiltmeters measures vertical fault movement by using a carpenter's level.
2. Creep Meters

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   • A creep meter measure horizontal fault movement using a wire and a weight.
3. Laser-Ranging Devices

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   • A laser-ranging device uses a laser to detect horizontal fault movement by timing how fast the laser beam takes to travel to a reflector and back.
4. GPS Satellites

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   • Ground-based receivers use the GPS satellite system to identify changes in land along faults. 

1. Mapping Changes Along Faults

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   • Scientists use seismic waves to detect a hidden fault.
2. Monitoring Changes Along Faults

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   • Faults that lock together will build up a lot of stress.
3. Trying to Predict Earthquakes

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   • It is hard for geologists to predict earthquakes because many unexpected things may happen such as an earthquake that stored up a lot of energy but not even happening.

1. Plate Boundaries and Faults

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   • Two plates meet at a fault. For example, the Pacific plate and the North American plate meets along the San Andreas fault.
2. Mapping Earthquake Intensity

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   • Geologists use modified Mercalli scale data to map the intensity of an Earthquake.
3. Historic Earthquakes

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   • Geologists study data from past earthquakes to estimate the probability of future earthquakes.

1. Shaking

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   • Shaking can cause a lot of damage including landslides and avalanches. The most violent jolts make take place kilometers away from the epicenter.
2. Liquifaction

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   • Liquefaction turns loose and soft soil into liquid mud, causing buildings to sink or pull apart.
3. Aftershocks

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   • An aftershock is a earthquake that may happen hours, days, or even months after the big, major earthquake before.
4. Tsunamis

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   • When an earthquake jolts the ocean floor, the ocean floor rises slightly, causing water to flow out of its way. The water may become a large wave called a tsunami.

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• Please take safety steps when an earthquake strikes. When the earthquake happens, drop, cover, and hold when you are inside.

1. Protecting Structures

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   • A base-isolated building is designed to reduce the amount of energy that reaches the building during an earthquake.
2. Making Utilities Safer

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   • Technology can prevent earthquakes from causing fires and flooding.