Monitoring Earthquakes

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• Seismic waves cause the seismograph's drum to vibrate. But the suspended weight with the pen attached moves very litle. There fore, the pen stays in place and records the drum's vibration.

1. Measuring Seismic Waves
2. Reading a Seismogram

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• To monitor faults, geologists have developed instruments to measure changes in elevation, tilting of the land surface, and ground moments along faults. 

1. Tiltmeters

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   • Measures tilting or raising of the ground. Used by geologists consist of two bulbs that are filled with a liquid and connected by a hollow stem. Each bulb contains a measuring scale to measure the depth of the liquid in that bulb.
2. Creep Meters

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   • Creep meter uses a wire stretched across a fault to measure horizontal movement of the ground. Geologists determine how much the fault has moved by measuring how much the weight has moved against a scale.
3. Laser-Ranging Devices

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   • A laser-ranging device uses a laser beam to detect horizontal fault movements. 
4. GPS Satelltes

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   • Scientists can monitor changes in elevation as well as horizontal movement along faults using a network of Earth-orbiting satellites called GPS.

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• Seismographs and fault-monitoring devices provide data used to map faults and detect changes along faults. Geologists are also trying to use these data to develop a method of predicting earthquakes.

1. Mapping Faults

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   • When seismic waves hit a fault, the waves are reflected off the fault. Seismographs can detect these reflected seismic waves.
2. Monitoring Changes Along Faults

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   • How rocks move along a fault depends on how much friction there is between the sides of the fault.
   1. Friction

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      • The force that opposes the motion of one surface as it moves across another surface
3. Trying to Predict Earthquakes

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   • Even with data from many sources, geologists can't predict when and where a quake will strike. Stress along a fault increases until an earthquake occurs. Sometimes stored energy builds up along a fault, but an earthquake fails to occur.