EarthquakeEarthquake is a movement of Earth's lithosphere that occurs when rock in the lithosphere suddenly shift, releasing stored energy. Energy is released during an earthquake by vibrations called seismic waves. When rocks are squeezed together, stretched or pulled them apart, pushed in different directions, stress is added. As tectonic plates move, they cause stress in the crust, which in turn produces faults and folds. Fault is a break in a mass of rock along which movement occurs. Fold is a bend in layers of rock/squeezed together but do not break. They tend to fold rather than break when they are under high temperature or pressure. Earthquakes occur because stress forces have exceeded the strength of rock.
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Focus• The location beneath Earth's surface where an earthquake begin
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Epicenter• The location on earth's surface directly above the focus
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Earthquakes produce three main types of seismic waves: P waves, S waves, and Surface waves
P Waves (Primary)• Longitudinal waves similar to sound waves
• Cause particles in the material to liberate in the direction of the waves’ motion=solids and liquids • Compress and expand the ground • Fastest seismic waves=first to be detected |
S Waves (Secondary)• Transverse waves
• Cause the particles in the particles in the materials to vibrate at right angles to the direction the waves move • Only solids |
Surface Waves• Waves that develop when seismic waves reach Earth’s surface
• Slower but produce larger ground movements and greater damage |
To measure earthquakes and pinpoint their epicenters, geologists record seismic waves using seismographs
Seismogram |
The record of an earthquake on a seismograph
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Richter Scale• The most well-known scale
• Rates based on measurements of the times and amplitudes of seismic waves |
Moment Magnitude Scale• The most useful scale for geologists
• A measure of the amount of energy released by an earthquake • 32-times increase in the energy release for each unit • Only solids • Only solids |
Modified Mercalli Scale• Ranges from 1 to 12. based on observations of the intensity of ground shaking and damage in the areas affected by and earthquake
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Most earthquakes are concentrated along plate boundaries, where many faults are found.