Earthquakes and Seismic Waves
An earthquake is the shaking that results from movement of rock beneath Earth’s surface.Key Idea: Earthquakes are caused by the forces of plate movements.
- Plate movement causes stress, which adds energy to rock and forms faults.
- Stress and energy increase until the rock slips or breaks, causing an earthquake.
Key Idea:
Earthquakes release an enormous amount of stored energy. Some energy travels in seismic waves.
Seismic waves are vibrations that are similar to sound waves. They travel through Earth carrying the energy released from an earthquake.
P Waves- Primary Waves / S Waves- Secondary Waves / Surface Waves
- First to arrive
- Compress and expand the ground like an accordion
- Can travel through solids and liquids
- Vibrate side to side or up and down
- Cannot move through liquids
- Can cause violent shaking
- Occur when P or S waves hit the surface
- Produce severe ground movement
- Can make the ground roll like the ocean
Focus / the area beneath Earth’s surface where rock under stress begins to break or move /
Epicenter / the point on Earth’s surface directly above the focus
Key Idea: Geologists use seismic waves to locate an earthquake’s epicenter.
- Use the arrival time of seismic waves to determine the distance from the epicenter at at least 3 seismographs
- Draw a circle around each seismograph location to show where the epicenter could be.
- The epicenter is located where the 3 or more circles intersect.
Key Idea: Earthquakes are measured by people’s observations and by using a seismograph.
Modified Mercalli Scale
- Based on people’s observations
- Rates amount of shaking and damage
- Based on earthquake’s size, or magnitude
- Measures the size of earthquake’s waves using a seismograph
- Rates total energy released by an earthquake
- Most commonly used
- Uses seismograph and other sources