Earth Science

Earthquakes

Merrill Chapter 14 Notes

14-1 Earthquakes and Plate Tectonics

 

Causes of Earthquakes:

  1. Rocks are squeezed by tremendous pressure = stress.
  2. Under this stress they can reach a limit where they will break.
  3. They break and move along surfaces called faults.

 

Forces on Rocks: 3 types: Compression, Tension, and Shear. These forces can cause rocks to move in several directions along a fault. These movements can create different landform types.

 

Types of Faults:

Normal Faults: Tension can pull rocks apart and create a normal fault.

            Ex. Sierra Nevada Mtns. Fault Block Mountains

 

Reverse Faults: the rocks above the fault surface are forced up and over (compression) the rocks below the fault surface.

            Ex.  Himalayan Mountains

 

Strike-slip Fault: the rocks on either side of the fault surface are moving past each other without much upward or downward movement. This is caused by a shearing force.

            Ex. San Andreas Fault

 

Most earthquakes, both large and small, are caused by plates moving over, under, and around each other.

 

14-2 Earthquake Information:

 

Seismic Waves: are generated by earthquakes and move like sound (mechanical) waves.

 

Focus: an area in the earth’s interior where energy that is released from an earthquake. (point of origin). The energy travels out from the focus in the form of waves.

 

Types of Seismic Waves:

  1. Primary Waves: as they travel through rock they cause the particles in the rock to move back and forth. The rock particles compress and stretch apart thus transmitting these wave through the rock.
  2. Secondary Waves: move through the earth by causing particles in rocks to move at right angles to the direction of wave. Very destructive.
  3. Surface Waves: Energy that reaches the earth generates waves that travel outward from the epicenter (the area on the surface that is directly above the focus)

 

Wave differences:

  1. Travel at different speeds.
  2. Primary are the fastest, surface waves are the slowest.

 

Mapping the Earth’s Interior:

MOHO Discontinuity: boundary between the crust and the upper mantle. Discovered by measuring the movement of seismic waves through the earth.

  1. Primary and surface waves slow down when they hit the plasticlike asthenosphere- speed up when they are in the solid earth again.
  2. Secondary waves- are not transmitted through liquid.
  3. Primary waves are slowed down or deflected by the liquid outer.

 

By studying the pattern of transmission of seismic waves, scientists have mapped the different layers of the earth’s interior. See Figure 14-11.

 

14-3 Destruction by Earthquakes

Measuring Earthquakes:

  1. Seismologists use an instrument called a seismograph to record the seismic waves.
  2. Height of the lines traced on the paper in a seismograph, represents the magnitude of the earthquake.
  3. The higher the magnitude the more destructive the earthquake.

 

Tsunamis: large sea waves produce by earthquakes. See figure 14-13.

 

Earthquake Safety:

 

14-4

Earthquake Preparation: