EARTHQUAKES

Rocks move along faults…  A fault is a fracture or break in the Earth’s lithosphere where blocks of rock move past each other.  Along some parts of the fault, the rock on either side may slide along slowly and constantly.  Along other parts of the fault, the rocks may stick or lock together.  Rocks bend as stress is put on them.  As the stress increases, the rocks finally break free.

Stress in Lithosphere = Earthquake  A sudden release of stress in the lithosphere causes an earthquake.  An earthquake is a shaking of the ground caused by sudden movement of large blocks of rock along the fault.  Most faults are located along plate boundaries, so most earthquakes occur in these areas.

 About 80% of the earthquakes occur in the belt around the edges of the Pacific Ocean.

What is the best known fault in this belt in the United States?  San Andreas Fault in California.  It forms part of the boundary between the North American Plate and the Pacific Plate.  Unlike many other faults, parts of the San Andreas Fault can be seen on the surface of the ground.

Strength of an earthquake  The strength of an earthquake depends in part on…  How much stress builds up before the rocks move  The distance the rocks move along the fault

3 Types of Faults  Normal Faults  Reverse or Thrust Faults  Strike-Slip Faults  The blocks of rock along different types of faults move in different directions, depending on the kinds of stress they are under.  More than one type of fault may be present along the same plate boundary.

Normal Faults  Earthquakes along normal faults are common near boundaries where tectonic plates are moving apart. (Divergent Boundaries) Pull Direction rocks move Example: Great Rift Valley of Africa

The Great Rift Valley

Reverse or Thrust Faults  Stress that presses rock together causes reverse faults. (Convergent Boundaries)  The Himalaya Mountains, which rise in the area where the Indian Plate is pushing into the Eurasian Plate, have may earthquakes along reverse faults. Direction rocks move Push

Strike-Slip Faults  These faults can occur where plates scrape past each other. (Transform Boundaries)  The San Andreas Fault is a strike-slip fault.

Earthquakes Release Energy  Earthquake energy travels outward in all directions – up, down, and to the sides.  The energy travels as seismic waves, which are vibrations caused by earthquakes.  All earthquakes start beneath Earth’s surface.  The focus of an earthquake is the point underground where the rock first begins to move.  Seismic waves travel outward from the earthquakes focus.  The epicenter is the point on the Earth’s surface directly above the focus.

Earthquakes produce 3 types of seismic waves…  Primary Waves  Secondary Waves  Surface Waves  Each type moves through material differently.  Scientists learn about Earth’s layers by studying the paths and speeds of seismic waves traveling through Earth.

Primary Waves, P Waves  These are the fastest waves that are the first to reach any particular location after an earthquake occurs.  Primary waves can travel through solids, liquids, and gases.  Primary waves = P waves

Secondary Waves, S Waves  Secondary waves travel through Earth’s interior at the same time as primary waves, but at about half the speed.  Therefore, they are the second waves to arrive at any particular location after an earthquake.  Secondary waves can travel through rock, but unlike primary waves they cannot travel through liquids or gases.  Secondary waves = S waves

Surface Waves  Surface waves move along Earth’s surface, not through its interior.  They make the ground roll up and down or shake from side to side.  Surface waves cause the largest ground movements and the most damage.  They travel more slowly than the other types of seismic waves.

Seismic waves can be measured…  There are seismic stations all over the world where ground movements are measured.  A seismograph is an instrument that constantly records ground movement.  The recording produced by a seismograph is called a seismogram.  By studying seismograms, scientists can determine the locations and strengths of earthquakes.

Seismograph and Seismogram  The height of the wiggles on the seismogram indicates the amount of ground movement produced by seismic waves at the seismograph’s location.

Interesting Facts…  Most injuries and deaths due to earthquakes are not directly caused by the movement of the ground.  They are caused by collapsing buildings and other structures and by fires.  After an earthquake, fires may start due to broken natural-gas lines, broken electrical power lines, or overturned stoves.

How do we measure an earthquakes magnitude?  The Richter Scale : An earthquake’s magnitude is based on how fast the ground moves at the seismic station.  The Modified Mercalli Intensity Scale: The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction.

 es/S_portfolio/Mercalli/Mercalli_Scale.swf es/S_portfolio/Mercalli/Mercalli_Scale.swf

Review Questions…  What is a fault?  How does an earthquake occur in the lithosphere?  What are the 3 types of faults? How are they different?  What are seismic waves?  What are the 3 types of seismic waves? How are they the same and how are they different?  How are seismic waves measured?  How do we measure an earthquakes magnitude?