Faults

Daily Dose of Destruction

The Earth's crust is constantly under stress.  And you thought you had it bad.  It is being squeezed, stretched, twisted, folded, lifted, and dropped 24 hours a day for millions of years.  Eventually the stress reaches its max and the Earth must give way so that the stress is released.  When this happens, rock breaks and faults are created.  Faults are cracks in rock where rock has moved on either side of that crack.  Movement on either side of fault move in opposite directions.  This fault movement causes vibrations in the crust.  Those vibrations are called earthquakes.

Earthquake fault

There are three main types of faults: normal, reverse, and strike-slip (transverse).  Normal faults happen when an area is being pulled apart by tensional forces.  As the crust is stretched part of the rock slides down the face on the other side of the fault.  Each side of the fault has a name: Foot Wall and Hanging Wall. These can be differentiated by asking "Which side of the fault is leaning?"  The leaning side is called the hanging wall, therefore the other side is called the foot wall.  The image below shows this.

Types of faults

hanging and footwall

The second type of fault is called a reverse fault (thrust).  These faults are caused when the Earth is being squeezed by compressional forces.  The rock is being pushed in opposite directions.  The hanging wall will be pushed up over the foot wall.

reverse fault

The third type of faults are known as strike-slip or transverse faults.  This happens when rocks slide past each other, parallel but in opposite directions.  This happens due to a force known as shear.  Think of shear forces as scissors or shears (that shear sheep).  They work because the two sides slide past each other.  There isn't much movement up or down.  You also wouldn't find hanging or foot walls in this case.  The San Andreas fault is an example of a strike-slip.

transverse fault

The power behind an earthquake is how much stress is built up before it is released.  The elastic limit is the limit to how far rocks can bend before breaking.  Imaging pulling a rubber band in opposite directions.  With enough force you can break the rubber band, but watch out because this can usually cause pain caused by the rebound of the rubber.  Earthquakes are similar.  They are stretched or squeezed until so much stress builds up that the rock will break and then snap back into place.  This is called the elastic rebound.

Elastic Rebound Video.jpg

In this next activity you are going to use Google drawing to demonstrate your understanding of the different types of earthquakes faults and hanging and foot walls.

SUSD5 Student version of the Google Drawing Earthquake Faults assignment

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