The Earth Shakes

What is an earthquake?

You may know that during an earthquake the ground rumbles, household items like dishes rattle, and walls sometimes shake.  The earth is constantly creating earthquakes, but what causes them?

The Earth consists of a thin outer layer called the crust which is mostly cold and brittle rock.  Large and small cracks called faults are located all over the crust.  Because these faults are burried deep in the ground and compressed together very tightly, they cannot be seen.  Faults are compressed by powerful forces and stay together for many years.  Eventually, the forces pushing the crustal pieces together break apart and move.  This sudden shift in the crust shakes all of the rock around the fault causing vibrations, called seismic waves, which travel outward in all directions.  The vibration of the earth caused by suddent shifts in the crust is called an earthquake.

What type of stress is associated with which faults?

When rocks are compressed with enough force, they can fracture (which means break) or fold (which means bend).

Geologists use the term anticline to describe the formation created when rocks fold upward and the term syncline to describe the formation created when rocks fold downward.  (Remember: vowels go together - anticline upward; constanants go together - syncline downward)

The action of rocks breaking and moving is called faulting.  Altough there are many different kinds of faults, this site limits the discussion of faults to three types.

1. Normal faults occur when one block of rock has slipped down in relation to the other.  Normal faults are produced by tension, as the rocks are pulled apart.  The Sandia Mountains in New Mexico is an example of a normal fault.  (Remember: Normal Faults - down, tension).
2. Reverse faults (also called thrust faults) occur when one block apears to have moved up over the other.  Reverse faults are caused by compression, as the rocks are pushed together.  Mount Gould in Glacier National Park is an example of a reverse fault.  (Remember: Reverse Faults - up, compression).
3. Strike-Slip faults (also called lateral faults) occur when the rock blocks move horizontal to each other.  Strike-slip faults are caused by shearing, as rocks slide past each other.  The San Andreas fault in California is an example of a strike-slip fault.  (Remember: Strike-Slip, Slide past each other, Shearing force)

    Modern Seismograph   

Today earthquake waves are measured by geologists with a seismograph.  The workings of a typical seismograph is very simple.

A horizontal rod is attached to a support pole.  Hanging from the horizontal rod is a wire or spring attached to a heavy weight.  On the other end of the weight is an ink pen that is free to swing from side to side when the ground shakes.  Directly underneath the pen is a peice of paper rolled around a cylinder or drum.  If the ground does not move, the rod does not swing.  Therefore, the pen stays in place and a smooth line is drawn.  If the ground shakes, the rod swings and the pen draws a zigzag line called a seismogram on the paper.  As the paper on the cylinder or drum rotates, the vibrations are recorded.  Earthquakes of high intensity are cause by more shaking and are recorded by sharper zigzags.