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Chapter 4 Earthquakes
Movement of the Earth’s crust results in a force called stress, which acts on
rock to change its shape or volume. This force can be stored within the
rock until it changes shape or breaks.
Three kinds of stress occur within rock: tension, compression and shearing.
Tension pulls on the crust so that it becomes thinner in the middle.
(illustration p. 119)
Compression squeezes rock until it folds or breaks. This can result in
metamorphic rock.
(illustration p.119)
Shearing occurs when forces cause rock to move in opposite directions
laterally. This can result in breakage along the weakest point of rock or it
can also release stress by bending.
(illustration p.119)
When rock breaks it forms a fault, which can then move in any number of
directions to release stress. There are three kinds of faults.
A normal fault occurs at an angle, so one section of rock is above another.
The lower rock is called the footwall. The upper rock is called a hanging
wall. In a normal fault, the footwall underneath moves up and the hanging
wall on top moves down. (illustration p. 120, 123)
A reverse fault is much like a normal fault, except the movement is in the
opposite direction. The hanging wall moves up and the footwall underneath
moves down. (illustration p. 120)
In a slip strike fault, the rock moves with little up or down motion. This is
found in a transform boundary. (illustration p. 120)
Compression forces can form anticlines and synclines and can eventually
result in a folded mountain range, like the Appalachians. Anticlines are the
high points of folded rock. Synclines are the low points in the folds.
(illustration p. 122)
Tension forces can form fault block mountains when the Earth’s crust is
uplifted. The Rocky Mountains are fault block mountains. (illustration p.
123)
An earthquake is caused when the Earth’s plates shift or move.
Earthquakes carry away the stored energy released in all directions like
ripples in a pool. The ripples are called seismic waves. The focus is the
point within the crust where the earthquake occurs, the epicenter is the
point directly above the focus on the surface. There are several types of
waves and ways to measure them.
P waves, or primary waves travel fastest through the crust and cause the
ground to compress and expand like an accordion. (illustration p. 127)
S waves, or secondary waves travel slower but can move side to side and
up and down. Unlike P waves, S waves cannot move through liquids.
(illustration p. 127)
Surface waves move more slowly than P or S waves, but can cause more
destruction. They can make the ground roll like ocean waves. Most
buildings are not constructed to withstand these forces. (illustration p. 127)
The differences between the times these waves arrive can help scientists
find the epicenter of where the earthquake occurred.
Earthquakes can be measured three ways: The Richter scale, which is
most common, the Mercalli scale, and the Moment Magnitude scale.
The Richter scale measures an earthquakes’ magnitude (power) based on
the size of its seismic waves. Each higher number increases the magnitude
by a factor of 32. So, an earthquake with a magnitude of 7.0 would be 32
times more powerful than an earthquake with a magnitude of 6.0.
The Mercalli scale measures earthquakes by the damage it does to
civilization. The higher the number in this 12 step scale, the greater the
damage.
The Moment Magnitude scale is used by geologists to measure total
energy released by an earthquake.
Seismographs are instruments used to record and measure seismic waves.
As the waves get farther away from the focus and the epicenter, their
strength diminishes.
Earthquakes can cause damage through shaking, liquefaction, aftershocks
and tsunamis. Liquefaction is when moist soil loses its strength to support
heavy structures and causes them to collapse.