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Earthquakes
Earthquakes are natural vibrations of the ground caused by
movement along gigantic fractures in Earth’s crust (or
sometimes by volcanic eruptions too). They can be
extremely destructive.
Fractures form when stress (the forces on the rocks)
exceeds the strength of the rocks.
There are three types of stress1. compression- squeeze rocks together
2. tension- pull rocks apart
3. shear- slide rocks past one another
Types of faults- (see page 497)
1. reverse faults- caused by compression stress
2. normal fault- caused by tension stress
3. strike-slip fault- caused by shear stress
Earthquake vibrations are called seismic waves. These
seismic waves can be felt by seismometers. These record
the waves as squiggly lines on a piece of paper (or
computer). The bigger the squiggles, the bigger the
earthquake.
Most earthquakes take place below ground. The exact
place where the earthquake starts is called the focus. Using
three seismometers, you can determine where the epicenter
of the earthquake was. The epicenter is where the focus
would be, if it were on Earth’s surface.
The amount of energy released during an earthquake is
known as its magnitude, which is measured by the Richter
scale. It is a logarithmic scale of seismic wave size, where
a 7 on the scale is 10 times as much as a 6, and 100 times
as much as a 5. Usually, as much as a 4 can be felt, where
a 6 might start creating damage. An 8 would cause huge
destruction.
Another way to measure earthquakes is by the damage it
causes using a Modified Mercalli Scale. See page 507.
Earthquakes cause many problems, such as building
failures (buildings collapse), collapsing of bridges and
overpasses, land and soil failure (meaning landslides),
tsunamis (huge waves that destroy coasts), and of course,
deaths by all of the above causes.
Look at the figure on page 510. This is where the most
earthquakes take place.
Some schools have earthquake drills, instead of tornado
drills.
If an earthquake happens, the best place to be is under a
desk or table, crouched down. If a table is not available,
stand in a doorway or near an inner wall. Inner walls are
usually the last to fall.