BACKGROUND:
Earthquakes are caused when stress within the crust of
the Earth builds up, causing an area of rock to "snap" along a
fault. This breaking causes a release of energy. This energy is measured by
how much "shaking" occurs. The shaking can vary in its intensity.
Some earthquakes shake everything (high intensity), and others are not felt
at all (low intensity). In these labs, we introduce two ways to measure this
shaking: the Richter scale and the Modified Mercalli scale. The Richter
scale is a mathematical measurement of the intensity of the ground shaking,
as measured on a seismograph. It is actually a measurement of the size of
the waves produced by the earthquake. The Modified Mercalli scale measures
how people feel and react to the shaking.
Seismic waves radiate outward from the focus
(point at which the earthquake starts). These waves cause damage in some
areas and but not in others. Many factors effect damage. One of the most
fundamental is distance: seismic energy is lost as waves travel through the
Earth, so the further you are from the epicenter of an earthquake, the less
shaking you will likely feel. Another important factor is the type of ground
through which the waves travel. For instance, if the waves shake sand
particles, the energy will tend to make the particles "settle."
This may cause the Earth’s surface to sink; large movements can occur,
damaging human structures. In contrast, if the wave past through hard, solid
rock, no settling occurs, and the movement will be less. Basically, the more
tightly bound a material is, the less it will settle in response to seismic
waves.
