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). Seismologists use several methods to measure the
intensity of an earthquake. This post lab focuses on two such scale.
First, 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 height (amplitude) of the waves produced by
the earthquake. The Richter Scale is an absolute scale; wherever an
earthquake is recorded, it will measure the same on the Richter Scale.
Second, the Modified Mercalli scales measures how people feel and react to
the shaking of an earthquake. It is a relative scale, because people
experience different amounts of shaking in different places. It is based on
a series of key responses such as people awakening, the movement of
furniture, and damage to structures. In general, the further one is from the
epicenter of an earthquake, the less shaking is experienced. When an
earthquake occurs, it is important for a student to have a way to interpret
the size of the event. Learning the Modified Mercalli scale can give
students this ability. It is sufficient to distinguish between small,
moderate, or large earthquakes. This knowledge can help a student determine
a course of action during and after the shaking.
PROCEDURE:
- Using the worksheet, review the Modified Mercalli and Richter Scales
with the students. Compare the two scales. If you live in an earthquake
prone area, you may wish to have your students recall earthquakes that
they have experienced. Ask them what Modified Mercalli scale rating they
would give it.
- Have the students complete the worksheet. They are asked to predict
the Modified Mercalli and Richter Scale magnitudes for a series of
earthquake descriptions.
ANSWERS:
1. IV - moderate: 4.3-4.8
2. X - disastrous; 7-7.3
3. II - feeble; 3.5-4.2
4. VI - very strong; 5.5-6.1
5. VIII - destructive; 6.2-6.9
Mercalli scale = measures intensity compared to human terms
Richter scale = strength of earthquake due to ground motion
