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Earth Science
Grade 7-11
Earthquake Magnitude & Scales Cheat Sheet
A printable reference covering earthquake magnitude, intensity, logarithmic scales, seismic waves, and the moment magnitude scale for grades 7-11.
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Earthquake magnitude and scales help students compare the size, energy, and effects of earthquakes. This cheat sheet explains why one earthquake can be much stronger than another even when the numbers look close. It is useful for reading earthquake reports, interpreting maps, and understanding hazards. Students need these ideas because earthquake data often uses logarithmic scales that can be misleading at first.
Key Facts
- Earthquake magnitude measures the size of an earthquake based on seismic wave data or fault movement.
- The moment magnitude scale, written Mw, is the main scale used by scientists for large modern earthquakes.
- Each increase of 1.0 in magnitude means the recorded wave amplitude is 10 times larger.
- Each increase of 1.0 in magnitude means about 32 times more energy is released.
- Energy comparison can be estimated with energy factor = 32^(magnitude difference).
- A magnitude 7 earthquake releases about 32 times more energy than a magnitude 6 earthquake.
- Intensity measures the shaking and damage at a specific location, so it can vary from place to place for the same earthquake.
- The Modified Mercalli Intensity scale uses Roman numerals I to XII to describe observed shaking and damage.
Vocabulary
- Magnitude
- Magnitude is a number that describes the overall size of an earthquake using seismic measurements.
- Moment Magnitude Scale
- The moment magnitude scale is a modern earthquake scale based on fault area, slip distance, and rock strength.
- Richter Scale
- The Richter scale is an older local magnitude scale based on seismic wave amplitude recorded by seismographs.
- Intensity
- Intensity describes how strongly an earthquake is felt and how much damage occurs at a particular location.
- Seismograph
- A seismograph is an instrument that detects and records ground motion from seismic waves.
- Epicenter
- The epicenter is the point on Earth's surface directly above where an earthquake starts underground.
Common Mistakes to Avoid
- Treating magnitude as a regular counting scale is wrong because magnitude is logarithmic, so a magnitude 7 earthquake is not just one unit stronger than magnitude 6.
- Confusing magnitude with intensity is wrong because magnitude describes the earthquake's overall size, while intensity describes local shaking and damage.
- Saying the Richter scale is always the best scale is wrong because scientists usually use moment magnitude for larger and modern earthquakes.
- Assuming the same earthquake has the same intensity everywhere is wrong because distance, rock type, soil, building design, and depth affect shaking.
- Ignoring units and scale labels is wrong because Mw, Richter magnitude, and Modified Mercalli intensity describe related but different earthquake information.
Practice Questions
- 1 An earthquake has magnitude 6.0 and another has magnitude 8.0. How many times larger is the wave amplitude of the magnitude 8.0 earthquake?
- 2 A magnitude 7.0 earthquake releases about how many times more energy than a magnitude 5.0 earthquake? Use energy factor = 32^(magnitude difference).
- 3 Two earthquakes differ by 0.5 magnitude. About how many times larger is the wave amplitude of the larger earthquake? Use amplitude factor = 10^(magnitude difference).
- 4 Why can two towns experience different earthquake intensity values during the same earthquake?