Earthquakes
Bill Menke
September 16, 2005
Summary
What is an earthquake
Why do earthquakes occur
How is size quantified
Where do earthquakes occur
How frequently do earthquakes occur
How do earthquakes cause damage
What is an earthquake
An earthquake is the shaking of the ground that is caused by sudden slip on a geological fault.
Why do Earthquakes Occur
Forces in the earth slowly build up to where they exceed the factors impeding fault motion, causing sudden slip on the fault.
Both friction and unbroken rocks are factors impeding fault motion.
Sudden slip causes earthquakes. Slow, steady slip (=creep), which can sometimes occur on faults, does not.
Analogy: breaking a twig
Step 1: Slowly bend twig. Energy, provided by your muscles, is stored in twig, because it is elastic.
Step 2: Keep bending (=straining). Eventually the force (=stress) is great enough to cause cracking (=faulting).
Step 3. The twig cracks. The stored elastic energy is used up by 1) tearing molecules apart (90%) and 2) making the cracking sound (vibrations, the earthquake)
Example: San Andreas fault
From the air, the fault really does look more-or-less line a line
Step 1
Step 2
Step 3
It really happens …
Forces are highest on Plate Boundaries
If one plate in moving in one direction …
And the other plate in moving in another direction …
Then the boundary between the two plates will be experiencing lots of force (=stress)
The Type of Plate Boundary Matters
Lithosphere – brittle part of earth – earthquakes happen here
Asthenosphere – viscous part of earth – no earthquakes
Convergent Plate Boundary
Convergent Boundary
Plates move towards one another
Oceanic Lithosphere: Subduction Zone (shown here)
Continents: Collisional mountain belt (e.g. Himalayas)
Extremely Large Earthquakes

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