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Earthquakes Part 1
• Earthquakes occur along faults
Sometimes when you pull on a drawer, it opens
smoothly. At other times, the drawer sticks shut. If you
pull the drawer hard enough, the drawer suddenly flies
open. This is how rocks along a fault act.
A fault is a break or fracture in earth’s lithosphere
(the crust and top of mantle), along which blocks of
rock slide past each other.
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Along faults, the rocks may slide past each other
smoothly, or they may stick. Stress builds up until the
rocks break free. A sudden release of stress in the
lithosphere causes an earthquake.
-An earthquake is a shaking of the ground caused by the
sudden movement of large blocks of rocks along a fault.
Where do earthquakes occur?
…Most earthquakes happen along tectonic plate boundaries.
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…80% of quakes happen in a belt around the edges of the Pacific
Plate. Because there are also many volcanoes around this belt it
is called the Ring of Fire.
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The best known fault on the Ring of Fire is the
San Andreas Fault in California. Some quakes are also
located on faults away from the plate boundaries and
near hot spots in the lithosphere
Faults are classified by how rocks move.
Rocks along faults move differently depending on what
kind of stress they are under.
1. Normal Faults: The block of rock above the fault slides
down relative to the other block.
-stress that pulls rocks apart causes this.
2. Reverse Faults: The block of rock above the fault moves
up relative to the other block.
-stress that presses rocks together causes this.
3. Strike-Slip Faults: Blocks of rocks move sideways on either
side of the fault.
- Horizontal stress causes this. The San Andreas Fault.
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Over time, movement of rocks along normal and reverse faults can push
up mountains and form deep valleys. Along strike-slip faults, rocks can be
seperated by hundreds of miles.
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Measuring Earthquakes
To measure the strength of an earthquake, scientist use
the Richter Scale. This measures the movement of ground
near the quake. A scale from 1 to 10.
Each number up means the quake is 10 times
stronger than the last number.
A 2 quake is 10 times stronger than a 1 quake.
A 3 quake would be 10 x 10 (100) times stronger.
A 6 quake would be 10 x 10 x 10 x 10 x 10 ( 100,000)
times stronger than a 1 quake.
• The Richter Scale
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• Plate Boundaries
Divergent
Convergent
Transform