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Chapter 6 Earthquakes
Ch. 6.1 Earthquakes and Plate
Tectonics
• Earthquakes—vibrations of the earth’s
crust.
• Earthquakes normally occur along faults,
or fractures in the crust where the rock
moves.
• A fault is normally “locked,” or pressed
together tightly, until stress overcomes the
pressure holding it together, and the rocks
suddenly grind past each other.
• The vibrations that result are felt and
recorded as earthquakes.
• The vibrations are released as energy
known as seismic waves.
• The movement of rock may cause
pressure on other rock, which then breaks
and releases more energy (aftershocks).
• Focus—the point along a fault where
slippage of rock (the rock breaks) first
occurs. Usually underground.
• Epicenter—the point on the earth’s surface
directly above the focus.
• Seismic waves radiate outward in all
directions from the focus.
• 90% of earthquakes have a shallow focus,
within 70 km of earth’s surface.
• 70 km to 300 km…intermediate focus.
• 300 km to 650 km…deep focus, in
subduction zones.
• The most damaging earthquakes usually
have a shallow focus.
• stop
• Because they occur in subduction zones,
intermediate and deep focus earthquakes
usually occur further inland than shallow
focus ones.
Elastic Rebound Theory
• Stress on rocks will slightly bend rock, but
too much stress will cause the rock to
break, and it will spring back to its original
shape.
Major Earthquake Zones
• Most earthquakes occur at lithospheric
plate boundaries.
• The earth has 3 major earthquake zones.
• 1. The Pacific Ring of Fire.
• 2. Along mid-ocean ridges.
• 3. The Eurasian-Melanesian mountain
belt.
Major Earthquake Zones
Pacific Ring of Fire
• Fault zones—group of interconnected
faults at plate boundaries. Example: San
Andreas fault, which extends almost the
length of California.
• A major earthquake occurred in the central
United States in 1812, far from any known
plate boundary.
• Evidence reveals an ancient but now
inactive plate boundary in the central U.S.
• Instead of breaking apart, the North
American continent is being compressed
by the actively spreading Atlantic Ocean.