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Earthquakes
Chapter 6.1
Earthquakes & Plate Tectonics
1. Earthquakes are vibrations of the earth’s crust.
a. Earthquakes occur when rocks under stress suddenly shift
along a fault.
b. The area along a fault where motion first occurs is called
the focus of an earthquake.
i. When an earthquake occurs seismic waves radiate outward in all
directions from the focus.
Earthquakes & Plate Tectonics
c. The point on the earth’s surface directly above the
focus is called the epicenter.
d. A fault zone is an area where there are groups of
interconnected faults.
i. For example, the San Andreas Fault zone is made up of many
faults and any movement could cause an earthquake within
that zone.
Elastic Rebound Theory
2. The elastic rebound theory states that the rocks on
each side of a fault are moving slowly.
a. If the fault is locked, not moving, then the stress in the fault
increases, and when the stress in the rocks increases past a
certain point the rocks fracture and spring back to their
original shape, or rebound.
Elastic Rebound Theory
b. When the rocks fracture they release energy in the
form of vibrations called seismic waves.
i. This release of energy increases the stress of other rocks
along the fault and causes them to fracture and spring
back.
ii. This spring back action is the cause of smaller tremors
called aftershocks.
Earthquake Zones
3. Most earthquakes occur along or near the edges of the
earth’s lithospheric plates.
a.The stress is greatest and the rocks experience the most
amount of strain at these areas of the earth.
Earthquake Zones
b. The earth has three major earthquake zones.
i. Pacific Ring of Fire
ii.Mid-ocean ridges
iii.Eurasian-Melanesian mountain belt