CHAPTER 7
7.1 Earthquakes occur
along faults
Difference between Earthquake&Stress
 Earthquake:
Shaking of the ground caused by :
- sudden movement of large rocks along a fault.
- sudden release of stress in
the lithosphere.
 Stress:
is the force exerted
to press, pull , or push an object against the other.
-As stress increases, the rocks break free.
Most
earthquakes occur along plate
boundaries.
The strength of an earthquake depends on:
How much stress applied,
before the rock moves.
How long distance the
rocks move along the
fault.
Why earthquakes don’t occur in the
asthenosphere?
 The
rock is hot enough to bend and flow in the
asthenosphere rather than breaking down.
 Faults:
a fracture, or break in Earth’s lithosphere,
where blocks of rock move past each other.
 Blocks
of rocks move in different directions,
depending on the kinds of stress they are under.
Faults are classified according to the
way the rocks on one side move with
respect to the other rocks on the other
side:
1- Normal faults.
2- Reverse faults.
3- Strike-slip faults.
Normal faults
 The
block of one rock moves down from the other
block.
 Stress that pulls rocks apart
causes normal faults.
 It’s common near divergent
boundaries where plates are
moving apart.
Like the Rift Valley of Africa.
Reverse faults
A
block of rock on one side
moves up relative to the
other block.
 Stress that presses rocks
together causes reverse
faults.
 Occur near convergent
boundaries (collision).
 Like Himalaya Mountains.
Strike –Slip Faults
 Stress
push blocks of rocks to move
sideways horizontally on either side
of the fault plane .
 This
causes earthquakes along
strike-slip faults.
 It
occur where plates scrape past
each other (Transform boundaries).

The San Andreas Fault is a strikeslip fault.