B48a
EARTHQUAKES
Shaking Crust
EARTHQUAKES
• vibrations produced by breaking of
lithosphere (crust and upper mantle)
• elastic limit = amount that rocks can
stretch and bend
–When surpassed --> rocks break
and move
–seismic waves (vibrations) sent out
in all directions
FAULT
• surface along which rocks move
(B7, Figure 19)
FOCUS V. EPICENTER
• FOCUS - point • EPICENTER -
below the surface point on surface
where rocks directly above the
actually break focus
(B7, Figure 20)
SEISMIC WAVES
• Body Waves
–primary
–secondary
–travel through
the earth
• Surface Waves
(B7, Figure 21)
–travel on the earth
PRIMARY SEISMIC
• P wave WAVES
• move out from focus
• longitudinal wave
–particles move
parallel to wave
direction
• fastest
• least destructive (B7, Figure 22)
SECONDARY SEISMIC
• S wave
WAVES
• move out from focus
• transverse wave
–particles move at
right angles to wave
direction
• slower than P waves
• more destructive than P(B7, Figure 23)
SURFACE WAVES
• L wave
• move out from
epicenter
• particles move in
ellipses compared
to wave direction
• slowest
• Most destructive
(B7, Figure 24)
LAG TIME
• LAG TIME = difference between first
S wave and first P wave arrival at a
seismic station
• longer lag time = further from epicenter
(B9, Figure 25)
TIME-TRAVEL GRAPH
• compares
distance from
epicenter to
lag time
(B9, Figure 26)
LOCATING AN
EPICENTER
• triangulation
• use information
from at least
three seismic
stations
• Draw 3 circles
• Intersection =
epicenter (B9, Figure 27)
RICHTER SCALE
• Measures amount • OBJECTIVE
of energy released SCALE
• Increase of “1” = • Not influenced
10 times more by other factors
energy released • Used to compare
earthquakes
MERCALLI SCALE
• Measures • SUBJECTIVE
amount of SCALE
damage done • Influenced by many
to man-made factors (height of
structures buildings, age, how
built, etc.)
• Used to determine
amount of $ for aid