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Earthquakes
5/23/2017
©1999 John Culpepper
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Earthquakes
• The movement of tectonic plates
creates stress on large areas of rock
• Stress within rocks can be relieved by
– Bending
– Stretching
– Breaking
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Earthquakes
• Occur when stress is relieved by rocks
breaking
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Earthquakes (cont.)
• Usually occur along faults and plate
boundaries
• Energy is rapidly released in all
directions (causing waves)
– Like a pebble dropped in a quiet pool of
water
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©1999 John Culpepper
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Types of Faults
• Normal Fault
– Hanging wall slides down the fault plane,
– Formed by divergent boundary
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Reverse Fault (Thrust Fault)
Hanging wall slides up the fault plane
Formed by convergent boundary
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• Strike-slip Fault
– Hanging wall and foot wall slide
horizontally to the fault plane,
– Formed by transform boundary
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Important Terms
• Fault- a weakness or
break in the Earth’s
crust
• Focus- the place
underground where the
break occurs (actual
location of the
earthquake)
• Epicenter- the location
on the Earth’s surface
just above the focus
(map location of an
earthquake)
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Seismic waves
• In an earthquake 3 kind of waves are
produced
– P-waves
– S-waves
– Surface waves
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P-waves (primary waves)
• Travel through
solids and liquids
• Fastest moving
seismic waves
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S-waves (secondary waves)
• Travel only through
solid material
• Move slower than Pwaves
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Surface waves
• Do the most
damage and move
the slowest
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Seismic waves are measured
by seismometers
• Seismic recordings
are called
seismograms or
seismographs
• Scientists who study
earthquakes are
called Seismologists
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Earthquakes (cont.)
• The exact time of an earthquake is
known as its origin time
• Earth’s outer core is liquid
– Only P-waves can travel through
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Earthquakes (cont.)
• Moho discontinuity -the boundary
between Earth’s crust and mantle
– Seismic waves speed up here
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Earth’s Interior
• Within the
Earth density,
pressure, and
temperature all
increase with
depth
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Faults
• Normal fault
– Found in areas
under tension
• Where sections of the
landscape are pulling
apart
Called Fault Block
Mountains
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Faults (cont.)
• Reverse fault
– Found in areas
under compression
• Zones of
convergence
Like the Himalaya
Mountains
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Faults (cont.)
• Strike-slip faults
– More horizontal
movement than
vertical
movement
Like the San Andreas
fault in California
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Shadow zone
• A zone on the opposite
side of the earth from
where an earthquake
has occurred where
seismic waves will not
be felt
– Due to the way that
seismic waves travel
through the different
materials within the
Earth
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Earthquake damage is
measured by 2 scales:
• Richter Scale 1-10 each increase of 1=10
fold increase in shaking (exponential)
• Mercalli Scale 1-7 based on the damage to
buildings and observed effects
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Deep, Intermediate, & Shallow
Earthquakes
• Deeper usually =
stronger
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• Shallow EQ’s occur
nearer the trench
and deeper occur
farther inland
• This is due to
subducting oceanic
crust
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EQ
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Typical
Seismogram
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How is an Earthquake’s Epicenter Located?
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Seismic wave
behavior
– P waves arrive
first, then S
waves, then L and
R
– Average speeds
for all these
waves is known
– After an
earthquake, the
difference in
arrival times at a
seismograph
station can be
used to calculate
the distance from
the seismograph
to the epicenter.
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How is an Earthquake’s Epicenter Located?
Time-distance graph
showing the average
travel times for P- and Swaves. The farther away a
seismograph is from the
focus of an earthquake,
the longer the interval
between the arrivals of
the P- and S- waves
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©1999 John Culpepper
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How is an Earthquake’s
Epicenter Located?
•
•
•
Three seismograph stations
are needed to locate the
epicenter of an earthquake
A circle where the radius
equals the distance to the
epicenter is drawn
The intersection of the
circles locates the
epicenter
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Earthquake Practice:
• How many seismic stations do you need to locate
the epicenter of an earthquake?
• An earthquake occurs 8000 km away, how long will it
take P-waves to reach you? S-Waves?
• P waves arrived at a seismic station at 12:03, Swaves arrived at 12:07. How far is the epicenter?
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