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Earthquakes
Plate Boundaries
Sketch the 3 type of
plate boundaries
Pacific-North American Plate Boundary
Earthquake Study
• Seismology – the study of earthquakes
(#27)
• Seismologist – person who studies
earthquakes
Where?
• Most occur near tectonic plate boundaries
Causes
• Deformation – bending, tilting, and breaking
of Earth’s crust (#28)
• Caused by plates pushing, pulling & sliding
• 2 kinds of deformation
– Plastic – bends like piece of soft clay
• No earthquake
– Elastic – stretches like a rubber band
• Earthquake
Elastic Rebound
• Elastic rebound – sudden return of elastically
deformed rock to its undeformed shape
– Like a stretched rubber band that breaks and returns
to its unstretched shape, now in 2 pieces
– Energy released as seismic waves
Normal
Reverse
Divergent
Convergent
Strike-slip
Transform
Body Waves
• Seismic waves – waves of energy that travel through Earth, away
from an earthquake in all directions (#29)
• Two types of body waves that travel through interior of Earth
– P waves
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Primary waves (#30)
Pressure waves (#30)
Back and forth motion (#30)
Go through solid, liquid, gas
Fastest wave
– S waves
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Secondary waves (#31)
Shear waves (#31)
Side to side motion (#31)
Go through solids only
Slower wave, 2nd to arrive
More Waves
• Surface waves – waves
that move in top few
km of the crust
– Up, down, circular
motion
– Back and forth motion
• Travel slower than
body waves
• More destructive
Draw pictures of P and S
Waves
Measurement
• Seismograph – instrument that
records ground vibrations to
find location and strength of
earthquake
• Seismogram – tracing of
earthquake motion created by
a seismograph
• Epicenter – point on Earth’s
surface directly above an
earthquake (#32)
• Focus – point on a fault where
earthquake motion first occurs
(#33)
Earthquake Location
Finding Epicenter – Step 1
• Scientists use the S-P Time Method
– Use a time-distance graph
– S and P curves are already calculated
– Determine distance away from epicenter for at least 3 seismograms
Finding Epicenter – Step 2
Earthquake Strength
• Richter Magnitude Scale –
– Created in 1930s by Charles Richter
– Magnitude measures strength of earthquake
by ground motion adjusted for distance from
epicenter
– Each unit represents 10x increase in strength
• 5.0 is 10x stronger than a 4.0
• 6.0 is 100x stronger than a 4.0
Richter Magnitude Scale
Earthquake Intensity
• Intensity – degree to which earthquake is
felt and amount of damage caused
• Modified Mercalli Intensity Scale
– Numerical scale from I to XII
– I not felt by people
– XII total destruction
• Intensity values highest near epicenter
Earthquake Hazards
• How likely area is to have a damaging
quake in the future
Earthquake Forecasting
Gap Hypothesis
• Areas on active faults that haven’t had many
earthquakes are likely to be sites for future
strong earthquakes.
– Known as seismic gaps
Earthquake Preparations
• Resistant buildings
– Mass damper
– Active tendon
system
– Base isolators
– Cross braces
– Flexible pipes
• Retrofitting
– Reinforce columns
– Fasten to foundation
What to do in an Earthquake
• Before shaking
– Put heavy objects down low
– Have an emergency meeting place
– Store emergency supplies
• During shaking
– Crouch or lie under a strong table or desk
– Outside – lay down away from buildings, trees, power
lines
– Stop car and stay inside
• After shaking
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Stay calm and think
Get away from danger areas
Prepare for aftershocks
Follow emergency plan
Earthquake Examples
• Normal
• Reverse
• Strike slip
Normal Fault Example
Dixie Valley-Fairview Peaks, Nevada earthquake
December 16, 1954
Thrust Fault Example
Thrust Fault Example
Strike-slip Fault Example
Strike-slip Fault Example
1906 San Francisco Earthquake
Earthquake Effects
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Ground shaking
Liquefaction – solid earth turns to liquid
Surface faulting
Landslides
Fires
Tsunami
Earthquake Effects - Ground Shaking
Loma Prieta, CA 1989
KGO-TV News ABC-7
Earthquake Effects - Ground Shaking
Kobe, Japan 1995
Earthquake Effects - Liquefaction
Source: National Geophysical Data Center
Niigata, Japan 1964
Earthquake Effects - Surface Faulting
Landers, CA 1992
Earthquake Effects - Landslides
Source: National Geophysical Data Center
Turnagain Heights, Alaska,1964 (upper left inset);
Santa Cruz Mtns, California , 1989
Earthquake Effects - Fires
Loma Prieta, CA 1989
KGO-TV News ABC-7
Earthquake Effects - Tsunamis
1957 Aleutian Tsunami
Photograph Credit: Henry Helbush. Source: National Geophysical Data Center