Earthquakes
Chapter 6
Elastic Rebound Theory
• Rocks on either side
of a fault move slowly
• When locked, stress
builds…until fault
ruptures
• Rocks fracture and
slip to new position
releasing vibrations create seismic waves
Relaxed
Stressed
Released
What are faults
• A fault is a crack in the Earth's crust along
which movement has occurred.
• Occur at plate boundaries or within a plate
• Consists of a
hanging wall (HW)
and a
foot wall (FW)
3 types of faults
• Normal:
– at divergent boundaries, HW moves
down, FW moves up
• Reverse (or thrust fault):
– at convergent boundaries: HW moves
up, FW moves down
• Strike-slip faults:
– At transform boundaries; movement is
horizontal
San Andreas Fault
Strike-slip fault
From shuttle (above)
From air (at right)
More San Andreas
Earthquake Vocab
• Focus: area along
fault where
slippage 1st occurs
• Epicenter: point on
surface directly
above the focus
Major EQ Zones – 1. Ring of Fire
2. Mid-Ocean Ridges
3. Eurasian-Melanesian Mts
Boundary of Eurasian-Indian-African plates
4. Fault Zones
• Faults along plate
boundaries – San
Andreas strike-slip
fault
• Faults within plates –
in middle of US 1812 New Madrid,
Missouri EQ
• New England, too?
• Nov. 2006 NH – 1.2
Recording EQs
• Seismograph:
instrument used
to record EQs
• Seismogram:
printout from a
seismograph
Types of Seismic Waves
• Primary “P”
• Secondary “S”
• Surface Waves
– Rayleigh “R”
– Love “L”
http://www.rsuw.daleh.id.au/html/seismic_waves.html
Primary “P” waves
•
•
•
•
“push-pull” waves
Fastest seismic waves; 1st to be recorded
Compression waves (like sound)
Cause movement in the same direction as
the wave motion
• Rocks vibrate backwards and forwards,
• P waves can travel through liquids and
solids
Secondary “S” waves
•
•
•
•
Shake or shear waves
Second to be recorded
Slower and more destructive than P waves
Rocks are moved from side to side as the
wave passes, moving at right angles to the
direction of wave motion
• Transverse waves, like water waves
• Move through solid only
Surface, “L” waves
• L for “Love” waves
• Slowest of the three EQ wave types
• L waves take the longer surface route around the
earth
• Have a rolling motion, side to side
• Cause great damage
• Behaves most like waves in a pond or on the sea
Other surface waves
• Rayleigh waves
• Rolls along the ground, moving ground up
and down
•
Animation: http://www.edcenter.sdsu.edu/ssc/3d/rayleighwave/rayleighwavetest2.mov
Power and ground type
•The type of rock will determine how fast
the wave travels, and will thus affect
amplitude of the waves
•Animation:
http://www.edcenter.sdsu.edu/ssc/3d/seismicpropagation2d/seismicpropagation-sm.mov
Using Seismic Waves to determine
Earth’s Interior
• P waves travel
through solids,
liquids and
gases
• S waves travel
through solids
only
• Outer core
must be liquid
(no S waves
pass)
Earthquakes, Tsunami and
animals
•
http://www.teachersdomain.org/asset/nat08_vid_waves/
Homework
• Read and take notes – pp 104-105
• Answer questions #1-4, pg 102 and
#3-5, pg 105