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
From shuttle (above)
From air (at right)
Strike-slip fault
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
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