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