March 31  Describe Stress vs. Strain  Describe Elastic Rebound  Describe the different types of faults

Earthquakes Forces inside Earth  When rocks break they move along FAULTS Applied forces cause rocks to undergo elastic deformation (stress) Applied forces cause rocks to undergo elastic deformation (stress) When elastic limits are passed, rocks break (strain) When elastic limits are passed, rocks break (strain) Rock on one side of a fault can move up, down, or sideways in relation to rock on the other side of the fault Rock on one side of a fault can move up, down, or sideways in relation to rock on the other side of the fault

Stress vs. Strain  Rubber Band Example You apply stress to elongate the rubber band You apply stress to elongate the rubber band Strain occurs when you apply to much stress and deform the material. Strain occurs when you apply to much stress and deform the material.

Elastic Rebound  Geologists think that earthquakes are the result of elastic rebound.  In this process, the rocks on each side of a fault are moving slowly. If the fault is locked, the rock deforms, and stress in the rocks increases.  When rocks are stressed past the point at which they can maintain their integrity, they fracture.  The rocks then separate at their weakest point along the fault and rebound, or spring back to their original shape.

Elastic Rebound

Faults  Faults occur because of the forces inside of Earth cause Earth’s plates move placing stress on or near plate edge Rocks will bend, compress, stretch, and possible break Rocks will bend, compress, stretch, and possible break Earthquake – vibrations produced by breaking rock Earthquake – vibrations produced by breaking rock Rocks break, move along the fault, return to original shapesRocks break, move along the fault, return to original shapes Rock on one side of a fault can move over, under or past each other along fault linesRock on one side of a fault can move over, under or past each other along fault lines

Faults Cont’d  Three types of forces act on rocks Tension Tension Normal fault – pulls material apartNormal fault – pulls material apart Compression Compression Reverse Fault – decreases the amount of materialReverse Fault – decreases the amount of material Shear Shear Strike slip fault – rocks on either side of the fault move past each other without much upward or downward motionStrike slip fault – rocks on either side of the fault move past each other without much upward or downward motion

Types of Fault

Normal Fault Reverse Fault

Types of Fault Strike Slip Fault

Features of Earthquakes  Seismic Waves Waves generated by an earthquake, can move the ground Waves generated by an earthquake, can move the ground forward and backward,forward and backward, up and downup and down side to sideside to side Focus – an earthquake’s point of energy release Focus – an earthquake’s point of energy release

Types of Seismic Waves  2 Types of Waves Body Waves Body Waves Primary (P):Primary (P): Secondary (S):Secondary (S): Surface Waves: Surface Waves: RayleighRayleigh LoveLove

Types of Seismic Waves  Body Waves Primary (P): cause particles in rocks to move back and forth in the same direction the wave is traveling Primary (P): cause particles in rocks to move back and forth in the same direction the wave is traveling Secondary (S): cause particle s in rock to move at right angles to the direction of wave travel Secondary (S): cause particle s in rock to move at right angles to the direction of wave travel

Types of Seismic Waves  Surface Waves Rayleigh Rayleigh Back to back motion on the surfaceBack to back motion on the surface Love Love Up and down motion on the surfaceUp and down motion on the surface

Types of Seismic Waves

Anatomy of an Earthquake

Epicenters  Point on the Earth’s surface directly above the earthquake  Speeds of Seismic waves allow scientists to determine epicenter Primary = fastest Primary = fastest Secondary = follow P Secondary = follow P Surface = slowest Surface = slowest

Finding the Travel Times of S and P Waves -submitted/pswavechart.swf -submitted/pswavechart.swf

Seismograph  Measures seismic waves  Consists of rotating drum of paper and a pendulum with an attached pen  The paper record of seismic event is called a seismogram

Horizontal and Vertical Seismographs

Reading Seismograms

Seismic Recordings and Earth’s Interior  Earth’s Internal Structure Lithosphere, Asthenosphere, Mantle, Core Lithosphere, Asthenosphere, Mantle, Core  Seismic Waves Change Speed and Direction when they move through material of different densities Change Speed and Direction when they move through material of different densities Density will increase with depth as pressure increase Density will increase with depth as pressure increase Shadow Zone Shadow Zone Do receive seismic wavesDo receive seismic waves The waves are bent or stopped by materials of different densitiesThe waves are bent or stopped by materials of different densities

Shadow Zone

People and Earthquakes  Magnitude: the amount of E released during an earthquake  Usually reported using 1 of 3 scales

Richter Scale Richter: based on the size of the largest seismic waves generated by the quake developed by American Charles Richter  each whole number increase represents an increase in seismic wave size, or amplitude, by a factor of 10 Ex: A 4.0 earthquake has 10 times the seismic waves as a 3.0 earthquake Ex: A 4.0 earthquake has 10 times the seismic waves as a 3.0 earthquake  the difference in E between two numbers on the Richter Scale is 32x Ex: A 4.0 earthquake has 32 times the E as a 3.0 earthquake Ex: A 4.0 earthquake has 32 times the E as a 3.0 earthquake Ex: How much more E is released during a 7.0 earthquake vs a 5.0 earthquake? Ex: How much more E is released during a 7.0 earthquake vs a 5.0 earthquake?

Other Magnitude Detectors  Moment Magnitude: based on several factors (not just largest seismic wave): size of fault rupture size of fault rupture amount of movement along the fault amount of movement along the fault rocks’ stiffness rocks’ stiffness  Modified Mercalli: based on intensity or damage done to the area uses structural damage & eye witness accounts to rate earthquakes uses structural damage & eye witness accounts to rate earthquakes uses Roman numerals I to XII to designate degree of intensity (p. 507 table 19-1) uses Roman numerals I to XII to designate degree of intensity (p. 507 table 19-1)