EARTHQUAKES

ROCK STRESS

With all the movement occurring on the earth’s crust, rocks go through a lot of stress. However rocks respond differently to different kinds of stress under different conditions. Rock stress is how rocks move when they respond to stress (defined as the force applied to an object). When stress causes rock to change shape, it has undergone strain or deformation. Four types of stresses act on rocks (only three are covered here).

COMPRESSION

• Compression squeezes rocks together, causing them to fault or fold, targeting the center of the rock. In horizontal compression stress, the crust can thicken or shorten; in vertical compression stress, the crust can thin out or break off.

COMPRESSION (CONT.)

• Compression is the most common stress at convergent plate boundaries, and can push rocks together or cause the edges of each plate colliding to rise. Mountains are a result of compression stress caused when two plates collide (e.g. Himalayas, Andes)

TENSION

• Tension is the opposite of compression. While compression forces the rocks and crust to collide and move together, tension pulls rocks apart. Rocks under tension lengthen or break apart.

TENSION (CONT.)

• Tension is the major type of stress at divergent plate boundaries, and can happen in two ways. Two separate plates can move farther away from each other, or the ends of one plate can move in different directions. The force of tension can form rift valleys (e.g. Álfagjá Rift Valley, East African Great Rift Valley).

SHEAR

• When forces are parallel but moving in opposite directions, the stress is called shear. Shear stress usually happens when two plates rub against each other as they move in opposite directions.
• The force of the stress can also push some of the crust in different directions.

SHEAR (CONT.)

• When the crust is pushed in different directions, a large part of the crust can break off, which makes the plate size smaller.
• Shear stress is the most common stress at transform plate boundaries. The friction of a shear stress at the edges of the plate can cause earthquakes.

FAULTS

Rock stress is constantly acting on the earth’s crust. When the stresses overpower the internal strength of the rock, the rock fractures, forming a fault (a break in a body of rock, marked by the movement of rock on each side of the break). Faults can occur in any direction with the blocks moving away from each other, and from both tensional and compressional forces. There are three types of faults (only two are covered here).

NORMAL

• Normal faults occur when tension acting in opposite directions causes the crust to drop down; one slab of the rock is displaced up, and the other slab down.
• Occurs when two bodies of rock are moving in opposite directions; forms divergent boundaries.

NORMAL (CONT.)

• Normal faults are classified by scientists as dip-slip faults; this is because they move in the direction of the downward plane.
• Tension pulls plates away from each other in normal faults; the depression formed between the plates is a rift valley. This was already explained in tensional stress; more examples include the Rhine Rift Valley and the Baikal Rift Valley.

REVERSE/THRUST

• In reverse faults, compression stress causes two bodies of rock to converge, or be pushed towards each other. One body is forced up and over the other body.
• Reverse faults are found at convergent plate boundaries.

REVERSE/THRUST (CONT.)

• Reverse faults are also classified by scientists as dip-slip faults; if the angle is shallow, the fault is classified as a thrust fault.
• Thrust faults are when two plates collide, compression forces pushing the edges of the plates up to form mountains. This structural deformation of the lithosphere/crust is called orogeny (e.g. Southern Alps, Pontic Mountains)

EARTHQUAKES

VIDEOS

• Hazards and damages from earthquakes: https://m.youtube.com/watch?v=TBCD8PVL0Io#fauxfullscreen
• Hazards and damages from tsunamis: https://m.youtube.com/watch?v=Wx9vPv-T51I#fauxfullscreen

EARTHQUAKES

• A sudden and violent shaking of the ground, sometimes causing great destruction, as a result of movements within the earth's crust or volcanic action.
• Hazards — liquefaction (when the ground acts like quicksand); damage from ground shaking; flooding (ruptures dams & levees); fires (broken gas our power lines); tsunamis

TSUNAMIS

• A long high sea wave caused by an earthquake, submarine landslide, or other disturbance.
• Hazards — objects and buildings crushed; large objects (e.g. ships & boulders) can be carried several miles inland; anything in the tsunami’s path (e.g. boats, buildings, cars, trees, power lines) disease (contaminates water & spreads disease

EARTHQUAKE SAFETY

• Staying safe during an earthquake: https://m.youtube.com/watch?v=BLEPakj1YTY#fauxfullscreen
• Stay indoors until the shaking stops
• Stay away from windows
• If in a car, slowly drive to a safe place, then stay in your car until the shaking stops

SAFETY (CONT.)

• If you’re in bed, stay there, protecting your head with a pillow.
• If you’re outdoors, find a clear spot away from anything that could collapse. Then, drop to the ground.
• If you’re outdoors, find a clear spot away from buildings, trees and power lines. Then, drop to the ground.

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BEING PREPARED

QUIZ TIME!

Go to the link below to take the quiz on Quizlet:
https://quizlet.com/378893849/flash-cards/

BIBLIOGRAPHY

• https://courses.lumenlearning.com/wmopen-geology/chapter/outcome-stress-and...
• http://www.physicalgeography.net/fundamentals/10l.html
• https://www.usgs.gov/faqs/what-a-fault-and-what-are-different-types?qt-news...

BIBLIOGRAPHY (CONT.)

• https://www.slideshare.net/mobile/sciencepowerpointcom/earthquakes-geology-...
• http://www.weatherwizkids.com/weather-safety-earthquake.htm
• https://m.wikihow.com/Prepare-for-an-Earthquake#Preparing_an_Emergency_Plan

THANK YOU