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Earth Science Chapter 5 Earthquakes Forces in Earth's Crust A force that acts on rock to change its shape or volume is stress 3 types of stress act on rock layers – Tension pulls on the crust –Compression squeezes rock –Shearing pushes a mass of rock in two opposite directions. Tension – Pulls on crust Rock becomes thinner in the middle Occurs at a spreading boundary Compression – Squeezes rock Rock becomes thicker in the middle Occurs at a colliding boundary Shearing– Pushes rock in two opposite directions Occurs at a sliding boundary Types of Force Faults & Folds Fault - a break in the rock layer of the crust where rock surfaces slip past each other. Most faults occur at plate boundaries 3 Types of Faults Depends on the type of Stress applied to the rock layer Foot wall & Hanging wall Picture yourself as a little person, if this little person could hang off of one of the walls then that wall is the hanging wall. If the little person could place their foot on the bottom of the wall then it is the foot wall Normal fault • Caused by tension • Hanging wall lies below the foot wall • Movement along vertical plane • Along spreading boundaries Reverse fault • Caused by compression • Hanging wall lies above the foot wall • Movement along vertical plane • Along colliding boundaries Strike Slip fault • Caused by shearing • Movement along horizontal plane • Along sliding boundaries Types of faults http://www.classzone.com/books/earth_science /terc/content/visualizations/es1103/es1103page 01.cfm Fold Rock Layers That Fold – Forces on the rock layer are not fast or strong enough to break or snap the rock but simply bends the rock layer Caused by Compression type forces Himalayas in Asia, Alps in Europe, and the north coast mountains of California are formed due to folding A fold in rock that bends upward into an arch is an anticline. A fold in rock that bends downward to form a valley is a syncline. Anticlines synclines Fault Types & Land Forms When two normal faults cut through a block of rock, fault movements may push up a fault-block Earthquake! A shaking and trembling that results from the sudden movement of part of the Earth’s crust. Similar to ripples from a pebble in a pond Usually occur along a fault More than 1 million per year on the planet Entire fault doesn’t all move at the same time. Energy is released at different places at any one time Earthquake Terms Focus: the point underground where the release of stress is located Epicenter: the point on the earth’s surface directly above the focus Aftershock: an earthquake that occurs after a larger earthquake in the same area. Liquefaction: earthquake's violent shaking suddenly turns loose, soft soil into a liquid-like slurry. Seismic Waves Seismic waves: carry energy from an earthquake away from the focus P waves S waves Surface waves Primary Waves: P waves –1. Fastest moving of the waves –2. Travel through solid, liquids and gases –3. “Push-pull” type of wave Primary Waves: P waves Secondary Waves: S Waves 1. Second to arrive 2. Travel through solid but not liquid or gas 3. “S” shaped waves 4. Rock particles move at right angles to the direction of the wave Secondary Waves: S Waves Surface Waves Slowest moving of the waves 2. Travel across the surface of the Earth 3. Originate on the earth’s surface 4. Cause the most damage of all the waves 1. Surface Waves: L Waves Seismic Wave Movements P waves travel thru solid & liquid. P waves arrive first then come the S waves S waves reflect off the liquid outer core Earthquake Strength & Intensity Richter Scale - rating of an earthquake's magnitude based on the size of the earthquake's seismic waves ( 1 – 10) Mercalli Scale - rate earthquakes according to the level of damage at a given place. ( I – XII) Moment Magnitude Scale - rating system that estimates the total energy released by an earthquake Richter Scale Most commonly used scale Each number represents a magnitude 10x greater than previous number. Mercalli Scale rate earthquakes according to the level of damage at a given place. ( I – XII) Seismograph A Seismograph detects and measures intensity of the earthquake – A Record of an earthquake is a seismogram To monitor faults, geologists use instruments to measure – 1. Changes in elevation 2. Tilting of the land surface 3. Ground movement along faults Instruments that Monitor Faults 1. 2. 3. 4. Tilt Meters – vertical movement Creep Meters – horizontal movement Laser-ranging Devices – horizontal movement GPS Satellites – vertical and horizontal movement http://www.powershow.com/view/122c7bYTY3Y/Monitoring_Faults_powerpoint_ppt_prese ntation Earthquake Safety Geologists determine earthquake risk by – Locating where faults are active Where past earthquakes have occurred Where most damage was caused How Earthquakes Cause Damage 1. Shaking – damage buildings, utility poles, fracture gas lines, cause landslides, avalanches Lose soil shakes more than solid rock – house built on sandy soil will shake more than the one built on solid rock Liquefaction – Where soil is full of moisture – as ground shakes, lose soil becomes liquid mud – foundation becomes weak 2. Aftershocks – Earthquake that occurs after a larger earthquake in an area – buildings are already weakened Tsunamis Tsunami - a large water wave caused by an under sea earthquake. When an earthquake jolts the ocean floor, plate movement causes the ocean floor to rise slightly and push water out of its way. Earthquake Safe Buildings 1. 2. Type of material Size of building Making buildings safer – Base isolated buildings Base isolators Tension ties Shear walls Cross bracing Tension ties Dampers Flexible pipes Earthquake song https://www.youtube.com/watch?v=sA6o Z4YgKCA Shake table – base isolation https://www.youtube.com/watch?v=kzVvd 4Dk6sw