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Forces in Earth’s Crust Name: ___________________________ Chapter 6 Notes: Earthquakes Stress Stress: A force that acts on rocks to change its shape or volume. Stress pushes, pulls, or twists the rock in Earth’s crust Types of Stress Tension: a force that pulls on the crust. It stretches the rock so it is thinner in the middle. Occurs at Divergent Plate Boundaries Compression: the stress force that squeezes rock until it folds or breaks Occurs at Convergent Plate Boundaries Landforms from Compression Mountain Shearing: a stress that pushes rock in opposite directions Occurs at Transform Plate Boundaries Landforms from tension Valley Faults Fault: is a break in the rock of the crust where rock surfaces slip past each other. Faults occurs along plate boundaries where forces of the plate motion push or pull the crust so much that the crust breaks. Hanging wall: block of rock that lies above the fault. Footwall: rock that lies below the fault. Types of Faults Normal Faults: fault in which the hanging wall has moved downward relative to the footwall Occur where plates diverge Ex: Along the Rio Grande (rift valley) Reverse Faults: Faults in which the hanging wall moves upward relative to the footwall Opposite of normal fault Occurs where plates converge Ex: in Canada & Northern Rocky Mtns Strike-Slip Fault: fault in which the rocks slide past each other without up and down motion Occurs at transform boundaries Changing Earth’s Surface The forces of plate movement have added mountains, anticlines, synclines, and plateaus. Folded Mountains: plate movement and collision of crust cause Earth’s crust to fold Anticline: the fold in the rock that bends upward Syncline: the fold in the rock that bends downward Examples of Folded Mountains Central Appalachian Mountains Himalayas in Asia Alps in Switzerland Fault Block Mountains: forms when 2 Normal Faults cut through a block of rock Plateau: large area of flat land elevated high above sea level Formed when forces uplift some areas without much folding Earthquakes and Seismic Waves: Earthquake Earthquake: shaking and trembling that results from the movement of rock beneath earth’s surface Caused by forces of plate movement. The movement causes stress in the crust At the fault Stress increases until the rock breaks Releases an enormous amount of energy Focus: the area beneath Earth’s surface where rock is under stress and breaks. Triggers an earthquake Usually about 100 km below the surface Epicenter: The point on Earth’s surface directly above the focus. Seismic Waves Seismic Waves: vibrations that travel through Earth carrying the energy released during an earthquake Types of Seismic waves P waves: the first wave to arrive Primary waves They compress and expand Like an accordion S waves: come after the P waves Secondary waves Vibrate side to side and up and down Cannot move through liquids Surface Waves: P waves and S waves that reach the surface They move slower but still cause severe ground movement because it has loose soil, sand, gravel, mud, and small rocks that can move, shift and slide. Measuring Earthquakes Scientists use 3 main methods to measure Earthquakes Mercalli Scale: rates earthquakes according to damage The same earthquake can have different Mercalli ratings Different amounts of ground motion at different places Richter Scale: rates earthquakes according to the magnitude of the seismic waves. Magnitude: is the number assigned to the earthquake based on the size. The magnitude tells how much energy was released by an earthquake. Moment Magnitude Scale: rating that estimates the energy released by an earthquake. Scientists study seismographs Seismograph: instrument that records and measures seismic waves These show the kinds of waves present and how strong they were. Comparing Magnitude For each point increased in magnitude, 32 times as much energy is released Earthquakes below a 3 are barely noticed. Magnitudes between 5-6 causes moderate damage. Magnitude 8 or greater causes great damage. Locating the Epicenter Geologists use the seismic waves to locate an earthquake’s epicenter Monitoring Earthquakes Geologists us seismograph Seismogram: the pattern of zigzag lines on paper as a record of an earthquake’s seismic waves Instruments used to monitor faults Tiltmeters Creep Meters Laser-Ranging Devices GPS Satellites Earthquake Safety Damage from Earthquakes Shaking Can cause landslides and avalanches Damage or destroy buildings, bridges, utilities, pipes S-waves and Surface waves can tear buildings apart Loose soil shakes more than solid rock Liquefaction Liquefaction: the result of the Earth’s shaking that turns soil to liquid mud. Aftershocks Aftershock: an earthquake that occurs after a larger earthquake in the same area. May strike hours, days, or months later Tsunamis Waves produced from earthquake under water Steps to Safety Drop, cover & hold Indoors: Go under sturdy table or inside corner Avoid windows, mirrors, wall hangings, furniture Outdoors: Go to open area Avoid vehicles, power lines, trees, building