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Earthquakes What Are Faults & What Causes Them?? • When subjected to stress (forces acting on rock) rocks will strain (deform). • Three different stresses(forces): • Compression: Force that pushes masses together (squeezes) • Tension: Force that pulls rocks apart • Shear: Force that have rocks slide past each other • • • • • When enough stress (force) accumulates rocks can deform no longer. The rocks will then fracture along surfaces called faults. Faults: cracks in rock where movement occurs The vibrations produced by faulting are called earthquakes. They have reached their elastic limit. Stress vs. Strain Elastic Rebound • Elastic energy stores up (i.e. stretched rubber band) due to stresses & friction btwn plates • When force exceeds friction, snap!!! It returns to original shape Faults • Normal: caused by tension (divergence), one block moves down relative to the other • Reverse: caused by compression (convergence), one block moves up relative to the other • Strike-slip: caused by shear stress (transform), no vertical movement. Blocks slide past each other Seismic Waves • Energy released by an earthquake is released in seismic waves. • 3 main types • P-waves (primary waves) • S-waves (secondary waves) • L-waves (surface waves) Seismic Waves • P-waves • Particles that move back and forth in the same direction as the wave. • Compressional waves (similar to sound waves). • Fastest moving • S-waves • Particles move at right angles to the direction of the wave. (transverse waves) • Moderate speed • L-waves • Surface waves roll around along the surface, like ocean waves • Do most of the damage during an earthquake. • Slowest moving waves Types of Waves Examples of Earthquakes Alaska – 1964 1. Subduction boundary Pacific under North American plate 2. Severe due to triggering of other breaks Result – 10,000 after shocks, tsunamis (600 km/hr and 21 m high) New Madrid (1811 & 1812) • Location – • Missouri Size – • 8.6, 8.4, 8.7 • Changed course of Mississippi River • 2 new waterfalls Significance! – • Not at a plate boundary • Cause: 3 faults deeply buried by Mississippi sediments • Class: mid-plate earthquakes (not many) Earthquakes and Earth’s Interior Seismology • Seismology: The study of earthquakes • Seismograph: instrument to measure earthquakes • Seismologists: scientist that study earthquakes Seismometer/Seismograph Locating an Earthquake Locating the epicenter - Triangulate between 3 or more seismographs Terminology • 1. Focus – • Where earthquakes actually occur below the surface • 2. Epicenter – • Point on earth’s surface directly above focus Focus & Epicenter • Focus: The point under the surface where the earthquake actually occurs. • Epicenter: The point on the surface directly above the focus Earth’s Structure Seismic waves allow us to “look” at Earth’s interior • P-Waves: can travel through liquids & solids • S-waves: can travel through solids only • Waves changes speed & direction when going through different materials • These changes indicate a different layer of Earth Earth’s Structure • P-waves are received on the other side of Earth • Due to p-waves ability to travel through all materials • S-waves don’t travel through the core • They can not pass through liquids • Thus there is an absence of them on the opposite side of the Earth (s-wave shadow zone) • This tells us the outer core is liquid Earth’s Structure Earth’s Structure