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EARTHQUAKES: WHY? AND HOW? EARTHQUAKES sudden movement or shaking of the Earth • Caused by plate tectonic stresses releases stored Energy! • Located at plate boundaries • Resulting in breakage of the Earth’s brittle crust Elastic Rebound Theory • Stresses build on both sides of a fault. • When stresses become too great, the rocks break and move. • This releases the built up energy and creates an earthquake. • Video Clip 3 Types of Stress • Compression- occurs at convergent boundaries (plates move toward each other) • Tension- occurs at divergent boundaries (plates move away from each other) • Shear- occurs at transform boundaries (plates are sliding past each other) PLATE TECTONIC STRESSES • Plate boundaries and faults (= cracks where plate sections are moving in different directions) cause friction as plates move • Plates in a fault zone have STICK-SLIP motion – Periods of no movement (stick) and fast movement (slip) – Energy stored as plates stick – Energy released as plates slip 3 Responses to Increasing Stress • Elastic Deformation- rock returns to original shape when stress is removed • Plastic Deformation- rock does not return to original shape when stress is removed • Fracture- rock breaks 3 Types of Faults –Normal Fault –Reverse Fault –Strike-Slip Fault Normal Fault • Result of plates moving apart. –The hanging wall moves downward in relation to the footwall. Normal Fault: Tetons in Wyoming • Plate boundaries are always faults, but not all faults are plate boundaries. Reverse Fault • Result of plates moving together. –The hanging wall moves upward in relation to the footwall. Reverse Fault: Sierra Madre Strike-Slip Fault • Result of plates sliding past each other. –The displacement is horizontal Strike-Slip: San Andreas Fault Parts of a Wave • Energy is transmitted in waves EARTHQUAKE WAVES • FOCUS = place deep within the Earth and along the fault where rupture occurs • EPICENTER = geographic point on surface directly above focus • SEISMIC WAVES produced by the release of energy – move out in circles from the point of rupture (focus) 2 types of Seismic Waves: surface & body (travel inside & through earth’s layers) 2 Types of Body Waves • Body waves travel faster than surface waves. • P waves: back and forth movement of rock; travel thru solid, liquid, gas • S waves: sideways movement of rock; travel thru solids only 2 Types of Surface Waves • Travel along the ground, outward from an earthquake’s epicenter. • Slowest of the seismic waves • Do most of the damage • Two Types of Surface Waves: – Love Wave – Rayleigh Wave Tsunami • Deadly ocean waves most often caused by undersea earthquakes. • Tsunami Awareness • NOAA DART