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Do Now: Earthquake Questions • How does an earthquake happen? • What are 3 basic types of faults? • What equipment is used to record/measure earthquakes? • What is the center of the earthquake called? How do we locate it? • What are the different types of quake waves? What are their characteristics? • Describe the scale used to measure quakes. Fault Types: Normal Fault – - Pieces of crust are pulled apart - Hanging Block slides down Thrust Fault – - Pieces are pushed together - One block pushed up fault line Strike-Slip Fault – - Blocks grind next to each other on same plane Earthquakes • Earthquake: shaking or trembling of the ground caused by the sudden release of energy, usually as a result of faulting, which involves the displacement of rocks along fractures • Aftershocks: earthquakes from continued adjustments along a fault; usually smaller than the initial quake Elastic Rebound Theory • Rocks undergoing deformation bend and store energy • When strength of rock is exceeded, they rupture and release energy – the earthquake • Rocks rebound to original, undeformed shape Fault Fence Original position Rupture and release of energy Deformation Rocks rebound to original undeformed shape Stepped Art Fig. 8-1a, p. 151 Seismology • Seismology: the study of earthquakes • Seismic waves: energy from earthquakes • Seismographs: detect, record, and measure earthquakes • Seismogram: record from a seismograph • Earthquakes occur along faults, where movement is stored as energy in rocks • Most faults related to plate movements Focus and Epicenter • • • • Focus: point where energy is first released Epicenter: point on surface above focus Shallow-focus: 0-70 km below surface Intermediate focus: 70-300 km below surface • Deep-focus: >300 km below surface • 90% less than 100 km below surface Earthquakes and Plate Boundaries • Divergent and transform boundaries: always shallow-focus • Convergent boundaries: – shallow-, intermediate-, and deep-focus – Beniorr-Wadati zones: foci along subducted plate Major Earthquake Regions • Plate boundaries: convergent, divergent, and transform • 80% in Pacific Ring of Fire • 15% in Mediterranean-Asian belt • 5% in plate interiors and ocean spreading ridges • Intraplate: from compression of plate along margins Seismic Waves • All waves generated by an earthquake • Body waves – P-waves – S-waves – Travel faster through less dense, more elastic rocks • Surface waves – R-waves – L-waves P-Waves • • • • Primary waves Fastest seismic waves Travel through solids, liquids, and gases Compressional/push-pull: expand and compress material, like sound waves S-Waves • • • • Secondary waves Slower than P-waves Travel only through solids Shear waves: move material perpendicular to direction of wave movement • Create shear stresses Undisturbed material Surface Undisturbed material Primary wave (P-wave) Direction of wave movement Wavelength Secondary wave (S-wave) Focus Stepped Art Fig. 8-7, p. 156 Surface Waves • Travel at or just below the surface • Slower than body waves • R-waves (Rayleigh waves) – Particles move in elliptical path, like water waves • L-waves (Love waves) – Faster than R-waves – Particles move back forth in horizontal plane perpendicular to direction of travel Earthquake Magnitude • Quantitative measure: amount of energy released • Richter Magnitude Scale: total amount of energy released at earthquake source • Measure amplitude of largest seismic wave • Logarithmic: each whole-number increase is a 10-fold increase in amplitude, but a 30-fold increase in energy Earthquake Magnitude, cont. • Richter Magnitude Scale underestimates energy of very large quakes – Only measures peak energy, not duration • Seismic-moment magnitude scale: – Strength of rocks – Area of fault rupture – Amount of movement of rocks adjacent to fault • 1964 Alaska earthquake: – 8.6 Richter – 9.2 seismic-moment