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Chapter 5: EARTHQUAKES &EARTH’S INTERIOR Earthquakes & earthquake hazards • Earthquake – Sudden release of energy • Seismology – Scientific study of earthquakes & seismic waves • Seismic waves – Shock waves traveling out in all directions from earthquake’s source Earthquake Hazards & Predictions • Primary hazard – ground motion – Collapsing buildings, bridges & other structures – Aftershocks Earthquake Hazards &Predictions • Secondary hazards – – – – Landslides Fires Ground liquefaction Tsunamis Earthquake Hazards &Predictions Earthquake Hazards &Predictions • Recent Tsunami – 2004 Sumatra – 2011 Japan – Magnitude 9.0 – Magnitude 9.1 – ~25 ft. – ~ 30 ft – 225,000 casualties – 15,000 casualties Earthquake prediction • Short-term prediction & early warning – Precursor phenomena – Foreshocks • Long-term forecasting • Paleoseismology – Study of prehistoric earthquakes Earthquake Readiness • Mercalli Intensity Scale – Based on observations & damage to structures – Subjective The Science of Seismology • Seismograph – Instrument to detect & measure vibrations of Earth’s surface • Seismogram – Record made by a seismograph Seismic waves • Body wave – Travels through Earth’s interior • Surface wave – Travels along Earth’s surface • Focus – Where rupture occurs & earthquake’s energy is first released • Epicenter – Point on Earth’s surface directly above earthquake’s focus Locating earthquakes • P (Primary) or Compressional waves: – Alternating pulses of compression & expansion – Pass through all states of matter • S (Secondary) or Shear waves: – Side to side or up &down movement – Pass only through solids – Travel slower than P waves Locating earthquakes • Surface or L (Love) wave: – Pass around Earth, not through it – Cause most ground shaking & damage – Slower than P & S waves Locating earthquakes • Finding the epicenter – S – P lag time – Triangulation (3 points needed) Measuring Earthquakes • Richter Magnitude Scale – Magnitude vs. Intensity – Based on amplitudes of seismic waves shown on seismograph – Logarithmic scale • 10 fold increase in amplitude = 1 unit increase • 32-fold increase in energy = 1 unit increase Measuring Earthquakes • Moment Magnitude Scale – Based on rupture size, rock properties, & amount of displacement along fault surface – Used for large earthquakes Studying Earth’s Interior • Seismic discontinuity – Boundary inside Earth where seismic wave velocities change abruptly Studying Earth’s Interior • 3 things can happen to seismic waves if they meet a boundary: – Refraction: waves bend as they pass from one material to another – Reflection: some or all of wave energy bounces back – Absorption: some or all of wave energy is blocked How geologists look into Earth’s interior • Direct observation – Drilling – Xenoliths How geologists look into Earth’s interior • Indirect observation – Magnetism – Astronomical observations • Density – Meteorites A Multilayered Planet • Crust – Outermost compositional layer of solid Earth, part of lithosphere • Lithosphere = brittle & solid – Thickness varies • 8 km (oceanic) • 45 km (continental) – Moho boundary A Multilayered Planet • Mantle – Middle compositional layer of Earth, between core & crust – Asthenosphere: mantle where rock is near melting • Plastic & weak – Mesosphere – Mantle-core boundary A Multilayered Planet • Core – Innermost layer • Where magnetic field is generated & most geothermal energy resides – Outer core (liquid) – Inner core (solid)
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