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Transcript
EARTHQUAKES AND EARTH’S INTERIOR Objectives • Explain the connection between earthquakes and plate tectonics. • Identify several earthquake-related hazards. • Define body waves and surface waves. • Explain how the materials in Earth’s interior affect seismic waves. • Define and describe the composition of Earth’s crust, mantle and core. Megathrust earthquakes Earthquakes and earthquake hazards • Seismology – The scientific study of earthquakes and seismic waves • Seismic waves – An elastic shock wave that travels outward in all directions from an earthquake’s source • Seismic creep Earthquakes and plate motion • The elastic rebound theory – Continuing stress along a fault – Results in buildup of elastic energy in the rocks – Energy abruptly released when an earthquake occurs Earthquakes and plate motion Earthquake Hazards and Predictions • Primary hazards – Collapsing buildings, bridges and other structures – Aftershock • Secondary hazards – Landslides, fires, ground liquefaction, tsunamis Earthquake hazards Landslide, Huascaran, Peru Open fissure, Golcuk, Turkey Earthquake hazards Fire, San Francisco, California Ground liquification, Niigata, Japan The Sumatra-Andaman Tsunami (2004) Earthquake prediction • Short-term prediction and early warning – Precursor phenomena – Foreshocks • Long-term forecasting • Paleoseismology – The study of prehistoric earthquakes – Seismic gaps Earthquake Readiness • Preparation and readiness to earthquakes key to reducing fatalities – Reinforced structures – Bolting wood-framed buildings to foundation – Protecting utility lines from movement – Education The Science of Seismology • Seismograph – An instrument that detects and measures vibrations of Earth’s surface – Advanced seismographs detect vibrations 10-8 of a centimeter • Seismogram – The record made by a seismograph The Science of Seismology Seismic waves • Body wave – Travels through Earth’s interior • Surface wave – Travels along Earth’s surface • Focus – Where rupture commences and an earthquake’s energy is first released Locating earthquakes • Compressional wave: – Wave consisting of alternating pulses of compression and expansion – Can pass through any medium (solids, liquids, gases) – P (or primary) wave • Shear wave: – Rock is subjected to side to side or up and down forces, perpendicular to wave’s direction of travel – S (secondary) wave – Not transmitted through water – Travel slower than P waves Locating earthquakes Locating earthquakes Locating earthquakes • Epicenter – The point on Earth’s surface directly above an earthquake’s focus Locating earthquakes Measuring Earthquakes • The Richter Magnitude Scale – A scale of earthquake intensity based on the recorded heights, or amplitudes, of the seismic waves recorded on a seismograph – A logarithmic scale—a 10 fold increase in amplitude for each unit • Moment Magnitude Scale – A measure of earthquake strength that is based on the rupture size, rock properties, and amount of displacement on the fault surface Measuring Earthquakes Richter magnitude 6 Richter magnitude 7 Richter magnitude 8 Studying Earth’s Interior • Seismic discontinuity – A boundary inside Earth where the velocities of seismic waves change abruptly Studying Earth’s Interior Studying Earth’s Interior • Three things can happen to seismic waves when they meet a boundary – Refraction: bent as they pass from one material to another – Reflection: some or all of the wave energy bounces back – Absorption: some or all of the wave energy is blocked How geologists look into Earth’s interior • Seismic tomography – Allows geologists to image inside of Earth • Direct observation – Drilling – Xenoliths • Indirect observation – Magnetism – Density Diamonds! A Multilayered Planet • Crust – The outermost compositional layer of the solid Earth, part of the Lithosphere – Thickness ranges between 8 kilometers (oceanic) and 45 km (continental) A Multilayered Planet A Multilayered Planet • Mantle – The middle compositional layer of Earth, between the core and the crust – Comprised primarily of olivine and pyroxene – Asthenosphere: mantle where rock is near melting – Mantle-core boundary: mesosphere A Multilayered Planet A Multilayered Planet • Core – Innermost layer, where the magnetic field is generated and much geothermal energy resides – Separated into outer core (liquid) and inner core (solid) Critical Thinking • If you were on a ship in the ocean, would you be able to feel an earthquake that occurred below you, on the ocean floor? • Do you think there is no limit to the magnitude of earthquakes? • What kind of wave would you expect to travel faster: a seismic wave or a tsunami wave?