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Chapter 5: Earthquakes and Earth’s Interior Copyright © 2012 John Wiley & Sons, Inc. All rights reserved. Learning Objectives Earthquakes and earthquake hazards • Explain the role of plate tectonics and elastic rebound in the occurrence of giant earthquakes. The science of seismology • Describe the methods and tools of seismology. Studying Earth’s interior • Describe Earth’s liquid core and the ways in which scientists study it. A multilayered planet • Discuss the composition of Earth’s crust, mantle, and core. © 2012 John Wiley & Sons, Inc. All rights reserved. Megathrust earthquakes This ancient stone monument marks the high-water point of a past tsunami. © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Seismology The scientific study of earthquakes and seismic waves Answers questions like: why are earthquakes so sudden? big ones cause catastrophic damage? why occur in same places? Seismic waves are shock waves releasing energy when earthquake occurs. Fig. 5.2a Offset orange groves Fig. 5.2b Vertical displacement, Alaska, 1964 © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards 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 Case Study: Fence crosses SA fault © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Earthquakes and Plate Motion Case Study: Proving the Elastic Rebound Theory © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Earthquakes and Plate Motion Case Study: Proving the Elastic Rebound Theory A = unstressed state B= stress build-up C= moment of frictional lock breaking, returning to unstressed state © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Earthquake Hazards and Readiness Primary hazards: cause damage during an earthquake • Collapsing buildings, bridges, and other structures • Aftershock Secondary hazards: after-effects • Landslides, fires, ground liquefaction, tsunamis • Can cause more damage than original quake © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Earthquake Hazards and Readiness Secondary Hazards Figure 5.3a Landslide, Huascaran, Peru Figure 5.3b Open fissure, Santa Cruz, CA © 2012 John Wiley & Sons, Inc. All rights reserved. © 2012 John Wiley & Sons, Inc. All rights reserved. © 2012 John Wiley & Sons, Inc. All rights reserved. © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Earthquake Hazards and Readiness Secondary Hazards Figure 5.3c Fire, San Francisco, California Figure 5.3d Ground liquification, Niigata, Japan © 2012 John Wiley & Sons, Inc. All rights reserved. http://www2.pvc.maricopa.edu/ssd/geog/outlines/GPH111/images/chap13_15/liquefaction_venez.jpg © 2012 John Wiley & Sons, Inc. All rights reserved. The Big Picture: How tsunamis are unleashed Figure 5.4 Process diagram © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Earthquake Hazards and Readiness Case Study: The Sumatra-Andaman Tsunami (2004) © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Earthquake Prediction Short-term prediction and early warning • Precursor phenomena • Foreshocks Long-term forecasting • Paleoseismology: The study of prehistoric earthquakes • Seismic gaps © 2012 John Wiley & Sons, Inc. All rights reserved. Evidence of ancient quakes © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquakes and Earthquake Hazards Earthquake Hazards and Readiness Preparation and readiness to earthquakes key to reducing fatalities • Reinforced structures • Bolting wood-framed buildings to foundation • Protecting utility lines from movement Figure 5.5a Port au prince, Haiti © 2012 John Wiley & Sons, Inc. All rights reserved. Earthquake Readiness Preparation and readiness to earthquakes key to reducing fatalities • Education Figure 5.5c Schoolchildren in Japan EQ drill © 2012 John Wiley & Sons, Inc. All rights reserved. The Science of Seismology Seismographs 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 Figure 5.7a Ancient Chinese seismograph © 2012 John Wiley & Sons, Inc. All rights reserved. The Science of Seismology Figure 5.7b How a seismograph works © 2012 John Wiley & Sons, Inc. All rights reserved. The Science of Seismology Seismic Waves Body waves •Travels through Earth’s interior Surface waves •Travels along Earth’s surface Focus •Where rupture commences and an earthquake’s energy is first released Figure 5.8 Seismic waves move outward in all directions from the focus. © 2012 John Wiley & Sons, Inc. All rights reserved. The Science of Seismology Seismic Waves 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 liquids or gases – Travel slower than P waves © 2012 John Wiley & Sons, Inc. All rights reserved. Locating Earthquakes Figure 5.10a Typical seismogram showing P-, S-, and surface wave first-arrivals © 2012 John Wiley & Sons, Inc. All rights reserved. The Science of Seismology Locating Earthquakes Epicenter •The point on Earth’s surface directly above an earthquake’s focus. •Using seismograms from at least three locations will produce intersecting circles to locate the epicenter. Figure 5.10b Using seismograms to locate an earthquake epicenter. © 2012 John Wiley & Sons, Inc. All rights reserved. The Science of Seismology 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 © 2012 John Wiley & Sons, Inc. All rights reserved. The Science of Seismology Measuring Earthquakes Modified Mercalli Intensity scale • Based on descriptions of what people feel, see, and damage that has occurred. • This scale differs with distance from the epicenter. Moment Magnitude 7.0; Mercalli IX Moment Magnitude 8.8; Mercalli VIII © 2012 John Wiley & Sons, Inc. All rights reserved. Measuring Earthquakes What a Geologist Sees Richter magnitude 6 Richter magnitude 7 Richter magnitude 8 © 2012 John Wiley & Sons, Inc. All rights reserved. Studying Earth’s Interior •We cannot directly study the Earth’s interior, so remote sensing is a technique that is widely used in geology. •Seismic waves and how they travel through the Earth is one of the best methods available. © 2012 John Wiley & Sons, Inc. All rights reserved. Studying Earth’s Interior Figure 5.11a Seismic waves in Earth’s interior © 2012 John Wiley & Sons, Inc. All rights reserved. Studying Earth’s Interior How Geologists Look into Earth’s Interior Three things can happen to seismic waves when they meet a boundary: • Refraction: waves are 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. © 2012 John Wiley & Sons, Inc. All rights reserved. Studying Earth’s Interior How Geologists Look into Earth’s Interior P- and S-waves • Travel differently but consistently through the Earth creating shadow zones Seismic discontinuity • A boundary inside Earth where the velocities of seismic waves change abruptly Figure 5.11b Seismic waves in Earth’s interior © 2012 John Wiley & Sons, Inc. All rights reserved. Studying Earth’s Interior How Geologists Look into Earth’s Interior Seismic tomography • Allows geologists to image inside of Earth using 3D technologies Direct observation • Drilling • Xenoliths Indirect observation • Magnetism • Density Figure 5.14a Earth’s magnetic field © 2012 John Wiley & Sons, Inc. All rights reserved. Diamonds: Messengers from the deep a. A special photo of diamond surface to identify variations in composition b. Oppenheimer diamond from S. Africa Figure 5.13 Diamonds © 2012 John Wiley & Sons, Inc. All rights reserved. c. Kimberlite pipe through which a diamond erupts to reach the surface A Multilayered Planet The Crust •The crust is the outermost compositional layer of the solid Earth, part of the lithosphere. •Thickness ranges between 8 km (oceanic) and 45 km (continental). •95% of crust is igneous or metamorphic derived from igneous rocks. •Mohorovcic (Moho) discontinuity marks boundary between crust and upper mantle (diff. densities and compositions, but similar physical characteristics). © 2012 John Wiley & Sons, Inc. All rights reserved. A Multilayered Planet Figure 5.15 Inside view of Earth © 2012 John Wiley & Sons, Inc. All rights reserved. A Multilayered Planet The Mantle •The middle compositional layer of Earth, between the core and the crust. •It is composed primarily of olivine and pyroxene. •Asthenosphere: mantle where weak, ductile rock is near melting but not molten. •Mesosphere: part of the mantle below the asthenosphere where rocks are stronger from being highly compressed. © 2012 John Wiley & Sons, Inc. All rights reserved. A Multilayered Planet The Mantle Lithosphere •The layer above the asthenosphere includes both the crust and uppermost part of mantle. •About 100 km thick. •These rocks are stronger and more rigid than the asthenosphere. •It acts as one unit that makes up the tectonic plates. © 2012 John Wiley & Sons, Inc. All rights reserved. A Multilayered Planet Figure 5.16 Seismic discontinuities in the mantle © 2012 John Wiley & Sons, Inc. All rights reserved. A Multilayered Planet The Core Core • Innermost layer, where the magnetic field is generated and much geothermal energy resides • Separated into outer core (liquid) and inner core (solid) • Composed of primarily iron-nickel metal © 2012 John Wiley & Sons, Inc. All rights reserved. A Multilayered Planet •Heat in the core is escaping very slowly up through the surface. •Release of heat from interior is driving energy for plate tectonics. http://www.gns.cri.nz/Home/Learning/Science-Topics/Earth-Energy/Geothermal-Energy/How-is-the-heat-created © 2012 John Wiley & Sons, Inc. All rights reserved. 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? © 2012 John Wiley & Sons, Inc. All rights reserved.