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Earthquakes Chapter 5 © 2011 Pearson Education, Inc. You will learn • What earthquakes are • Where earthquakes occur • How fault ruptures generate earthquakes • How we measure and study earthquakes • What earthquake hazards are • How scientists attempt to predict earthquakes • How people mitigate earthquake hazards © 2011 Pearson Education, Inc. Earthquakes Defined • Trembling, shaking, or vibration of Earth. • Tectonic movements within lithosphere causes earthquakes and most occur along preexisting faults. • Movements generate seismic waves that travel outward in all directions from fault. • Seismic waves originate at focus and migrate outward; epicenter marks projection of focus directly to surface. Figure 5-2 Movement on Faults Causes Earthquakes © 2011 Pearson Education, Inc. Where Earthquakes Occur— Transform Boundaries • Motions along transform plate boundaries cause many large earthquakes globally • Transform plate boundaries are complex—San Andreas fault zone is a system of many connected faults that allow plates to move past each other • Main San Andreas fault cuts to basal crust and extends from a divergent plate boundary in the Gulf of California northward to near Cape Mendocino, where it merges with an offshore transform fault. Figure 5-3 The San Andreas Fault Zone Large fault system = transform plate boundary. © 2011 Pearson Education, Inc. Where Earthquakes Occur— Convergent Boundaries • Largest earthquakes on record occurred along subduction zones, where one plate slips beneath another • Reverse/thrust faults are common Figure 5-4a The Largest Earthquakes Figure 5-4c Tsunami generated from May Occur at Convergent Plate Boundaries 1960 Chilean quake of 9.5 magnitude was also devastating to Japan. © 2011 Pearson Education, Inc. Where Earthquakes Occur— Divergent Boundaries • Pulling apart or extension of the lithosphere and normal faulting along these oceanic divergent plate boundaries causes many shallow, smallto-moderate-size earthquakes along MORs • Pull-apart basins or rifts are also common on land with some quakes larger than Figure 5-5a those at the MORs Earthquakes at Divergent Plate Boundaries Earthquakes are common in the East African rift zone, where normal faulting marks new divergent plate boundaries. © 2011 Pearson Education, Inc. Basin and Range— A Divergent History Figure 5-6 Basin and Range Province—Crust Extended (a) The Basin and Range is centered on Nevada. (b) Normal faults occur between long mountain ranges and intervening valleys, or basins. (c) Displacement on normal faults generates many small-to-moderate size earthquakes and creates surface scarps. © 2011 Pearson Education, Inc. Where Earthquakes Occur—Not all along Boundaries: Intraplate Locations • Central Mississippi Valley experiences many small-tomoderate-size intraplate earthquakes. • Map shows epicenters of earthquakes recorded since 1974. Figure 5-7 Earthquakes in the Mississippi Valley © 2011 Pearson Education, Inc. Elastic Rebound Theory Figure 5-8 The Elastic Rebound Theory Explains Fault Rupture and Earthquake Generation Sudden release of elastic strain that built up along a fault releases seismic energy, causing earthquakes (right). Theory developed from observations of displacements on San Andreas fault in 1906 such as the offset fence line on left. © 2011 Pearson Education, Inc. Creepy Faults Figure 5-9 The Hayward Fault Creeps Part of the San Andreas Fault system, the Hayward Fault goes from goalpost to goalpost across the UC Berkeley football stadium (left). The creeping movement on the fault is very slowly tearing the stadium apart (right). © 2011 Pearson Education, Inc. Earthquakes and Faults— Body Waves • Body waves travel through the interior of Earth (i.e., through rock). • Body waves include Primary (P) (a) and Secondary (S) (b) waves • P waves are known also known as compressional (think P for Pressure) waves • S waves are known also as Shear waves • P waves can be transmitted through both liquid and solids • S waves can be transmitted only through solids (shear cannot be transmitted through liquid) Figure 5-10a and b How Seismic Waves Pass through Rock © 2011 Pearson Education, Inc. Earthquakes and Faults— Surface Waves • When body waves approach the Earth’s surface, a part of their energy is converted into surface waves • Surface waves travel along Earth’s outer edges rather than through its interior, as in the case of body waves • Love waves (c) and Rayleigh waves (d) are surface waves Figure 5-10c and d How Seismic Waves Pass through Rock © 2011 Pearson Education, Inc. Investigating Earthquakes • Satellites in space are used to measure changes in land surface in seismic zones such as upwarps or downwarps • Seismometers are used to measure magnitudes of earthquakes and locate fault ruptures associated with quakes • Seismometers measure vertical and horizontal earth vibrations • A seismogram is a recording of ground motions caused by seismic waves Figure 5-11 Earthquake Vibrations Are Recorded by a Seismometer © 2011 Pearson Education, Inc. How Big Was That Quake?—Magnitude Figure 5-13 Amount of Energy Released by Earthquakes of Different Magnitudes Comparing amount of energy released by earthquakes to energy associated with other natural and human-produced phenomena helps us understand how incredibly powerful they can be. The figure also gives the energy equivalents of moment magnitudes. © 2011 Pearson Education, Inc. Quantifying the Magnitude • Moment magnitude, Mw (the “w” stands for “work”) = numerical scale of the amount of energy released by an earthquake. • Mw calculated on the basis of: 1) the total area of the fault rupture, 2) how far the rocks move along the fault during quake, and 3) the strength of the rock that ruptures. • These 3 aspects of an earthquake are related to long-period seismic waves recorded on seismograms. a good estimate of the seismic energy released by earthquakes, especially very large ones. • On the Mw scale a change of one represents a change of 31.6 times the power released. © 2011 Pearson Education, Inc. Intensity—the Degree of Shaking Mercalli Scale can be used to make a comparison between the shaking and damage © 2011 Pearson Education, Inc. Modified Mercalli Intensity Maps Figure 5-14 Mercalli Intensity Maps of Two Famous Bay Area Earthquakes • Maps show that the 1906 earthquake (far left) was much more extensive and destructive than was the 1989 Loma Prieta quake in California. • Red areas show highest intensities between IX and X. © 2011 Pearson Education, Inc. Locating Earthquakes Figure 5-15 Locating the Epicenter of an Earthquake P and S arrival times and triangulation with 3 or more stations is used to locate the epicenter. • Analyzing the difference in arrival times of P and S waves at several different seismometers allows the epicenter to be identified. • Since travel time of P and S waves is different the distance away from a given recording station can be determined. • Depths of quakes can also be determined using a 3-D analysis similar somewhat to method for finding the epicenter. © 2011 Pearson Education, Inc. Building Design and Earthquakes Figure 5-16 Earthquakes Don’t Kill People—Collapsing Buildings Do (a) Bam, Iran, before earthquake of December 26, 2003, (top) and afterward (below), reduced to piles of mud bricks. Magnitude 6.6 earthquake caused 41,000+ deaths. (b) Northridge, California, earthquake of 1994, though equally strong, claimed only 57 lives. © 2011 Pearson Education, Inc. Earthquake Hazards— Ground Shaking Factors to consider: • Magnitude—quake size • Distance from focus—shallow or deep? • Site geology—consolidated or not? © 2011 Pearson Education, Inc. Earthquake Hazards—Ground Displacement and Failure Factors to Consider: • Liquifaction • Slope Failure • Surface Ruptures • Crustal Deformation Figure 5-17 Kobe, Japan Liquefaction of soft fill after a 6.9. Figure 5-18 Earthquakes May Trigger Landslides Peru devastated by a large mud and rock flow caused by a 1970 earthquake in the Andes. © 2011 Pearson Education, Inc. Surface Ruptures Figure 5-19 Surface Rupture Surface rupture can permanently scar the landscape and cause great damage. Views captured after the great Anchorage, Alaska earthquake of 1964. © 2011 Pearson Education, Inc. Earthquake Hazards—Tsunami • On December 26, 2004, a subduction zone earthquake of Mw 9.1— 3rd largest ever recorded—occurred off coast of Sumatra, Indonesia • 225,000 people in 11 countries perished in this great tsunami Figure 5-20 Earthquakes May Cause Tsunami (a) Tsunami form when water is displaced above a sudden offset on the seafloor; (b) As they enter shallow water, tsunami slow down and gain height. © 2011 Pearson Education, Inc. Tsunami Destruction— December 26, 2004 Figure 5-21 Tsunami Devastation—before (on left) and after (on right). Note nearly complete wiping out of landscape. © 2011 Pearson Education, Inc. Earthquake Hazards—Fire • Greatest hazards to urban areas stricken by earthquakes • Great 1906 San Francisco earthquake—fires burned 3 days, devastating 12 square kilometers (4.6 mi2) • Great 1923 Tokyo earthquake, tens of thousands of people perished in fires. Figure 5-22 Fires Commonly Are Major Secondary Effects of Earthquakes 1906 San Francisco fires (left), 1995 Kobe, Japan, fires (right) © 2011 Pearson Education, Inc. Earthquake Predictions • Ability to accurately predict the size, location, and time of earthquakes remains elusive even with today’s technology and information. • Limited success of prediction based on a variety of possible warning signs or precursors such as animal behavior. • Prediction has been superseded by a more achievable goal of making long-term, lessspecific earthquake forecasts. © 2011 Pearson Education, Inc. Seismic Gaps Figure 5-23 Earthquake history: Strike-Slip Fault in Turkey Quakes migrated E to W over 60 yrs; lack of activity in W identified a seismic gap—marking vicinity of a Mw 7.6 quake in 1999. Figure 5-24 Damage in Izmit, Turkey—1999 17,000 deaths © 2011 Pearson Education, Inc. Recurrence Intervals and Predictions • Records of past earthquakes, including the size of the rupture, allow us to identify where future earthquakes are likely to occur. • The big question is “When?” • Long-term records of quake size and characteristics establish the average time, or recurrence interval. FIGURE 5-25 Waiting for the Big One Parkfield, California, residents were ready for the predicted earthquake, but it arrived 15 yrs late. © 2011 Pearson Education, Inc. Making Earthquake Forecasts Figure 5-26 Looking to the Future Earthquake forecasts give probability that a quake will occur on a fault per specific time period. Globally few areas have been sufficiently studied to justify a forecast. Two areas of exception however are: San Francisco Bay Region (a) and Los Angeles area (b). © 2011 Pearson Education, Inc. Mitigating Earthquake Hazards through Mapping Figure 5-27 The USGS National Shaking Hazard Map Colors on map show levels of horizontal shaking that have a 10% chance of being exceeded in a 50-yr period. Shaking = % acceleration of falling object due to gravity (g). Highest hazards are orange to red zones with 16–32+% s. • USGS maintains national maps showing peak horizontal ground motion accelerations expected from earthquakes • Maps are part of the National Earthquake Hazard Reduction (NEHRP) program • Mapping reveals major seismic hazard zones such as western states and New Madrid region © 2011 Pearson Education, Inc. Engineering for Earthquakes • Geologists mapped fault location and size of expected earthquakes on it. • Engineers designed pipeline for crossing the fault. The design worked. • Pipeline withstood 5.5 m. • (18 ft) of horiz. and 1 m (3 ft) of vert. surface displacement caused by the November 3, 2002, Mw 7.9 earthquake on Figure 5-28 Trans-Alaska Pipeline fault. System (TAPS) Crosses the Active StrikeSlip Denali Fault © 2011 Pearson Education, Inc. Being Prepared for Earthquakes Being prepared includes: • Have a planned emergency response • Having an early warning system • Providing public education and training for preparedness Figure 5-29 Map of Shaking Intensity Created Immediately after the Northridge Earthquake in California Emergency services to tens of thousands of people in the Northridge area were efficiently provided and allocated for based on the produced map. © 2011 Pearson Education, Inc. Can We Improve the Tsunami Warning System? • Communication is critical throughout the world in the event of a large earthquake occurring in the ocean. • Despite a network of warning systems, the December 26, 2004, tsunami severely affected many countries around the Indian Ocean and killed over 225,000 people. • This was more than any other tsunami in recorded history. © 2011 Pearson Education, Inc. SUMMARY • Movements in the dynamic geosphere cause earthquakes—most along plate boundaries but significant intraplate quakes also occur • Earthquakes present many hazards, in some cases even far away from where they occur such as via development of tsunami • Elastic rebound theory explains the relationship between earthquakes, tectonic stresses, and faulting. © 2011 Pearson Education, Inc. SUMMARY (cont.) • Seismic energy moves as body waves or surface waves • Seismometers record ground motions from seismic waves • Scales: Richter, and more accurate Moment Magnitude (Mw), ranks earthquakes based on ∑seismic energy released, and Modified Mercalli Intensity • Major hazards include liquefaction, permanent surface displacements, landslides, tsunamis, and fires © 2011 Pearson Education, Inc.