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Chapter 4 Plate Tectonics and Earthquakes Lecture PowerPoint 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plate Tectonics and Earthquakes China, May 12, 2008: • Major earthquake great natural disaster – Many poorly constructed buildings collapsed • Nearly 90,000 people killed Insert Figure 4.1 here Figure 4.1 Plate Tectonics and Earthquakes Most earthquakes can be explained by plate tectonics: • Divergent plate boundaries – Divergent motion and high temperatures cause rocks to fail easily in tension – Earthquakes are small and generally non-threatening • Transform plate boundaries – Plates slide past each other in horizontal movement, retarded at irregularities in plate boundaries – Energy required to move plates is released as large earthquakes • Convergent plate boundaries – Great amounts of energy are required to pull a plate back into the mantle or push continents together – Largest earthquakes are generated at convergent boundaries Plate Tectonics and Earthquakes Figure 4.2 Plate Tectonics and Earthquakes Examine example of Pacific plate: • Created at spreading centers on eastern and southern edges, producing small earthquakes • Slides past other plates on transform faults (Queen Charlotte fault, Canada; San Andreas fault, California; Alpine fault, New Zealand), generating large earthquakes • Subducts along northern and western edges, generating enormous earthquakes Figure 4.3 Spreading Center Earthquakes Iceland • Volcanic island fed by hot spot along the mid-Atlantic ridge • Swarms of moderate earthquakes too small to destroy buildings or kill people Figure 4.4 Figure 4.5 Spreading Center Earthquakes Red Sea and Gulf of Aden • Young spreading center, new ocean basin • Hot upper mantle under Africa melts and uplifts crust, which gravity then pulls apart and downward, creating pull-apart basins (or rift valleys) • As down-dropped pull-apart basins widen, are flooded by ocean to form new arm • At south end of Red Sea, three pull-apart basins meet at triple junction • Spreading has split Arabian plate from Africa • East African Rift Valley may someday split ‘Somali’ plate from African plate Figure 4.6 Spreading Centers and Earthquakes Figure 4.7 Spreading-Center Earthquakes • Spreading center underlies Gulf of California, extends north into U.S., including Salton Sea, Coachella and Imperial Valleys • Salton Trough: – High heat flow, glassy volcanic domes, boiling mud pots, geothermal reservoirs, earthquake swarms from moving magma – One of most earthquake-active U.S. areas – Brawley seismic zone – often hundreds of small earthquakes over few days (swarms), continually releasing stress rather than building for big earthquake – Other nearby faults generate larger earthquakes, up to magnitude 7 Figure 4.9 Convergent Zones and Earthquakes • Largest earthquakes • Three types of convergence: – Ocean-ocean: older, denser oceanic plate is subducted – Ocean-continent: oceanic plate is subducted – Continent-continent: both plates are too buoyant to be subducted; continental upheaval results Subduction Zone Earthquakes Subduction Zones: • Sites of great earthquakes • Shallow earthquakes: – Compressive movements of overriding plate and subducting plate – Pull-apart movements where subducting plate bends downward – Most damaging earthquakes • Inclined plane of deep earthquakes, defining descending slab of oceanic lithosphere – Rigid interior of slab can stay cold enough to generate earthquakes down to depths of 700 km – Most seismic energy is dissipated before reaching surface Subduction Zone Earthquakes Seismic-Gap Method: • If some segments of a fault have moved recently, it is reasonable to expect that unmoved portions will move next, to fill the gaps • Yields expectations, not guarantees; segments may move in two or more earthquakes before adjacent unmoved segments move once Figure 4.11 Figure 4.10 Subduction Zone Earthquakes Indonesia, 2004 to 2009 • Indian-Australian plate collides with Indonesia at 5.3-5.9 cm/yr frequent, huge subduction-caused earthquakes, frequent tsunami • December 26, 2004: 1,200 km long fault ruptured over 11 minutes • March 28, 2005: 400 km rupture south of earlier rupture • Additional quakes in 2007, 2008 and 2009 • Earthquake cluster is under way • More seismic gaps to fill Subduction Zone Earthquakes Tokyo, Japan, 1923 • One of world’s most deadly disasters (probably about 144,000 people killed) • Series of earthquakes, with principal one worst of year globally • Tsunami 11 m high hit city • Fires raced through city for 2½ days, destroying 71% of Tokyo and all of Yokohama – 38,000 people were killed by fire, crowded into a park that was consumed by fire from three sides • Region SW of Tokyo has had five very strong earthquakes in last 400 years – about every 73 years, most recent in 1923 Subduction Zone Earthquakes Insert Table 4.1 Subduction Zone Earthquakes • Subduction of Pacific plate under Alaska creates truly large earthquakes (not San Andreas fault) • The Good Friday Earthquake, Alaska, 1964 – Major subduction movement created magnitude 9.2 earthquake, as Pacific slab shoved under Alaska in seven lurches – Seven minutes of shaking induced avalanches, landslides, ground settling and tsunami, killing 131 people – Relatively low loss of life because area is sparsely inhabited, few people downtown, low tide and warm weather Subduction Zone Earthquakes Mexico City, 1985 • Subduction of Cocos plate under North America created magnitude 8.1 earthquake, with magnitude 7.5 and 7.3 aftershocks • Earthquake was 350 km from Mexico City, but intense damage • 8,000 people were killed by building collapses • Earthquake was expected Michoacan seismic gap • Guerrero seismic gap still exists Figure 4.13 Subduction Zone Earthquakes Mexico City, 1985 • Earthquakes Don’t Kill, Buildings Do • 5,700 buildings in Mexico City were damaged, while small towns closer to the epicenter had much less damage Figure 4.12 Figure 4.14 Subduction Zone Earthquakes Mexico City, 1985 • Earthquakes Don’t Kill, Buildings Do • Resonance between seismic waves, soft lake-sediment foundations and improperly designed buildings, all vibrating most intensely at 1 to 2-second periods, destroyed many buildings Figure 4.16 Figure 4.15 Subduction Zone Earthquakes Pacific Northwest, The Upcoming Earthquake • Cascadia subduction zone – small plates subducting under North America • Why no major earthquakes in last 200 years in Pacific Northwest? – Has subduction stopped? • No, because volcanism still occurs Figure 4.17 – Are plates subducting so smoothly that earthquakes don’t occur? • Probably not, since subducting lithosphere is so young Subduction Zone Earthquakes Pacific Northwest, The Upcoming Earthquake • Similar subduction setting to Japan and Chile – Japan’s subduction zone relieved stress with two magnitude 8.1 earthquakes in 1944 and 1946 – Chilean subduction zone generated world’s largest measured earthquake in 1960, magnitude 9.2 – 1960 Chilean events would be comparable to earthquake series that shifts entire Cascadian subduction zone • Could Cascadia have a magnitude 9 earthquake? – Yes, it has and will again Subduction Zone Earthquakes Earthquake in 1700 • Last major earthquake in Pacific Northwest was about magnitude 9, in 1700 – Trees in forests drowned, died by salt-water flooding when ground dropped in 1700 – Tsunami hit Japan in 1700 – might have been caused by magnitude 9 earthquake along Washington-Oregon coast • Effects of magnitude 9 earthquake: Figure 4.18 – Three to five minutes of intense shaking – Tsunami 10 m high, 15 to 40 minutes after earthquake Convergent Zones and Earthquakes Continent-continent collisions • India has moved 2,000 km north into Asia since initial contact – Pre-collision, Indian and Asian crusts were 35 km thick – Crust under Tibetan plateau is 70 km thick, highest continental area on Earth Figure 4.19 Convergent Zones and Earthquakes Continent-continent collisions • India continues to move 5 cm/year into Asia, along 2,000 km front, affecting India, Pakistan, Afghanistan, Tibetan Plateau, eastern Russia, Mongolia and China with great earthquakes, and pushing parts of China to east and southeastern Asia farther to southeast Figure 4.20 Figure 4.21 Convergent Zones and Earthquakes • China, Pakistan and India, 2008, 2005 and 2001 – Caused by India’s collision into Asia – May 2008 earthquake in Sichuan, China killed 87,500 people – October 2005 earthquake in Kashmir, Pakistan killed 88,000 people – January 2001 earthquake in Gujarat, India killed more than 20,000 people – Seismic gaps (such as 600 km long section of central Himalayas) throughout region – Rapid population growth in region, sporadic seismicsafety codes, poor construction practices – None of recent earthquakes were direct hit on major cities Convergent Zones and Earthquakes • Shaanxi province, China, 1556 – Deadliest earthquake in history: about 830,000 people killed – Soft soil (with little cohesion) made caves practical homes, but shaking caused ground and caves to collapse • Tangshan, China, 1976 – Deadliest earthquake in recent history: more than 240,000 people killed – Dense mining city of 2 million, with most buildings of mudbrick built under lenient building codes 93% of residential buildings collapsed – Earthquake weather: • No connection between fault movements many kilometers underground and weather above the surface • Shaanxi and Tangshan earthquakes during opposite kinds of weather Transform Faults and Earthquakes Arabian Plate: • Spreading in Red Sea and Gulf of Aden pushes Arabian plate into Eurasia, uplifting mountains and creating large earthquakes Figure 4.22 The Arabian Plate Continent-continent collision earthquakes: • Iran, 1962-2009 – In last 48 years, 10 earthquakes killed over150,000 people in Iran – December 2003: Construction methods of mud-brick walls with heavy roofs caused collapsing homes to kill 41,000 people – “Earthquakes don’t kill people, buildings do” The Arabian Plate Transform fault earthquakes: • On western side, Arabian plate slides past African plate at edge of Mediterranean Sea along Dead Sea fault zone – Runs through Holy Land – Steps in fault zone have created pull-apart basins that hold Dead Sea, Sea of Galilee – Dead Sea fault zone generates magnitude 6-7 earthquakes about every 100-200 years Figure 4.23 The Arabian Plate Transform fault earthquakes: • On western side, Arabian plate slides past African plate at edge of Mediterranean Sea along Dead Sea fault zone Figure 4.24 Transform Faults and Earthquakes • Horizontal movements cause major earthquakes • Turkey, 1999: – Segment of North Anatolian fault ruptured for 120 km in magnitude 7.4 earthquake near Izmit, followed weeks later by rupture to the east in magnitude 7.1 earthquake – Residential buildings on soft ground, adding sand to concrete resulted in buildings collapsing during shaking – 19,000 people killed, $20 billion in damages Transform Faults and Earthquakes • Turkey, 1999: – As Arabian plate pushed in Eurasia, Turkey is pushed westward, rotated counter-clockwise in escape tectonics along the North Anatolian fault, which runs for 1,400 km along the Black Sea – Since 1939, North Anatolian fault has ruptured in 11 earthquakes greater than 6.7, from east end of fault to west • Unique, semi-regular pattern • Next event? Probably to west of Izmit, closer to Istanbul • Probably within next 30 years Transform Faults and Earthquakes • Turkey, 1999 Figure 4.26 San Andreas Fault Tectonics • Opening of Atlantic Ocean basin pushes North and South America west • Farallon plate subducts beneath North America • By 28 million years ago, enough of the Farallon plate has subducted under North America for the Pacific spreading center to make contact with North America Figure 4.27 San Andreas Fault Tectonics • Remaining spreading centers to the north and south are connected by a transform fault ancestor of San Andreas fault • In last 5.5 million years, Gulf of California has opened 300 km, tearing peninsular California (Baja California and California west of the San Andreas fault) away from the North American plate and on the Pacific plate Figure 4.27 Side Note: San Francisco , 1906 Home to about 400,000 people • Intense ground shaking for about one minute in early morning hours • Damage much worse in areas constructed on artificial fill rather than rock or consolidated sediment • Many fires broke out and water lines were destroyed, making fire-fighting impossible – ten times more damage than shaking San Andreas Fault Tectonics • Different sections of the fault behave differently: – South of 1906 section, between San Juan Bautista and Cholame: • Frequent small to moderate earthquakes and creep (millimeters per year of ongoing offset) – South of creeping section, between Cholame and San Bernardino: • Locked section with recent deficit of earthquake activity • Most recent large rupture in 1857 Fort Tejon earthquake of magnitude 7.9 – South of locked Fort Tejon section: • Complex zone with locked sections San Andreas Fault Tectonics World Series (Loma Prieta) Earthquake, 1989 • Just prior to scheduled start of Game 3 of the World Series, at Candlestick Park of San Francisco • Rupture of southernmost 42 km of 1906 section • Fault rupture was deep, did not offset ground surface • Fault movement was vertical and horizontal, not just horizontal • Rupture lasted only 7 seconds (short for Figure 4.33 magnitude 7.0 earthquake) San Andreas Fault Tectonics World Series (Loma Prieta) Earthquake, 1989 • Occurred at bend in San Andreas fault near Santa Cruz Mountains’ highest peak, Loma Prieta – Bend in fault uplifted Santa Cruz Mountains • At bend, fault plane is inclined at 70 degrees (not vertical), so fault movement shifted Pacific plate 1.9 m to north, and uplifted Santa Cruz Mountains 36 cm Figure 4.32 San Andreas Fault Tectonics World Series (Loma Prieta) Earthquake, 1989 • Mainshock was magnitude 7.0, numerous aftershocks • Loma Prieta region was seismic gap before earthquake – Mainshock and aftershock filled in seismic gap • Destruction was much less than could have been, as shaking lasted only 11 seconds – 67 people killed, 3,757 people injured, 12,000 people homeless – $6 billion in damages Figure 4.34 San Andreas Fault Tectonics World Series (Loma Prieta) Earthquake, 1989 • Earthquakes don’t kill, buildings do • Near epicenter, damage was mostly to older and poorly designed buildings • Seismic waves were amplified by soft muds and artificial-fill foundations around San Francisco Bay • Ground motion on soft-sediment sites was 10 times stronger than at nearby rock sites • Farther from epicenter, considerable damage to poorly sited or poorly designed structures – Marina District – Interstate 880 San Andreas Fault Tectonics World Series (Loma Prieta) Earthquake, 1989 • Marina District Figure 4.35 San Andreas Fault Tectonics World Series (Loma Prieta) Earthquake, 1989 • Marina District – Built on fill from debris from 1906 earthquake • Amplified shaking • Deformation and liquefaction of artificial-fill foundations • Soft first-stories – ground floors of buildings cleared out to make room for parking, leaving building without adequate support so rest of building pancakes onto first floor San Andreas Fault Tectonics World Series (Loma Prieta) Earthquake, 1989 • Interstate 880 – 42 people killed when upper deck collapsed onto lower deck Figure 4.36 San Andreas Fault Tectonics World Series (Loma Prieta) Earthquake, 1989 • Interstate 880 – Section where collapse occurred was built on soft muds (other sections on sand and gravel survived) – Freeway’s natural resonance was amplified by shaking of muds – Could have been predicted by collapses of freeways in 1971 San Fernando earthquake Figure 4.37 San Andreas Fault Tectonics Bay Area Earthquakes – Past and Future • Historic records for last 225 years (good records only last 160 years) – Major earthquake recurrence interval is more than 160 years • 19th century earthquake history quite different than 20th century – 19th century: Seven greater than 6 magnitude earthquakes before 1906 earthquake – 20th century: No large earthquakes until 1970s Figure 4.39 San Andreas Fault Tectonics Bay Area Earthquakes – Past and Future Pattern 1: common large earthquakes vs. rare giant earthquakes • Plate tectonic movement is satisfied by either one magnitude 8 earthquake in a century or one magnitude 6-7 earthquake every decade Pattern 2: East-west pairings of earthquakes • In 1836, Monterey Bay area had earthquake about magnitude 6.5; in 1838, San Francisco peninsula had earthquake of magnitude 6.8 • In 1865, Santa Cruz had a large earthquake; in 1868, Hayward had a Figure 4.39 magnitude 6.9 earthquake San Andreas Fault Tectonics Bay Area Earthquakes – Past and Future Pattern 3: northward progression of earthquakes • Earthquakes of 1865 and 1868 on the Hayward fault were preceded by five moderate earthquakes that moved northward up the Calaveras fault – Area of Hayward fault today is populated by 2 million people, with schools, hospitals, city halls, houses and the University of California built on the fault itself San Andreas Fault Tectonics Kobe, Japan, 1995 Versus Oakland, California, 20?? • Most expensive earthquake in history • Magnitude 6.9 earthquake with 50 km long rupture of Nojima fault and 100 seconds of shaking • Tile roofs and little lateral support caused collapse of buildings causing 6,308 fatalities • More than 140 fires resulted Figure 4.41 San Andreas Fault Tectonics Kobe, Japan, 1995 Versus Oakland, California, 20?? • Hayward fault has 27% probability of causing magnitude 6.7 or greater earthquake before 2032 • San Francisco Bay region, 62% probability of 6.7 magnitude earthquake on a fault before 2032 • Hayward fault and Nojima fault: – About 50 km long, densely populated areas, with large areas of weak ground materials – Both capable of generating magnitude 7 earthquakes Figure 4.43 End of Chapter 4