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Geotechnical Earthquake Engineering by Dr. Deepankar Choudhury Professor Department of Civil Engineering IIT Bombay, Powai, Mumbai 400 076, India. Email: [email protected] URL: http://www.civil.iitb.ac.in/~dc/ Lecture - 7 Module – 3 Engineering Seismology IIT Bombay, DC 2 Plate Tectonics IIT Bombay, DC 3 Layers of the Earth Crust: Continental crust (25-40 km) Oceanic crust (~6 km) Mantle Upper mantle (650 km) Lower mantle (2235 km) Core Outer core: liquid (2270 km) Inner core: solid (1216 km) Values in brackets represent the approximate thickness of each layer IIT Bombay, DC 4 Layers of the Earth The earth is divided into four main layers: Inner core, outer core, mantle and crust. The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulphur (S). The inner core is under such extreme pressure that it remains solid. Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner. The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle, so it can fracture in earthquakes. IIT Bombay, DC 5 Continental drift Theory that continents and plates move on the surface of the Earth proposed by Alfred Wegener in 1915. Alfred Wegener IIT Bombay, CE 684, CE 402, DC 6 Maps by Wegener (1915), showing continental drift IIT Bombay, DC 7 Theory of continental drift IIT Bombay, DC 8 Evidence for continental drift • • • • • Matching coastlines Matching mountains Matching rock types and rock ages Matching glacier deposits Matching fossils IIT Bombay, DC 9 Evidence for continental drift Matching coastlines IIT Bombay, DC 10 Evidence for continental drift Matching mountain ranges IIT Bombay, DC 11 Evidence for continental drift Matching rock types and ages of rocks IIT Bombay, DC 12 Evidence for continental drift Matching glacier deposits 300 million years ago IIT Bombay, DC 13 Theory of Plate tectonics • The theory of Plate tectonics was proposed in 1960s based on the theory of continental drift. • This is the Unifying theory that explains the formation and deformation of the Earth’s surface. • According to this theory, continents are carried along on huge slabs (plates) on the Earth’s outermost layer (Lithosphere). • Earth’s outermost layer is divided into 12 major Tectonic Plates (~80 km deep). These plates move relative to each other a few centimeters per year. IIT Bombay, DC 14 Earth’s magnetic field Basic Data used in formulating plate tectonics: Magnetic stripes on the seafloor. Magnetic field of Earth reverses on semiregular basis. Minerals act like compass needles and point towards magnetic north. “Hot” rocks record the direction of the magnetic field as they cool. IIT Bombay, DC 15 Tectonic plates of Earth IIT Bombay, DC 16 Tectonic plates of Earth Subduction zone Strike-slip (transform) IIT Bombay, DC Uncertain plate boundary Ridge axis faults 17 Major Earthquakes IIT Bombay, DC 18 Types of plate boundaries Divergent plate boundaries: where plates move apart Convergent Plate boundaries: where plates come together Transform plate boundaries: where plates slide past each other IIT Bombay, DC 19 Types of plate boundaries Divergent (Tension) Convergent (Compression) Transform (shearing) IIT Bombay, DC 20 Types of plate boundaries IIT Bombay, DC 21 Divergent Plate Boundaries • Plates move away from each other (tension) • New lithosphere is formed • normal faults • Causes volcanism • not very explosive IIT Bombay, DC 22 Divergent Plate Boundaries Upwarping Continental Crust IIT Bombay, DC 23 Divergent Plate Boundaries Rift Valley IIT Bombay, DC 24 Divergent Plate Boundaries Linear Sea IIT Bombay, DC 25 Convergent Plate Boundary • • • • • Plates move toward each other (compression) lithosphere is consumed reverse/thrust faults and folds Mountain building explosive volcanism IIT Bombay, DC 26 Ocean- Continent convergent margin • Ocean-continent plates collide • Ocean plate subducts below continent • Forms a subduction zone • Earthquakes and volcanoes IIT Bombay, DC 27 Ocean-ocean convergent margin • 2 oceanic plates collide • One plate dives (subducts) beneath other • Forms subduction zone • Earthquakes and volcanoes IIT Bombay, DC 28 Continent-continent convergent margin • 2 continental plates collide • Neither plate wants to subduct • Collision zone forms high mountains • Earthquakes, no volcanoes example: Himalayas IIT Bombay, DC 29 Transform plate margin • Two plates slide past each other • strike slip faults. • Lithosphere is neither consumed nor destroyed. • Earthquakes, no volcanoes • Responsible for most of the earthquakes IIT Bombay, DC 31 What drives plate movement? • Ultimately: heat transported from core and mantle to surface • Heat transported by convection • Core is ~5,000°C and surface is ~0°C • Where mantle rises: rifting • Where mantle dives: subduction zones IIT Bombay, DC 32 Types of Faults and Seismic Waves IIT Bombay, DC 33 Elastic Rebound Theory • This theory was discovered by making measurements at a number of points across a fault. • Prior to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake. Elastic Rebound Theory Sequence of elastic rebound: Stresses Sequence of elastic rebound: Bending Sequence of elastic rebound: Rupture Sequence of elastic rebound: Rebound Sudden Slip by Elastic Rebound • Stresses (force/area) are applied to a fault. • Strain (deformation) accumulates in the vicinity of friction-locked faults. • Strain accumulation reaches a threshold and fault slips suddenly • Rupture (slip) continues over some portion of the fault. Slip is the distance of displacement along a fault. Fault A fracture (crack) in the earth, where the two sides move past each other and the relative motion is parallel to the fracture. 90˚ dip = vertical fault plane, 0˚ strike = north parallel fault plane