1. January 24, 2008 1 Earthquake Engineering GE / CEE - 479/679 Lecture 2. Plate Tectonics January 24, 2008 John G. Anderson Professor of Geophysics

2. January 24, 2008 2 Handout Today Chapter 2. Plate Tectonics –Students are advised to read this as your textbook on this topic. –Solve exercises 3, 4, 5, 6, and 8. –For extra credit, solve exercises 9, 11. –Exercises are due Jan. 31.

3. Source: Geological Society of America web: http://www.geosociety.org/science/timescale/timescl.htm

4. Source: Geological Society of America web: http://www.geosociety.org/science/timescale/timescl.htm

5. January 24, 2008 5 http://www.physics.mcgill.ca/~crawford/PSG/PSG12/204_97_L12.2_earthxn.html Crust - rocks (granite, basalt); density ~ 3 g/cm 3 Lithosphere - "rock-sphere" - rigid layer (includes crust) Asthenosphere - "weak-sphere" (lower v seismic waves) Convection currents flow in the 'plastic' asthenosphere, and the plates of the lithosphere ride on these. Mantle - iron-rich, mainly solid rock Outer core - liquid iron and nickel Inner core - solid iron, nickel Distances are in kilometers.

6. January 24, 2008 6 The radii for the tops of the major layers are more accurate on this plot.

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9. January 24, 2008 9 http://topex.ucsd.edu/marine_topo/gif_images/global_topo_small.gif

10. January 24, 2008 10 http://www2.ocean.washington.edu/oc540/lec02-1/

11. January 24, 2008 11 http://www2.ocean.washington.edu/oc540/lec02-1/

12. January 24, 2008 12 Why believe plate tectonics? Geography - matching continental boundaries. Geology - units match where continental boundaries match. Paleomagnetism –Characteristics of the magnetic field –Polar wander –Mid-ocean ridge magnetic anomalies. –Past and future Earth.

13. January 24, 2008 13 Why believe plate tectonics? Ocean drilling results. Hot spots Earthquakes Direct measurements

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26. http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html

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31. January 24, 2008 31 Direct measurement using the Global Position System

34. January 24, 2008 34 Engineering applications Context to understand seismicity Quantitative estimate of earthquake rates Seismic gaps

35. January 24, 2008 35 Distance between two points on the Earth’s surface Where: (φ, ψ) = (latitude, longitude) of selected point on plate boundary (φ 1, ψ 1 ) = (latitude, longitude) of pole of relative rotation

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38. January 24, 2008 38 u = relative rate of plate motion a = radius of Earth, 6370 km Δ = distance from pole of rotation, in degrees ω = rotation rate of plates about their pole (a different use of the symbol ω)

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41. January 24, 2008 41 Seismic Moment Definition of Seismic Moment M 0 =μAD –μ is the shear modulus of the rock –A is the area of the fault on which slip takes place –D is the average slip on the fault

42. January 24, 2008 42 Application to Japan Accept for the time that the plates in Japan converge at u=10.6 cm/yr. –This is a horizontal convergence rate. –. The last large earthquake at Tokyo was 1923. –Time past is (2005-1923)=84 years. Then accumulated slip is: –10.6 cm/yr*82 yr=890 cm.

43. January 24, 2008 43 Application to Japan (cont.) Then accumulated slip is: –10.6 cm/yr*82 yr=890 cm. Suppose the fault is 200 km long and the seismogenic zone is from 0 to 30 km depth. –The subduction zone dips, lets say at 40 o –Width = 30 km/cos(dip)=40 km M 0 =μAD = 4*10 11 dyne/cm 2 * 200 km * 40 km * (10 5 cm/km) 2 * 890 cm = 2.8 * 10 28 dyne-cm

44. January 24, 2008 44 Moment Magnitude M W =(2/3) log M 0 -10.73 This is the preferred magnitude scale in the seismological community.

45. January 24, 2008 45 Application to Japan (cont.) M 0 =μAD =2.8 * 10 28 dyne-cm M W =(2/3) log M 0 -10.73 =8.24 Conclusion, by these assumptions, there is enough strain accumulated to cause a M W =8.2 earthquake under Tokyo.