Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Age of the Earth wikipedia , lookup
Seismic inversion wikipedia , lookup
History of geomagnetism wikipedia , lookup
Major explorations after the Age of Discovery wikipedia , lookup
History of geology wikipedia , lookup
Earthquake engineering wikipedia , lookup
Magnetotellurics wikipedia , lookup
Earthquake Engineering GE / CEE - 479/679 Lecture 2. Plate Tectonics January 24, 2008 John G. Anderson Professor of Geophysics January 24, 2008 1 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. January 24, 2008 2 Source: Geological Society of America web: http://www.geosociety.org/science/timescale/timescl.htm Source: Geological Society of America web: http://www.geosociety.org/science/timescale/timescl.htm Crust - rocks (granite, basalt); density ~ 3 g/cm3 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. http://www.physics.mcgill.ca/~crawford/PSG/PSG12/204_97_L12.2_earthxn.html January 24, 2008 5 The radii for the tops of the major layers are more accurate on this plot. January 24, 2008 6 January 24, 2008 7 http://topex.ucsd.edu/marine_topo/gif_images/global_topo_small.gif January 24, 2008 9 http://www2.ocean.washington.edu/oc540/lec02-1/ January 24, 2008 10 http://www2.ocean.washington.edu/oc540/lec02-1/ January 24, 2008 11 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. January 24, 2008 12 Why believe plate tectonics? • • • • Ocean drilling results. Hot spots Earthquakes Direct measurements January 24, 2008 13 January 24, 2008 14 January 24, 2008 15 January 24, 2008 16 January 24, 2008 17 January 24, 2008 19 January 24, 2008 22 January 24, 2008 24 January 24, 2008 25 http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html January 24, 2008 28 January 24, 2008 30 Direct measurement using the Global Position System January 24, 2008 31 Engineering applications • Context to understand seismicity • Quantitative estimate of earthquake rates • Seismic gaps January 24, 2008 34 Distance between two points on the Earth’s surface cos sin sin 1 cos cos 1 cos( 1 ) Where: (φ, ψ) = (latitude, longitude) of selected point on plate boundary (φ1, ψ1) = (latitude, longitude) of pole of relative rotation January 24, 2008 35 January 24, 2008 36 u a sin 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) January 24, 2008 38 deg radians 6 my 5 cm o u 0.98 10 6370 km 10 sin 77 180 deg my y km January 24, 2008 39 Seismic Moment • Definition of Seismic Moment • M0=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 January 24, 2008 41 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. January 24, 2008 42 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 40o – Width = 30 km/cos(dip)=40 km • M0=μAD = 4*1011 dyne/cm2 * 200 km * 40 km * (105 cm/km)2 * 890 cm = 2.8 * 1028 dyne-cm January 24, 2008 43 Moment Magnitude • MW=(2/3) log M0-10.73 • This is the preferred magnitude scale in the seismological community. January 24, 2008 44 Application to Japan (cont.) • M0=μAD =2.8 * 1028 dyne-cm MW=(2/3) log M0-10.73 =8.24 Conclusion, by these assumptions, there is enough strain accumulated to cause a MW=8.2 earthquake under Tokyo. January 24, 2008 45