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

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Presentation transcript:

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

January 24, 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.

Source: Geological Society of America web:

Source: Geological Society of America web:

January 24, 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.

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

January 24,

January 24,

January 24,

January 24,

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

January 24,

January 24,

January 24,

January 24,

January 24,

January 24,

January 24,

January 24,

January 24,

January 24,

January 24, Direct measurement using the Global Position System

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

January 24, 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

January 24,

January 24, 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,

January 24, 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

January 24, 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 –Time past is ( )=84 years. Then accumulated slip is: –10.6 cm/yr*82 yr=890 cm.

January 24, 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 * dyne-cm

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

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