02-Plate-Tectonics
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Transcript 02-Plate-Tectonics
Earthquake Engineering
GE / CEE - 479/679
Lecture 2. Plate Tectonics
January 24, 2008
John G. Anderson
Professor of Geophysics
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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.
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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
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The radii for the tops
of the major layers
are more accurate on
this plot.
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http://topex.ucsd.edu/marine_topo/gif_images/global_topo_small.gif
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http://www2.ocean.washington.edu/oc540/lec02-1/
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http://www2.ocean.washington.edu/oc540/lec02-1/
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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.
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Why believe plate tectonics?
•
•
•
•
Ocean drilling results.
Hot spots
Earthquakes
Direct measurements
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http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html
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Direct measurement using
the Global Position System
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Engineering applications
• Context to understand seismicity
• Quantitative estimate of earthquake rates
• Seismic gaps
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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
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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)
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deg radians 6 my
5 cm
o
u 0.98
10
6370
km
10
sin
77
180 deg
my
y
km
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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
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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.
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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
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Moment Magnitude
• MW=(2/3) log M0-10.73
• This is the preferred magnitude scale in the
seismological community.
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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.
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