plate rotations and velocities - uc berkeley …seismo.berkeley.edu/~rallen/eps122/lectures/l03.pdf2...
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EPS 122: Lecture 3 – Plate rotations and velocities
Plate rotations and velocities Reading: Fowler Ch 2
EPS 122: Lecture 3 – Plate rotations and velocities
Plate boundaries
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EPS 122: Lecture 3 – Plate rotations and velocities
Plate velocities A flat Earth
AvB The velocity of plate B with respect to plate A
AvB = - BvA
Mid-ocean ridges: • Also called spreading centers • The half spreading rate is the spreading
rate on one side of the ridge
Subduction zones: • Carat points down dip • Velocity vector on subducting
plate
Transform faults: • Also called strike-slip faults • Right- or left-lateral
• Drawn with half arrow heads
EPS 122: Lecture 3 – Plate rotations and velocities
Calculating relative plate velocities A flat Earth
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EPS 122: Lecture 3 – Plate rotations and velocities
Calculating relative plate velocities A flat Earth
CvB = ?
EPS 122: Lecture 3 – Plate rotations and velocities
Calculating relative plate velocities A flat Earth
CvB = ?
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EPS 122: Lecture 3 – Plate rotations and velocities
Latitude, longitude, great and small circles Rotation poles and vectors
Latitude
• small circles
• on a plane perpendicular to the Earth’s rotation pole
Longitude
• great circles
• pass through rotation pole
• on a plane that contains the rotation axis
EPS 122: Lecture 3 – Plate rotations and velocities
Euler Euler’s fixed point theorem:
“The most general displacement of a rigid body with a fixed point is equivalent to a rotation about an axis through that fixed point”
…a plate is a rigid body
…if the earth is spherical, plates rotate around the center of the Earth
Therefore, any displacement of a fixed point on the surface of the Earth can be represented a rotation about a suitably chosen axis passing through the center of the Earth
Bullard et al, 1965
rotation axis
rotation pole
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EPS 122: Lecture 3 – Plate rotations and velocities
Angular velocity
2D
3D
Right-hand rule
A reminder….
EPS 122: Lecture 3 – Plate rotations and velocities
Plate velocity
plate velocity:
v = R sin
Average radius of the Earth R = 6371 km
Plate velocity depends on the angle from the rotation axis
When quoting plate velocities, give the velocity at 90° i.e. the maximum
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EPS 122: Lecture 3 – Plate rotations and velocities
Strike of transform faults Measuring present day plate motions
Provides location of poles orientation of rotation vector
EPS 122: Lecture 3 – Plate rotations and velocities
Strike of transform faults Measuring present day plate motions
Provides location of poles orientation of rotation vector
Can we obtain pole location from this data?
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EPS 122: Lecture 3 – Plate rotations and velocities
Strike of transform faults Measuring present day plate motions
Provides location of poles orientation of rotation vector
Can we obtain pole location from this data?
…start to see problems
EPS 122: Lecture 3 – Plate rotations and velocities
Variation in spreading rate Fit data to obtain rotation vector
i.e. pole location and rotation velocity
Measuring present day plate motions
plate velocity:
v = R sin
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EPS 122: Lecture 3 – Plate rotations and velocities
Focal mechanism from earthquakes
Provides location of poles orientation of rotation vector
Measuring present day plate motions
EPS 122: Lecture 3 – Plate rotations and velocities
Plate boundary motions
1906 San Andreas
1995 Kobe, Japan
San Andreas at San Juan Bautista, California - Today
Measuring present day plate motions
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EPS 122: Lecture 3 – Plate rotations and velocities
Wallace Creek A record of plate motion
Measuring present day plate motions
EPS 122: Lecture 3 – Plate rotations and velocities
Wallace Creek Measuring present day plate motions
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EPS 122: Lecture 3 – Plate rotations and velocities
Wallace Creek Measuring present day plate motions
EPS 122: Lecture 3 – Plate rotations and velocities
Wallace Creek
= 34 mm/yr
Measuring present day plate motions
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EPS 122: Lecture 3 – Plate rotations and velocities
Matt d’Alessio
EPS 122: Lecture 3 – Plate rotations and velocities
Northern CA
Time till LA reached SF:
Los Angeles: on the Pacific Plate
San Francisco: on the North American Plate
500 km
5 cm/yr = 10 million years
LA
Total Pacific-North America plate motion:
50 mm/yr
Daily GPS observations
Measuring present day plate motions
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EPS 122: Lecture 3 – Plate rotations and velocities
d’Alessio et al, 2005
EPS 122: Lecture 3 – Plate rotations and velocities
Satellite based observations Daily GPS observations
Slope gives average velocity of station to the north
SCIGN – southern CA
Total Pacific-North America plate motion:
50 mm/yr
Measuring present day plate motions
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EPS 122: Lecture 3 – Plate rotations and velocities
NUVEL-1A Measuring present day plate motions
Combine multiple observations and solve for best-fitting rigid plate motions
• 277 ridge spreading rates – use distance of last magnetic
reversal from ridge
• 121 ocean transform-fault azimuths
• 724 earthquake slip vectors
Eurasia fixed
EPS 122: Lecture 3 – Plate rotations and velocities
NUVEL-1A Comparison with GPS observations
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