Plate rotations and velocities Plate boundaries

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EPS 122: Lecture 3 – Plate rotations and velocities

Plate rotations and velocities Reading: Fowler Ch 2


Plate boundaries

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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


Calculating relative plate velocities A flat Earth

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CvB = ?



CvB = ?

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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


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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Angular velocity

2D

3D

Right-hand rule

A reminder….


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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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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Can we obtain pole location from this data?

…start to see problems


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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Focal mechanism from earthquakes




Plate boundary motions

1906 San Andreas

1995 Kobe, Japan

San Andreas at San Juan Bautista, California - Today


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Wallace Creek A record of plate motion



Wallace Creek Measuring present day plate motions

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Wallace Creek Measuring present day plate motions


Wallace Creek

= 34 mm/yr


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Matt d’Alessio


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


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d’Alessio et al, 2005


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


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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


NUVEL-1A Comparison with GPS observations

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Global plate motion modeling Seismicity

Seismicity Plate geometry

…problems

Global distributions of continuous GPS

Documents

Plate rotations and velocities Plate boundaries