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USGS CMG InfoBank: Measuring Deformation

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Comment: 03:08 - 04:33 (01:25)
Source: Annenberg/CPB Resources - Earth Revealed - 8. Earth's Structures
Keywords: alidade, "Scott Paterson", deformation, strike, dip

Our transcription: To accurately measure and record deformation, geologists use a small instrument, which combines a compass with a hand level.
This instrument called a "pocket alidade" measures two aspects of the orientation of any tilted layer, the "strike" and the "dip."
One of the things we always want to do with structures is to measure their orientation in the field, and the way we usually do that is to measure two angles.
Those angles are called the "strike" and the "dip" of a surface.
I'm going to take the upper block and just remove it and get rid of it.
Then, we can look at the surface of the fault itself right here, which is a planer feature, and our two angles again are the "strike."
The strike is measured from a horizontal line lying within that plane to true north, so it's an angle between that line and whatever direction north is and it fixes the orientation of the plane in this direction.
The other angle that we need to measure is what's called the "dip," and that's just the angle between a line perpendicular to the strike line and the horizontal plane.
It's an angle from the horizontal down to the plane itself, and it fixes the orientation of the plane in this direction.
So the strike is from true north and the dip is from horizontal.

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