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First Motion

By Earthquake Hazards Program 2021 (approx.)
First motion diagram: a circle split into quadrants with hollow white circles in the upper left and lower right and solid black circles in the upper right and lower left. There is an arrow pointing down and another pointing right from the upper left quadrant, and an arrow pointing up and another pointing left from the lower right quadrant.
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Detailed Description

First motion mechanism. On a seismogram, the first motion is the direction of ground motion as the P wave arrives at the seismometer. Upward ground motion indicates an expansion in the source region; downward motion indicates a contraction.

Sources/Usage

Modified from UC Davis