Focal Mechanism


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Focal Mechanisms

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Focal Mechanisms

After an earthquake occurs, seismologists create graphics of focal mechanismsinformally referred to as beach balls, to show the faulting motions 
that produced the earthquake. Focal mechanisms, also known as fault-plane solutions, are based on the direction of the first arriving P wave, and are used to show direction of movement on a fault using great circles with 2 intersecting curves. 
When the ground shifts abruptly as it does during an earthquake, energy in the form of seismic waves radiates in all directions.  The P waves curve through the Earth's interior up to the surface, pushing and pulling the ground from beneath the seismometer.  A P wave that lifts the ground corresponds to a crest on the seismogram; a P wave that pulls the ground inward corresponds to a trough.  Seismologists can examine the first motion of the P waves to reconstruct the initial forces that triggered the event (see:

 mp4 (41 MB)


Directed by Robert Butler, geophysicist, University of Portland

Graphics & Animation:  Jenda Johnson
Narration: Roger Groom, Mount Tabor Middle School, Portland OR
Dr. Geophysics character: Richard Blakely, geophysicist, U.S.G.S.
Music:  "Constellation" by Water Strider (
"Diagonal Stride" by Fadin' by 9 ( written & fiddled by Seth Moran, seismologist U.S.G.S.


Animations and videos are made in partnership with Earthscope and Volcano Video & Graphics.

Please send feedback to Jenda Johnson.