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An Earthquake Rattled His Store. Then He Noticed What The Sand Pendulum Created

September 21, 2016

When a magnitude 6.8 earthquake shook Olympia, Washington, in 2001, shop owner Jason Ward discovered that a sand-tracing pendulum had recorded the vibrations in the image below.

Credit: Jason Ward

For anyone wondering, this is how a sand pendulum works:

Ward says the rose-like shape was formed by the trembling pendulum during the 45 seconds the quake shook the Pacific Northwest.

Credit: Jason Ward

You can see two distinct pendulum motions here that relate to the type of seismic wave and their timing. The primary wave, or P wave, occurs first and is a short, vertical (up and down) movement. This is marked in the sand by the diamond shaped pattern in the background. Then there is pause (depending on how far you are from epicenter), giving the pendulum time to get back towards center. Then the secondary wave, or S wave, hits. The S wave is a more varied in its movements and last for a much longer time. This is the worm nest looking thing in the middle that has overwritten the center of the P wave pattern.

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