The Trembling Earth

now at blogs.agu.org/tremblingearth

Liquefaction in an aftershock

I stumbled upon a fascinating video a couple of weeks ago showing what happens to sand volcanoes as seismic waves pass. This would have fit in either my aftershocks or my liquefaction posts from last week, but it’s both stunning and unique so I decided it deserves a post of its own.

 

This New Zealand man is filming sand blows forming from liquefaction of a school yard in the 22 Feb M6.3 earthquake, when a large aftershock hits. Shaking from the aftershock isn’t immediately visibly apparent, but shrieks and metallic rattling herald its onset. Once the sharp noisy shaking is over, the ground continues undulating, apparent only in the fissures opened from liquefaction. As the seismic waves from the aftershock compress, dilate, and otherwise jostle the ground, the fissures feeding the sand blows open and close, alternately squirting silty water out and drawing it back down–along with the air above.

Quite a phenomenal illustration of the ground’s undulation. The sight of this lends credence to some alleged eye-witness reports of the 1811-1812 Missouri earthquakes, which claim that as the ground heaved, “fissures opened and slammed shut… spewing water high into the air.” Reports like this (especially from random websites) tend to suffer from the telephone syndrome, in which subtle modifications by a series of reporters lead to misinformation. So take the accounts in that link with a grain of salt; perhaps I’ll compile legitimate resources on that fascinating earthquake sequence in a future post.

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