The Trembling Earth

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Tag Archives: sand volcano

Amazing liquefaction in Tokyo

A new video from Japan [embedded below] shows liquefaction occurring at a scale and scope that I haven’t seen before in video footage. The video is from Urayasu  town, Chiba Prefecture–an industrial suburb of Tokyo that appears to be sited on made land adjacent to Tokyo Bay. No wonder it sloshes so heavily: made land (fill) is particularly susceptible to liquefaction. We just can’t pack things down the way nature can over millennia.

The video starts in the aftermath of the March 11, 2011 M9.0 Tohoku earthquake, where sediment-filled ground water is bubbling up through gaps in the pavement, or any other fractures that represent escape routes. Within minutes, the M7.9 aftershock hits, and you can see light poles, trees, and buildings shaking violently. All the while, the engineered infrastructure sloshes and bobs, essentially floating on a thick package of fluid-saturated mush. The differential swaying you observe is effectively the dramatic reduction of seismic wave velocity in the loose, fluid-supported substrate. The sound you hear is largely the metal guard rails creaking as they’re stretched and bent. The oscillations last for a very long time.

Liquefaction commonly accompanies large earthquakes in areas where a shallow groundwater table supports suspension of soft soils when it’s shaken. I’ve posted videos of the phenomenon before. It occurs during any earthquake that strikes an area with the right mix of water-saturated sediments. It was widespread throughout Christchurch in each of their big jolts in 2011, and occurred pervasively in the shoreline areas of Japan during their monstrous 2011 quake. Fractures opened during shaking provide conduits for this newly mobile soil slurry to escape surface-ward under the weight of dry material above, producing sand volcanoes, like the one pictured in the righthand image of this blog’s banner.

After watching these videos, it’s clear why this phenomenon results in such destruction, particularly to pipelines and underground utilities. Water main failures may add to the fluid pressure mobilizing all the soil.

When you hear about the danger of living on “fill” and the destruction of liquefaction, these illustrative clips should come to mind.

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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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