Soil Liquefaction

While I was doing some general research on the earthquake's impact on Japan's (and affected areas') landscape, I came across this unfamiliar concept: soil liquefaction.

"Agence France-Presse reportsthat the parking lot at the Tokyo Disneyland theme park was covered with "water-logged segments from the ground." At first, officials blamed the tsunami, until they realized that the soil itself had turned to liquid. (The Tokyo Disneyland site was a setup for soil liquefaction for two reasons: it is located in Tokyo Bay, and it is built atop a landfill. Both of these are associated with water-logged soils."
(from here)

I then Youtubed it, and found several fascinating videos of soil liquefaction actually happening in real time in the aftermath of the earthquake.

This is a video of the impact of the tremors in the Chiba prefecture in Japan. The other one can be viewed here (unfortunately, embedding has been disabled, which means I can't put it here).

So, what exactly is soil liquefaction and how does it occur? As the name suggests, soil liquefaction is essentially soil suddenly developing the consistency and nature of a liquid, or in other words, 'liquifying'. More technically,

"[l]iquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading ... [i]t occurs in saturated [completely filled with water] soils ... [w]hen liquefaction occurs, the strength of the soil decreases and the ability of a soil deposit to support foundations for buildings and bridges is reduced ..."
(from Wikipedia, the world's most trusted source!)

This phenomenon is most likely to occur in saturated, loose (i.e. low density) sandy soils, since it has a greater tendency to compress when weight is applied, as opposed to denser sands, which tend to expand in volume. When the soil is saturated by water, the gaps between soil grains are filled, resulting in the water increasing in pressure. Since water flows from zones of high pressure to low pressure, it usually moves upwards through the soil towards the ground surface. However, if the weight is rapidly applied and/or significantly large and/or repeated often (in this case specifically, by multiple, successive tremors), the water is unable to flow out in time and pressure builds to such an extent that the soil structure is lost. With that loss the strength and integrity of the soil disappear as well, and it is observed to behave like a liquid.

This is a computer simulation of soil behaviour as it becomes more and more saturated.

This is a result of the 1964 Nigata earthquake, where whole buildings have tilted or outright toppled over due to soil liquefaction.

In the case of Japan, soil liquefaction manifested itself chiefly through land instability - the cracking and movement of the ground, particularly evident through the fractures and ruptures in pavements and roads. This also had, unsurprisingly, the effect of tilting or even toppling some buildings; those gave way after the foundations on which they stood lost the soil structure necessary to support the weight of the buildings. Given the intricate physics and architecture behind the construction of such buildings, soil liquefaction often renders the buildings utterly unserviceable and uninhabitable, if not outright dangerous.

This is a picture of some of the effects of soil liquefaction in various prefectures in Japan.

Soil liquefaction predominantly happens in coastal areas or areas where the land had previously been reclaimed due to their susceptibility to flooding. On that note, soil liquefaction is different from flooding primarily because flooding occurs on the ground surface, and though water is eventually absorbed downwards by the soil, the rate at which it happens is slow enough that the soil is never saturated, and there is no build-up of pressure. Soil liquefaction occurs only when the soil is engulfed, almost, by water, from under or within, and it happens with such speed and violence that it is unable to seep upwards out of the soil in time. This definitely affects the physical landscape of the area given the havoc it wreaks on both the natural and manmade landscapes.

This article illustrates an example of soil liquefaction occuring in Aiba, an inland city, again in Chiba prefecture. Due to the tilting or sinking of buildings into the soil, soil liquefaction "has rendered over 100 homes uninhabitable", though "the damage was mostly confined to one neighborhood". The reason for soil liquefaction in this city was discovered by the city authorities, who found out from aerial photographs taken 62 years ago that the area on which the city stood was once a pond/marsh, later filled in with soil. This reclamation explains the occurrence of soil liquefaction.

One positive aspect of soil liquefaction, however, is the tendency for subsequent earthquakes or tremors to be significantly dampened and hence mitigated. Given the advance of technology, there have been methods invented to mitigate the effects of soil liquefaction, all of which are essentially based on various techniques of soil compaction to increase the density of the soil in order to lower the chances of soil liquefaction occurring.

Paleoliquefaction or paleoseismology, the study of liquefaction features left by prehistoric earthquakes, can reveal important and significant information about such earthquakes, which occurred before any records or accurate measurements were or could be kept and taken.

Personally, I found this little-known but quite destruction side effect of earthquakes fascinating. It made me consider how the physical landscape is very much in a dynamic equilibrium, and while it is really easy to assume that the physical landscape is unchanging and permanent, simple shifts or imbalances in a factor could result in the entire equilibrium collapsing, and humans are affected because our construction, infrastructure planning (and in general civilisation!) are prepared only for one state of the physical landscape without full knowledge of possible changes or problems - until they happen. Land reclamation and the construction of buildings seem like ingenious solutions to human problems of space constraints and population increase, but they also come with flaws with regards to their interaction with possible states of nature. On a larger scale, of course, this lesson can be extended to the presence of nuclear facilities within Japan, given the knowledge that Japan is a disaster-prone country. In situations like this, human technology and knowledge always seem to be running damage control.