March 4, 2011

Liquefaction in New Zealand

Posted by Austin Elliott

One of the ubiquitous features of the nice flat sedimentary plains where people tend to prefer building cities, suburbs, or at least vast agricultural tracts, is that they generally liquefy when subjected to strong shaking. The results include serious instability of the “solid” ground, flooding, and ejection of silty water in places it would certainly be best not to have water being ejected.

Here’s an example from Christchurch

Here’s a video a clever New Zealand man made to demonstrate liquefaction in action as he cleaned up the mess around his house:

So why does this happen?

Imagine the layers of sand, gravel, and silt that constitute the subsurface of many cities. They’re not consolidated into hard rock yet–you can still dig through them with a tractor or even a shovel–so there is pore space between the individual grains. Water infiltrates this pore space, and tends to sink down and pool at the lowest possible position thanks to gravity. The top of this pooled groundwater is a roughly flat surface we know as the “water table.” Above it water may cling to grains and reside in the pore space, but any gaps between grains are mostly either cemented shut or filled with air.

When seismic waves pass through these water-saturated layers (i.e., an earthquake happens) they compress grains and momentarily reduce pore space, increasing the pressure on the water and driving it to squeeze into new positions. As this happens repeatedly during the passage of seismic waves, the water squeezes upward and becomes more evenly distributed throughout the rock, prying grains apart and thus suspending them so that the whole batch of material begins to behave like a liquid. In a way it’s akin to mixing oil/water/milk with dry flour to make gooey batter. Sort of.

This New Zealand child demonstrates how jostling water-saturated silt changes the way it behaves–inducing liquefaction in the product of liquefaction, which many of us might call quicksand:

The liquefied ground suffers a substantial loss of strength, allowing things jostling around in/on it to sink in. Dramatic video exists of this phenomenon occurring allegedly during an earthquake in Japan in 1964:

When liquefaction occurs within a material that is unconfined on one side (e.g., land atop a hill or coastal bluff), it results in lateral spreading.

Liquefaction during the 1964 Good Friday earthquake in Alaska results in lateral spreading collapse of the coastline

This is common along coastlines and the banks of rivers during earthquake shaking, and is the phenomenon that results in surreal scenes like this one from the Good Friday earthquake in Alaska, 1964:

Disaggregation of the ground after the 1964 Alaska earthquake

Liquefaction is common in large earthquakes, and especially prevalent in the flat-lying sedimentary basins that are ideal sites for cities and farms. If it’s inevitable, it’s at least nice to see why we’re so much at risk of it.