Serendipitous quake unearths explanation

23 February 2016

Serendipitous quake unearths explanation

Liquefaction caused by the Valentine’s Day earthquake in Christchurch may hold the answer to lingering questions for Lincoln University soil scientists and GNS Science specialists puzzled by enigmatic features in the soil at numerous liquefaction sites.

Their research into prehistoric liquefaction, or paleoliquefaction, received an unexpected boost when new liquefaction appeared at a Kairaki Beach site, the latest site to be studied in a long-term project aimed at improving liquefaction hazard awareness.

The site resembles an archaeological dig, with gridded areas marked out by string and perfectly rectangular trenches, painstakingly dug out to reveal an unusual and distinctive soil profile showing repetitions of convoluted soil layers.

Associate Professor Peter Almond of the Lincoln University Soil and Physical Sciences Department says they’d seen these features before but couldn’t be sure if what they were looking at was in fact paleoliquefaction as they didn’t know how the unusual profiles had formed.

“We’d seen features like this in the bottom of trenches at QEII and Wainoni Parks while studying the effects of the 2010 and 2011 earthquakes. They clearly predated those earthquakes, but we didn’t see anything similar resulting from the recent liquefaction. There hasn’t been much research done in coastal dune environments and no-one has documented exactly what to look for as evidence of past liquefaction. It didn’t help that the soil in those parks had been significantly modified. With fresh liquefaction at Kairaki we have new insights. We have found blisters and depressions we can dissect layer by layer to build a 3D picture of the structure and use it as a model for interpreting features we see elsewhere.”

Associate Professor Almond says the team’s careful excavations at Kairaki Beach have revealed that liquefaction in sand country is prone to forming collapse features that can preserve the old topsoil, which is then covered by sand blow material during the liquefaction event, on which another topsoil forms. “That’s the origin of the weird soil layering we couldn’t understand.” Multiple generations of these features points to Kairaki Beach being hypersensitive to liquefaction.

“This is about the northern-most locality for liquefaction caused by the Valentine’s Day earthquake, while nearby Kaiapoi had no evidence of liquefaction as far as we know, despite it being badly affected in September 2010 and February 2011. We suspect there are probably so many events of past liquefaction recorded here that we may never be able to disentangle them all.” Local residents probably do not need much convincing of the sensitivity of the area to liquefaction, with some properties having sand blows and flooding during every major earthquake since 2010.

Associate Professor Almond says paleoliquefaction provides a good off-fault record of earthquakes and ground acceleration, and that knowing what features to look for will make it easier to identify sites with prehistoric seismic activity. “Scientists studying past earthquakes often dig trenches across faults to decipher how many times and when the fault has ruptured. However, where faults generating earthquakes have no surface trace, such as those that produced the September 2010 Darfield earthquake, paleoliquefaction is an alternative fingerprint of past earthquakes, and importantly, one that can also tell us how strong the shaking was.”

The Canterbury earthquakes have created new opportunities for studying liquefaction and training a new generation of scientists specialising in paleoliquefaction research. Monica Bucci arrived at Lincoln University from Italy in 2013 to begin her PhD research, which is funded by the Natural Hazards Research Management Platform, and she has worked closely with Sarah Bastin, a PhD candidate at the University of Canterbury. Both students’ research is contributing to global understanding of a potentially devastating natural hazard.

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