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Kaikoura quake produced record ground shaking measurement


10 APRIL 2017


Last November's magnitude 7.8 Kaikoura earthquake produced the strongest ground shaking ever recorded for an earthquake in New Zealand, according to new research out this week.

Most of the ground motions recorded by seismic instruments in the upper South Island were strong, but comparable to recordings for previous earthquakes.

However, an instrument located at Waiau in North Canterbury, a short distance from the epicentre at Culverden, recorded a vertical acceleration of approximately 3g, or three times the acceleration due to gravity. The previous high of 2.2g was recorded in the magnitude 6.3 Christchurch earthquake of February 2011.

By way of comparison, passengers taking off in an airliner experience about 0.1g. A person on a roller coaster ride would experience more than 1g, although they would be firmly strapped in.

Lead author of the study in the publication Seismological Research Letters, seismologist Anna Kaiser of GNS Science, said the Waiau instrument happened to be directly over the rupture of the Humps Fault, which was one of the first of more than 20 faults to rupture during the earthquake.

Dr Kaiser said scientists use a measure called 'peak ground acceleration' or PGA to record how strongly the ground shakes during an earthquake. It is measured in both horizontal and vertical components.

"The pattern of ground accelerations across the upper South Island would have been quite complicated because of the many different faults that ruptured, and also other factors like the type of soil at a particular location," Dr Kaiser said.

The next highest PGA values were recorded at Kekerengu (1.2g) and Ward (1.3g) at the northern end of the rupture, more than 100km northeast of where the fault rupture started.

The research showed that overall more energy was released north of Kaikoura than further south. Ground motions in the northern part of the rupture were also likely to be enhanced by the ‘directivity effect’.

This is where seismic waves travelling through the ground at about 3km a second become stacked on top of each other as the rupture moves towards you, producing stronger shaking than if the rupture was moving away.

Dr Kaiser said although Kaikoura was in the central part of the overall rupture, it was further from any individual mapped surface ruptures than Waiau and Ward.

"PGA is a good measure of how ‘sharp’ the shaking was, but there are other things that are important to consider, such as the duration of the ground shaking, the strength of the rolling type of motions, and also the permanent movement of the land."

The seismic record from Waiau was so unusual and so exceptionally high that scientists were initially sceptical and wondered if the instrument was affected in some way, Dr Kaiser said.

However, the maker of the instrument - Canterbury Seismic Instruments Ltd - checked it thoroughly and found it was working properly at the time of the earthquake.

The instrument is attached firmly to the concrete floor of an implement shed on a farm in Waiau. It is part of a national network of about 180 such instruments operated by GeoNet. Designed to measure the strongest shaking in damaging earthquakes, they are sited in buildings and other structures such as bridges.

Dr Kaiser said the Waiau instrument may have recorded a type of ‘trampoline effect’ sometimes observed during large earthquakes.

"This can sometimes arise from interactions of the near-surface soils or structures with underlying stiffer material. It was also observed at some seismic instruments close to Christchurch during the February 2011 Christchurch earthquake."

She said the maximum upward force was a short spike of energy lasting less than a second, rather than sustained shaking at 3g.

The paper titled 'The 2016 Kaikoura New Zealand earthquake: Preliminary Seismological Report', discusses how different parts of central New Zealand experienced different types of damage because of the way the ground shook.

It highlights that it took more than 90 seconds for the earthquake to rupture the whole series of faults, and the strongest burst of energy release occurred in the northern part of the rupture over a minute after the earthquake first started. It also discusses landslide and tsunami impacts and the aftershock forecasts in the wake of the main shock on 14 November 2016.

Close to the faults that ruptured, ground accelerations were extremely high and ground shaking would have felt ‘sharper’, with sudden violent jolts as well as a range of other ground movement.

"This mixture of ground shaking can be potentially damaging to a wide range of structures. In Wellington, ground shaking would have been less ‘sharp’ and more like a rhythmic back-and-forth or rolling motion.

Dr Kaiser said this was mostly because the earthquake occurred some distance away from Wellington.

"Deep soft soils also enhance this type of ground shaking, so the earthquake would have been experienced quite differently if you were on rock or on softer material close to the waterfront for example."


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