Kaikoura quake records strongest ground shaking in NZ history
The Kaikoura quake produced the strongest ground shaking ever recorded for an earthquake in New Zealand, a new study has found.
GNS seismologist and the lead author of a new study Anna Kaiser said the study was the latest step in figuring out how the magnitude-7.8 quake went down.
It's already been described as one of the most complicated quakes in New Zealand's history thanks to more than 20 fault ruptures, now it's also been labelled the greatest when it comes to ground acceleration.
The 2016 Kaikoura, New Zealand, Earthquake: Preliminary Seismological Report looked at rupture patterns, permanent earth movements and ground motions caused by the quake.
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PEAK GROUND ACCELERATION
While most of the ground motions recorded by seismic instruments in the upper South Island were strong, they were comparable to recordings for previous earthquakes.
However, an instrument located at Waiau in North Canterbury, a short distance from the epicentre near Culverden, recorded a vertical acceleration about 3g, or three times the acceleration due to gravity.
Until then, the greatest ground acceleration recorded was the 2.2g recorded in the magnitude-6.3 Christchurch earthquake of February 2011.
Peak ground acceleration - or PGA - records how strongly the ground shakes during an earthquake. It is measured in both horizontal and vertical components.
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.
Kaiser 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.
"This particular instrument was was basically right on top of the first fault to rupture... it was right in the thick of it."
The seismic record from Waiau was so unusual that scientists were initially sceptical and wondered if the instrument was affected in some way, Kaiser said.
However, the maker of the instrument - Canterbury Seismic Instruments - 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, these instruments are in buildings and other structures such as bridges.
DIFFERENT TYPES OF SHAKING
The pattern of ground accelerations across the upper South Island would have been quite complicated because of the many different faults that ruptured, and other factors like the type of soil at a particular location, 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 100 kilometres from the epicentre.
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.
In Wellington, ground shaking would have been less sharp and more like a rhythmic back-and-forth or rolling motion.
"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," she said.
MORE THAN 90 SECONDS
It took more than 90 seconds for the earthquake to rupture the whole series of faults.
The strongest burst of energy release occurred in the northern part of the rupture more than a minute after the quake started.
The paper also discussed landslide and tsunami impacts and the aftershock forecasts in the wake of the main shock.
Kaiser said studies into the quake were ongoing and this paper was an overview of research gathered during the month following the quake.
Getting a better understanding of what happened during the quake would help authorities plan for different types of future quake scenarios, she said.