Shaking under scrutiny
Extreme shaking in the February 2011 Christchurch earthquake may not be as strong as first thought if a re-examination by scientists reveals a similar effect at play in the 2016 Kaiko¯ ura quake.
Scientists are withholding judgment on the outcomes of their investigation, in which they will look again at instruments and sites that recorded severe shaking at 12.51pm on February 22, 2011, to ensure they accurately measured the actual ground movement.
A paper in this week’s Scientific Reports journal concluded that a rogue reading from an instrument in a farm shed at Waiau exaggerated the peak ground accelerations (PGA) in the Kaiko¯ ura quake, making them more than double those of the second-highest recorded in the quake zone.
GNS Science says it is important that recordings made during large earthquakes are now reviewed to check their veracity.
Peak ground accelerations of more than twice the force of gravity (g) – 2.2g – were recorded at Heathcote Valley Primary School, a figure that put the magnitude-6.2 Christchurch quake into the world’s top three in terms of severe shaking.
The quake’s magnitude was originally considered to be 6.3.
GeoNet’s page on the February 22, 2011, quake shows high ground accelerations were also recorded at Pages Rd pumping station (1.88g) and Hulverstone Drive pumping station (1.07g).
The anomalous Waiau reading, of 3.2g, was recorded close to the epicentre of the November 14, 2016, magnitude-7.8 Kaiko¯ ura quake.
This week’s paper said analysis revealed the slab foundation of the farm shed, containing bags of fertiliser, had lifted from the ground during the quake and ‘‘bounced several times’’, creating a ‘‘flapping effect’’ that did not accurately reflect the true ground shaking.
The authors found the actual level of shaking at that site was about 1.5g. Co-author Dr Yoshihiro Kaneko, a GNS Science seismologist, said the Waiau reading had been discounted from the outset and not included in developing engineering standards or seismic hazard models, including assessing the probability of liquefaction and landslides.
‘‘Scientists regarded the Waiau measurement with scepticism right from the start, so it has had little or no impact.’’
The ground motion recorder had been working accurately but it was the behaviour of the shed that affected the readings, he said.
When asked if the findings might affect the extremeness of the Heathcote and other readings, Kaneko said the assessment would look for any signs those sites could have made ‘‘anomalous measurements’’.
‘‘GNS Science is planning a process of due diligence that will involve assessing strong-motion sites in the Christchurch area where large PGA figures were recorded in 2011. This process is in the very early stages and, until it is completed, it would be premature to make any judgments on the 2011 Heathcote PGA recording.’’
The authors said their conclusions had ‘‘important implications for how strong-motion sensors in New Zealand and elsewhere are installed to ensure the recordings of actual ground motions, rather than soil-slab interactions’’.