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Maps updated with additional station data 11am,
I think we can all agree there has been a lot of shaking over the past few days. Scientists from GNS Science and University of Auckland, GeoNet field technicians and our platform team, have been busy at work collating the data to figure out exactly how much shaking occurred. While we still haven't retrieved all of data, here is what we have so far.
Ground shaking caused by the Kaikoura Earthquake reached over MMI VIII near the fault rupture, on the Modified Mercalli Intensity Scale and is considered severe. In Wellington, some areas experienced shaking up to MMI VI-VII, which is considered strong. This is based on the current Shakemap calculation, which includes both measured data and estimated values. This may be updated as more information is acquired.
The strongest ground shaking measured by GeoNet instrumentation so far was a peak ground acceleration of (PGA) 1.3g in Ward. Since publishing this story, data has come in from the WTMC station in Waiau with a vertical PGA over 3g. As yet, we're unsure if this is a reliable value, it may have been contaminated by other effects.
The highest ground accelerations extend in a band trending northeast from the epicentre. This aligns with where the largest fault movements have occurred, based on field surveys.
In the Wellington region, PGAs exceeded 0.2g in parts of the CBD and Lower Hutt. This is similar to that experienced during the Cook Strait sequence (earthquakes of M6.6), however the duration of shaking was significantly longer (around four times) and there was more long period energy.
In Christchurch, the recorded PGAs were lower (less than 0.1g) even though the city is closer to the epicentre. This is related to the fault rupturing from the epicentre northwards away from Christchurch. This is our understanding based on what we know so far about the faults.
We are still acquiring strong motion data from the closest stations to the rupture.
Shakemap is a combined model of the estimated ground shaking based on instrumental observations from GeoNet sites and ground motion predictions based on the current fault models. As we get more information from InSAR (Interferometric Synthetic Aperture Radar) the fault models are likely to be updated.
Shakemap also takes into account soil conditions that can amplify shaking in certain areas. This can be seen on the map as areas experiencing higher shaking despite being further from fault rupture.
The trip in their own words:
Kekerengu and Waipapa Bay
“The first fault rupture we saw was the Kekerengu Fault. The fault shifted State Highway 1 and the railway line by at least 2.3 m horizontally and 1 m vertically. We followed the fault rupture from the beach inland for 2km to Bluff Station. Here, the earthquake had dislocated hills, fences, roads, buildings and the river bed, some by as much as 10m horizontally. A house was also spectacularly moved off its foundations - the occupants were shaken up, but otherwise okay. This was as far as we flew along this fault so the surface rupture could well have continued past this point.
We continued flying south. At Waipapa Bay the road and railway line were shifted by 1m. Further south, we came to the Hope Fault. From the air it looked like this has offset the road by about 1m. We didn’t follow the Hope Fault inland on this flight, but we’ve heard from other scientists who did fly along here earlier, that they didn’t see any disturbance of the land
Hundalee, Emu Plains, and The Humps fault zone (yes, someone long ago named a fault 'The Humps')
Then it was on to the Hundalee Fault. Again, we saw the fault had disturbed State Highway 1 and the railway line as well as the beach south of Kaikoura.
We then flew to the epicentre area (The Humps fault zone and Emu Plains) and found some fault ruptures on the north side of the Waiau River, some on previously unknown faults. It was less clear exactly how these faults had moved compared to the others further north, but it looked like some horizontal movement had occurred. At the western end these ruptures had affected some small streams.”
Field trips like this one can only find where fault lines have broken through to the surface. It’s likely that other faults below the surface (or longer lengths of the faults mentioned above) have also moved. Other investigations including satellite images will help to show any subsurface fault movements.
Now that the wind and rain have subsided, additional teams are out in the field - they’re hoping to cover areas that Nicola and Pilar didn’t get to.
Fault Map by: Litchfield NJ, Barrell DJA, Begg JG, Benson A, Berryman KR, Clark KJ, Cochran UA, Cox SC, Gasston C, Glassey, PJ, Heron DW, Howarth JD, Langridge RM, Little T, Lukovic B, Nicol A, Pettinga J, Ries WF, Rowland J, Stirling MW, Townsend DB, Upton P, Van Dissen RJ, Villamor, P. 2016. 14th November 2016 Kaikoura Earthquake. Preliminary earthquake geology observations. GNS Science. http:dx.doi.org/10.21420/G2MW2D