Kaikoura a big shock with big surprises for scientists
When the first pictures of the impact of the November 14 earthquake started emerging, a stunned world realised just how astonishing an event it had been.
Great gashes were ripped through the landscape, giant landslips had come down, a ‘‘wall’’ more than three metres high rose across a field, there was land where there used to be sea, roads were ripped apart, and large new lakes formed where rivers had been dammed.
Then scientists started talking about the number of faults involved, culminating in a paper in March that said the quake ruptured at least 12 major crustal faults plus another nine lesser faults. Starting in North Canterbury, the rupture moved north for more than 170km, straddling two distinct active fault domains - the North Canterbury fault zone and the Marlborough fault system. On the Ke¯kerengu¯ Fault, pieces of the Earth’s crust were displaced relative to each other by up to 25m at a depth of about 15km. Parts of the South Island moved more than 5m closer to the North Island and there was extensive uplift along the coastline, with one faultbounded block being pushed up 8m.
The study’s authors, led by GNS Science geodesy specialist Ian Hamling, said the complex quake defied many assumptions about the degree to which earthquake ruptures were controlled by individual faults.
‘‘The number of faults was certainly a big surprise,’’ Hamling said.
One intriguing aspect of the quake was the way it looked to have jumped as much as 15km between some faults, when global studies of past quakes had shown ruptures were generally stopped if faults were separated by more than 5km.
Hamling said data showed there were actually hidden faults at depth that essentially connected the rupture from south to north. That did raise issues for earthquake hazard models - which estimate the strength of shaking that can be expected in an earthquake because without those hidden faults the apparent gap would be too large to allow a thoroughgoing rupture.
‘‘On top of that, the stress changes induced by the earlier part of the rupture also further promoted the northern segments to fail so it may be that the rupture could have jumped even if the hidden structures weren’t there,’’ Hamling said. ‘‘It would be highly unlikely that something as complex would happen in the same location within our lifetimes. That said, there are other areas around New Zealand with complex networks of faults and we don’t know if they could break in a similar manner.’’
The magnitude 7.1 Darfield earthquake in September 2010 was another fairly complex event in a place where it wasn’t expected ‘‘so we are certainly still learning’’.
He was working on a Marsden Fund project to try to integrate satellite radar observations with GPS to look for evidence of potentially hidden faults across the South Island.
There has been debate about how much of the earthquake’s energy was produced by surface faulting and how much was produced in the subduction zone where the Pacific tectonic plate is being pushed below the Australian Plate. From his perspective, most studies from New Zealand and overseas pointed toward the subduction zone having a minimal component, Hamling said.