Rather than the initial reports of a single 7.5-magnitude earthquake based near Hanmer Springs the now-named Kaikoura quake is believed to be a multiple rupture involving four faults, with the two-minute long event actually comprising of a reverse fault thrust near Culverden, and a series of strike slip thrusts near Kaikoura heading northward towards Wellington.
[Note: since this was written, the November 14 earthquake has been upgraded to magnitude 7.8.]
The reverse fault near Culverden thrust the northern South Island (Australian plate) up over the Pacific Plate, a straight up and down motion creating several surface ruptures. The strike slip faults were caused by two fault planes sliding past each other in opposing directions, though these occurred on a few different faultlines, including several offshore locations. The 7.5-magnitude refers to the energy released by the whole event, not quakes on individual faults.
So far, GeoNet has identified that the Kekerengu Fault at the coast, the Hope Fault, the Hundalee Fault and the newly-discovered fault at Waipapa Bay, all contributed to the 7.5M earthquake almost simultaneously.
As it was not a single earthquake event involving one faultline, it has been difficult for seismologists, geologists and tsunami specialists alike to pinpoint exactly what has been happening. Caroline Holden, GNS Science duty officer, noted in a live Q & A on stuff.co.nz on Wednesday; “this is an earthquake that occurred possibly along 200 km of fault rupture! Therefore it takes more time to carefully understand the earthquake process and its impacts on the ground”.
Maps of the reverse fault thrust show the proximity to known faults, while others show the location of ruptured land and the breadth of quake activity on November 14.
Shifting earth means some areas have been moved up to four metres, which is well understood as several GNS Science monitoring stations were moved by the quake.
GNS scientists confirmed that the reported aftershock probabilities from GeoNet had been close to what had later been observed, indicating that the aftershocks were following a fairly standard pattern. The aftershock probabilities “include likelihood of earthquakes on neighbouring mapped and unmapped faults as well as on the subduction interface,” said Caroline Holden. The latest update was on Tuesday afternoon, when GeoNet predicted that there was 37% chance of at least one M6-6.9 quake within the next day, according to new calculations.
Seismologists at GNS aren’t the only ones poring over recent events, their volcanologists have been keeping a close eye on volcanic activity around the country. But luckily our volcanoes seem to have ignored the Kaikoura quakes.
In response to the tsunami alert, Ken Gledhill, director of GeoNet, said that “because we do not have a 24/7 monitoring centre we have to wake people and get them out of bed to look at complex data and make serious calls very quickly.” In response to how people acted when faced with the possible tsunami, he said that “for those people who took those brave steps in the middle of the night of the tsunami, I applaud your efforts. YOU DID THE RIGHT THING. For people who were further away and waited but left once told to evacuate, YOU ALSO DID THE RIGHT THING”.
On the Kaikoura coast, which has experienced most of the destruction from both the quake and subsequent landslides, a massive upthrust of the seafloor has had devastating effects on marine life exposing shellfish. A similar situation was caused by an earthquake in Chile in 1985, where the “affected sites experienced catastrophic mortality of many species”, according to Jeff Shima, a marine ecologist from the Victoria University of Wellington. “Recovery took years.”
The landslides have cut off Kaikoura from the north and south on SH1, damaged the railway and damaged the famous Ohau seal breeding colony.
Excellent summaries of the science behind the earthquakes can be found from Radio NZ’s Alison Ballance and NZ Herald’s Jamie Morton.
Nov 14
GeoNet: M7.5 Kaikoura Earthquake: Latest updates
NZ Herald: Explained: The science behind the quake
Stuff.co.nz: Seismologists record 2 metre shift south of Marlborough
Science Media Centre (AU): Magnitude 7.5 earthquake – Expert Reaction
NZ Herald: How has the earthquake warped New Zealand?
The Conversation (AU): What happened in New Zealand’s magnitude 7.5 earthquake
Nov 15
NZ Herald: Monday’s earthquake lifted seabed around Kaikoura ‘by at least a metre’, scientists confirm.
RadioNZ: The Science Of… Earthquakes
GeoNet: M7.5 Kaikoura Quake: What we know so far
Radio NZ: Risk of tsunami remains after massive quake
NZ Herald: 7.5 quake: Five big questions answered
The Spinoff: Kaikoura aftershocks: the latest probability of another big shake
Science Media Centre: Coping with disaster – Expert Q&A
Geonet: A Message from GeoNet’s Director: Responding to the Monster
Science Media Centre: Earthquake effect on tourism – Expert Q&A
Nov 16
GeoNet: We haven’t forgotten the volcanoes; fortunately, they are staying quiet
Stuff.co.nz: Live Chat: GNS officer Caroline Holden shares new information about quakes
Stuff.co.nz: Raised seabed off Kaikoura coast baffles marine experts
Science Media Centre: Marine uplift from Kaikoura earthquake – Expert Reaction
NZ Herald: North Canterbury quake explained from a scientist’s perspective
GeoNet:Ruptured land: observations from the air
The Spinoff: This stunning map shows that six faults – at least six – ruptured in the big Kaikoura quake